ORIGINAL PAPER

On the use of a steerable main landing gear for crosswind landingassistance

Dennis Vechtel • Ute Marita Meissner •

Klaus-Uwe Hahn

Received: 7 November 2013 / Revised: 17 February 2014 / Accepted: 18 March 2014

� Deutsches Zentrum fur Luft- und Raumfahrt e.V. 2014

Abstract Several crosswind-related incidents show that

landing under heavy crosswind conditions can be chal-

lenging for pilots and may pose a threat to aviation or at

least lead to higher pilot workload and/or irregularities in

operations. For transport aircraft the common approach

technique for crosswind landings is the so-called crabbed

approach with wings level and a windward heading cor-

rection. This technique requires alignment of the aircraft

with the runway prior to touchdown in order to keep lateral

loads of the landing gear and tyres as low as possible and to

maintain the controllability on ground after touchdown.

The German Aerospace Center Institute of Flight Systems

has used the idea of steerable main gears and developed a

crosswind landing assistance system. During approach all

gear struts are automatically aligned with the runway so

that no decrab manoeuvre is required. On ground the

assistance system uses each steerable landing gear, differ-

ential braking, and the aerodynamic control surfaces to

control and stabilise the aircraft. After touchdown the air-

craft is automatically aligned with the runway centreline

and the still existing crab angle is slowly reduced. A

simulator study with pilots in the loop using a model of a

typical medium range transport aircraft was conducted in

order to evaluate the benefits of such a landing technique.

The study revealed that not only the aircraft controllability

could be improved by landing in crabbed motion under

strong crosswind conditions, but that the side forces acting

on the landing gear can be reduced significantly as well. It

was also shown that the use of steerable main landing gears

is able to enlarge the spectrum of autoland operations,

which is relatively limited at present in terms of maximum

allowable crosswinds. All together the system has shown to

be able to improve flight safety, lower the risk of weather-

related delays due to go-arounds or diversions, and it also

reduces structural loads on the landing gear during touch-

down and landing.

Keywords Crosswind landing � Steerable landing gear �Pilot assistance � Simulator study

Abbreviations

CLAS Crosswind landing assistance system

ECAM Electronic centralised aircraft monitoring

EFCS Electronic flight control system

EFIS Electronic flight instrument system

FCOM Flight crew operating manual

FCS Flight control system

FCU Flight control unit

ILS Instrument landing system

NDB Non-directional beacon

NTSB National Transportation Safety Board

UAV Unmanned aerial vehicle

VOR VHF omnidirectional radio range

List of symbols

Fx Longitudinal force

Fy Lateral force

This paper is based on a presentation at the German Aerospace

Congress, September 10–12, 2013, Stuttgart, Germany.

D. Vechtel (&) � K.-U. Hahn

German Aerospace Center (DLR), Institute of Flight Systems,

Lilienthalplatz 7, 38108 Brunswick, Germany

e-mail: dennis.vechtel@dlr.de

K.-U. Hahn

e-mail: k-u.hahn@dlr.de

U. M. Meissner

Auenweg 36, 50996 Cologne, Germany

e-mail: dr.r.meissner@t-online.de

123

CEAS Aeronaut J

DOI 10.1007/s13272-014-0107-2

Fz Vertical force

FS Spring force

FD Damping force

fO Oleo deflection

ny Lateral load factor

pT Tyre pressure

rK Yawing rate in track-fixed coordinates

uK Longitudinal velocity in track-fixed coordinates

vK Lateral velocity in track-fixed coordinates

g Steering angle

lB Brake friction coefficient

lR Roll friction coefficient

s Skid angle

1 Introduction

Crosswind landings are part of every pilot’s daily practice.

Whilst landing with light and moderate crosswind com-

ponents is a natural part of daily airline operations, heavy

crosswind conditions are highly demanding for pilots and

may pose a considerable threat to aviation.

Increasing air traffic in addition to capacity constrains at

airports represent a challenge especially at large international

airports. The implementation of new runways (under con-

sideration of crosswind aspects) is difficult. Today already

existing runways are not always optimal with respect to

crosswind aspects (Fig. 1 with Copenhagen as a good

example for an airport with frequent crosswind conditions).

They are often exposed to strong crosswinds and this even

applies to newly opened runways. This problem may increase

in future since in many areas a change of the main prevailing

wind direction can be observed during the last years.

Also, the number and strength of storms seem to

increase in the northern hemisphere. Since 2007 the num-

ber of storms which reached hurricane strength is above the

long-term average [1]. Consequently, a National Trans-

portation Safety Board (NTSB) inquiry revealed an annual

amount of about two dozen crosswind-related incidents in

the past few years (Fig. 3).

Thus, technical solutions, which make air traffic less

dependent on strong crosswind conditions, can contribute

to flight safety. Airports can benefit from fewer annulments

and the advantages for airlines are a possible reduction of

unwanted diversions.

1.1 Crosswind landing techniques

There are two basic approach techniques accomplished for

landing an aircraft under crosswind conditions, which can

also be combined (Fig. 2).

Crabbed approach In crosswind conditions the aircraft

can be landed with a crab angle for correction of the wind drift

to align the aircraft’s track with the runway centerline. This

crabbed approach is conducted with wings level and wind-

ward heading correction as shown in Fig. 2, left-hand side.

Sideslip approach Another technique for landing in

crosswind conditions is to land without crab angle as

shown in Fig. 2, right-hand side. Here, the aircraft heading

is aligned with the runway centerline. The pilot generates a

sideslip angle combined with a bank angle applying into-

wind aileron deflection and with rudder input against wind

(cross-control).

Depending on the strength of the crosswind component,

there are limiting factors for a safe landing of an aircraft in

crosswind. The limits for pitch attitude and bank angle are

Fig. 1 Distribution of wind direction and wind speed at Copenhagen

airport between 1991 and 2010 in relation to existing runways Fig. 2 Crabbed and sideslip approach [2]

D. Vechtel et al.

123

the limiting parameters for preventing a tail strike, as well

as engine or wing tip contact (geometrical limitations). The

aileron/rudder effectiveness defines the limitation for the

aircraft’s capability to keep a steady sideslip angle in

crosswind conditions (aerodynamic limitations). The geo-

metrical and aerodynamic limitations are the main handi-

caps for the sideslip technique without crab angle.

However, there are limiting factors for landing with a

crab angle and wings level, too. With a large crab angle

during touchdown large side forces have to be transmitted

between the main gear tyres and the runway surface.

Especially on dry runways such excessive lateral loads on

landing gear and tyres can lead to damages of the main

landing gears. Therefore, the crabbed approach technique

requires alignment of the aircraft with the runway with a

decrab manoeuvre just before touchdown on the runway.

For these reasons, Airbus recommends for landing under

strong crosswind conditions a crabbed approach with a

combination of crab angle (maximum 5�) and wing down

(maximum 5�) [3]. However, the crab angle is the pre-

dominant factor for landings under strong crosswind

conditions.

1.2 Incidents and accidents

As a consequence of all above-mentioned limiting factors

to handle crosswind landings, the pilot’s effort to com-

pensate for crosswind is always a challenge in spite of

routine crosswind training. The following factors are often

involved in crosswind landing incidents and accidents:

unawareness of changes in landing conditions with time,

for example changes in wind direction, wind velocity or

gust increase (Fig. 3). For the pilot’s acceptance of the

prevailing conditions the consequences of a diversion to

another airport with less crosswind conditions plays a role

as well.

The analysis of crosswind-related incidents and acci-

dents show that typical damages on aircraft after a hard

landing on the runway, runway landing excursions and

overruns are engine contact with the runway or wing tip

contact causing tyres attrition and overload (Fig. 4), broken

landing gears, damage to the fuselage, engine nacelles or

propeller blades.

The range of damages includes damages of the winglets,

substantial airframe damages by impact forces and post-

crash fire. Crosswind incidents and accidents affect airport

operations and result in personnel injuries as well as

damages of aircraft, facilities and ground-support

equipment.

Another contributing element is low visibility, which

makes runway alignment at touchdown more difficult. In

such dynamic situations like crosswind landings factors

like lack of time for all-embracing situational awareness,

evaluate and control the aircraft attitude and flight path are

crucial. Thus, the pilot has to be very concentrated.

Additionally, other factors like wet or contaminated run-

way might contribute to an increase in the pilot’s workload.

There are many flight deck tools, procedures and train-

ing aids to assist the pilots to handle such situations.

However, a steerable landing gear allowing for a landing

with crab angle will contribute to improve the situation at

airports and support the attempt to overcome the increasing

air traffic, to reduce the pilot’s workload and ensure a

simplified and safe landing.Fig. 3 Weather, wind and crosswind-related accidents from 1994 to

2007 (NTSB)

Fig. 4 Airbus A340 tyres No. 4 and No. 8 burst on landing under

heavy crosswind conditions at London Heathrow Airport on February,

25th, 2006 [4]

On the use of a steerable main landing gear

123

1.3 Existing main gear steering solutions

Several different applications of steerable main landing

gears exist. However, besides the Boeing B-52 none of the

existing solutions enables crosswind assistance.

Boeing 737 The two main landing gears of the B737 are

two-wheel bogies. They are equipped with a shimmy

damper connected with the torque link to damp and reduce

side forces during landing [5]. With this concept side forces

lead to small steering movements of the main gear wheels.

However, a systematic steering control of these main

landing gears for crosswind landing is not possible.

Boeing 747 The B747 is equipped with a body gear

steering system. Behind a pair of main landing gears at the

wings (wing landing gear), a second pair of main landing

gears is mounted at the body of the aircraft (body landing

gear). The wing gear struts are not steerable. The body gear

struts are equipped with a hydraulic steering cylinder and

are steerable up to 13� left or right. Above a speed of

20 knots the gear steering of the body gear struts is swit-

ched off.

Airbus A380 Like the B747 the A380 is equipped with

two pairs of main landing gears [6]. The wing gear struts

have two axes and the body gear struts have three axes. All

main landing gears of the A380 are not steerable as a

whole, but the aircraft has a steering mechanism to steer

the rear pair of the six wheel main gears.

The main gear steering systems of both B747 and A380

are solely designed to enhance the ground manoeuvrability

by reducing the turning radius. For this feature only a few

wheels of the main landing gears are made steerable. For

landing in crosswind conditions with crab angle the

undercarriages of the B747 and the A380 cannot be used

without modifications as in this case all wheels have to be

aligned with the centerline of the runway, hence must be

steerable. To the authors’ knowledge such functionality

was applied only once in aeronautical history, namely at

the Boeing B-52 Stratofortress.

Boeing B-52 The undercarriage of the B-52 consists of

one pair of two-wheel bogies located at the front of the

aircraft and one pair of two-wheel bogies located near the

wings. The steering angle of each pair of landing gears can

be adjusted manually by the pilot. For manoeuvering the

aircraft on ground, only the front gears can be steered. In

this case the aft pair of landing gears remains fixed.

However, for crosswind landing and take-off all landing

gears can be steered in concert in order to enable crabbed

landings (Fig. 5).

Considering the existing main gear concepts there is an

increase in additional weight and effort required to get all

wheels of an aircraft steerable. Additional weight could be

expected in the magnitude of a few hundred kilograms,

which could probably be counteracted by a possibly lighter

weight of the steerable gear (see Sect. 4.2). At least for

two-wheel bogies, like those of present mid-range aircraft,

it appears feasible to apply a steering mechanism with

acceptable effort. As such steering system is safety critical;

it has to fulfil the required safety standards. However, all

required technologies are available today.

2 Landing gear modelling

The simulations of the steerable main landing gear and the

crosswind landing assistance system were performed with a

six-degrees-of-freedom simulation model of an A320-like

aircraft. The undercarriage of the A320 consists of a nose

gear and two main landing gears, each twin-tyred. The

mathematical landing gear model provides the forces and

moments acting on and about the centre of gravity. It fol-

lows a mathematical description given by Fischenberg [7],

who has once designed it for a simulation model of the

VFW614. The original gear model was adapted to the

A320 landing gear in terms of geometry and characteristics

and additional changes in order to provide steering func-

tionality of the main gear struts.

Each of the three gear struts is modelled as an oleo-

pneumatic damper. The oleo force consists of two parts, a

nonlinear spring force and a damping force. The dynamics

of the tyre itself are mostly neglected except for the

maximum braking coefficient which is a function of the

tyre pressure.

The model provides steering functionality to all three

landing gear struts. As the A320 main landing gear does

not have any steering functionality, the dynamics of the

main gear steering actuation was assumed to be the same as

for the nose gear [8]. This means that the steering angle gand the steering rate _g are limited to

• gMG ¼ �75�

• _gMG ¼ �12�=s.

It is questionable whether these dynamics are necessary.

However, it was not the focus of this study to evaluate the

required dynamics of the main gear steering for the

envisaged purpose.

Fig. 5 Crosswind landing of B-52 aircraft (photo: Michael Hall)

D. Vechtel et al.

123

The mathematical descriptions given in the following

for the calculation of gear forces and moments are nearly

the same for every gear strut. Only parameters like, e.g. the

maximum oleo deflection differ for the nose gear and the

main gear struts. For each gear strut three forces in the

three spatial directions are calculated.

The vertical force of each gear strut is calculated by a

nonlinear spring force FS and a velocity dependent oleo

damping force FD.

FZ ¼ FS þ FD ð1Þ

The spring force is a function of oleo deflection fO.

FS ¼ f fOð Þ ð2Þ

The spring force characteristics as applied for the main

gear struts are shown in Fig. 6. The vertical dashed-dotted

line on the right side of the figure shows the maximum

deflection of the main gear strut of 0.47 m [8]. The max-

imum deflection of the nose gear strut is 0.43 m [8].

The oleo damping force is a quadratic function of the

compression rate for each direction. The constant values

for compression and decompression K1 and K2 differ by a

factor of 2.

FD ¼ K1 � _f 2O

�K2 � _f 2O

: Compression

: Decompression

�ð3Þ

The longitudinal force is a function of the vertical force

multiplied by the sum of the brake friction and the roll

friction coefficients.

Fx ¼ Fz lB þ lRð Þ ð4Þ

It has to be considered that brakes are only applied to the

main landing gear. Hence, the longitudinal force of the

nose gear is solely a function of the vertical force multi-

plied by the roll friction coefficient. The roll friction

coefficient is a fixed value for speeds above a threshold of

10 kts. Below a speed of 10 kts the roll friction coefficient

is calculated as a function of the longitudinal speed com-

ponent uk.

lR ¼0:03

0:03þ 0:002 � 10� ukð Þ: uk [ 10 kts

: 0\uk � 10 kts

�ð5Þ

The brake friction coefficient for the main landing gear

is a function of the maximum brake friction coefficient

lBmax and the brake power applied by the pilot kb, where

kb 2 0. . .1½ �.lB ¼ �0:03þ 0:94 � lBmaxð Þ � kb ð6Þ

The maximum brake friction coefficient is a function of

the runway condition. Four different runway conditions can

be chosen in the developed simulation model: dry, wet,

flooded and icy. For the simulations described in this paper

dry runway conditions were chosen. The tyre pressure pT is

also taken into account as an influence of the maximum

brake friction coefficient. For dry runway the maximum

brake friction coefficient is calculated by

lBmax ¼ 0:912 � 1� 0:0011 � pTð Þ � 0:00079 � uk ð7Þ

The lateral gear force is a function of the vertical force,

the lateral friction coefficient lS and the skid angles.

Fy ¼ �Fz � lS � sign sð Þ ð8Þ

The skid angle is the difference between the wheel

steering angle and the wheel drift angle (Fig. 7), hence

depends on the wheel steering angle g, the velocity com-

ponents uk and vk, the yawing rate rk as well as the lateral

and longitudinal position of the respective gear strut xCG

and yCG.

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.50

2

4

6

8

10

12

14 x 105

Oleo Deflexion [m]

Ole

o S

prin

g F

orce

[N]

Fig. 6 Spring force characteristics (main gear struts)

Fig. 7 Steering angles [9]

On the use of a steerable main landing gear

123

s ¼ g � sign uk � rk � yCGð Þ � � � �

� � � � arctanvk þ rk � xCG

uk � rk � yCGj j

� � ð9Þ

Usually only the nose wheel is steered, but for the

application of a main gear steering the mathematical

description of the nose wheel is applied to the main gears

as well.

The lateral friction coefficient lS is a function of the

skid angle s and the maximum lateral friction coefficient.

For reasons of simplification the factor uf is introduced.

uf ¼olS

os � tan sð ÞlS;max

ð10Þ

This factor is used to distinguish different calculations

of the side friction coefficient

lS ¼lS;max � uf � 0:148 � u3

f

�� ��lS;max

�::

ufj j\1:5ufj j � 1:5

ð11Þ

For dry runway the maximum side friction coefficient is

the same as the maximum brake friction coefficient

lS;max ¼ lB;max ð12Þ

The total gear forces are the sum of all forces of each

gear strut. Based on the three forces acting on each gear

strut the total gear moments are calculated with the

respective lever arms. The forces and moments are fed into

the equations of motion of the six-degrees-of-freedom

aircraft simulation [10, 11].

3 The crosswind landing assistance system (CLAS)

Contrary to the B-52, where the pilots have to adjust the

steering angle of the main landing gear manually, modern

controllers and assistance systems can widely enlarge the

scope of steerable landing gears. For crosswind landings

not only the sometimes challenging decrab manoeuvre can

be neglected by steering the main gear in the runway

direction, an automatic assistance system can also stabilise

the aircraft on ground with the aim to guide it to the run-

way’s centerline. Such assistance systems similar to sta-

bilisation programs known from the automobile industry

are a consequent continuation of present aircraft systems

like, e.g. the A380’s ‘‘Brake-To-Vacate’’ system, which

uses the brakes for optimal roll out towards the desired exit

of the runway.

As a first development step towards such a fully inte-

grated, comprehensive stabilisation system DLR Institute

of Flight Systems developed a CLAS using steerable main

landing gear functionality. Crosswind landing assistance

system can make use of all available controls such as the

landing gear steering, the brakes and all aerodynamic

control surfaces in order to assist the pilot under adverse

weather conditions. However, braking is not optimised at

this state of development compared to the above-men-

tioned ‘‘Brake-To-Vacate’’ system. A system like CLAS

enables the pilot under manual flight to fully concentrate on

flight path control, on the flare and controlling the aircraft

on the runway, while CLAS assists and stabilises the air-

craft. Under autopilot operations CLAS could vastly

enlarge the envelope of the autoland system, such that

automatic landings could not only be performed in case of

low visibility but also under different adverse weather

situations such as strong crosswind.

Crosswind landing assistance system is developed as a

self-contained modular controller. In order to enable an

easy implementation into the complex flight control system

(FCS) of the A320-like aircraft simulation used for the

evaluation of CLAS, the current system is implemented in

parallel to the regular FCS and switches from the control

commands of the FCS to those of CLAS during flare. This

should prevent possible interactions between the regular

aircraft controller and CLAS. For future applications

CLAS should be fully integrated into the aircraft’s flight

control system, which was not possible at this state of

development. The current implementation of CLAS into

the flight control system and the aircraft simulation is

shown in Fig. 8.

Crosswind landing assistance system can make use of all

available data from which the aircraft position and attitude

can be obtained, such as radio navigation aids (VOR, NDB,

ILS, etc.), inertial and satellite navigation data, air data,Fig. 8 Implementation of CLAS

Fig. 9 Schematic system architecture (drawings from [8])

D. Vechtel et al.

123

radar altimeter data, etc. A schematic description of the

system architecture is shown in Fig. 9.

The primary task of CLAS is to keep the aircraft safely

on the runway and to guide it to the centreline. In addition,

CLAS slowly aligns the aircraft with the runway orienta-

tion during rollout.

4 Evaluation of results

After various offline simulations during the development

phase the assistance system and the aircraft model with

steerable main landing gears was implemented in a flight

simulator in order to enable testing of crabbed landings

with pilots in the loop in a realistic environment. Especially

the evaluation of the piloted simulations concerning the

pilot’s acceptance of such landing technique was a major

focus of the simulator campaign.

The simulator campaign was conducted in the EFCS

(electronic flight control system) engineering simulator

operated at DLR Institute of Flight Systems (Fig. 10). The

simulator is a fixed-based simulator with a generic glass-

cockpit comprising sidesticks, six displays for the EFIS

(electronic flight instrument system), an FCU (flight con-

trol unit) and ECAM (electronic centralised aircraft mon-

itoring) control panel, as well as a touch-screen

representing the overhead panel.

The biggest advantage of the simulator is that it enables

a relatively simple implementation of arbitrary aircraft and

system models or adaptation of existing models. Especially

for the testing of new systems the simulator offers good

opportunities for pilot-in-the-loop simulations and rating of

new system functionalities by pilots.

However, the simulator also comprises several limita-

tions in the degree of realism. First of all, the missing

motion impression degrades the pilot’s perception and his

flying performance. Furthermore, the controls like the

sidesticks, pedals and thrust levers are similar but not

identical to those in a real aircraft, which might also have

an influence on the pilot’s ability to handle highly dynamic

situations like decrab manoeuvres. It should also be men-

tioned that the wind model in the simulation does not

comprise a gradient in ground-effect. Nevertheless, the

simulations are considered to be representative enough at

this first investigation step. This was also confirmed by the

pilots.

4.1 Autoland tests

First of all, crabbed landings were tested under autopilot

operations in order to evaluate the general operational

capabilities of the system.

The crosswind landing capabilities and the reliability of

the utilised autopilot cannot be quantified in relation to the

original A320 autoland system. For this reason the opera-

tional limitations of the original A320 autoland system

were taken as reference for the evaluation of crabbed

landings under autopilot operations. These limitations are

relatively narrow in terms of the maximum allowable

crosswind component. The flight crew operating manual

(FCOM) of the A320 gives a maximum crosswind com-

ponent for CAT II or CAT III automatic approach landing

and roll out of 20 kts [12]. It is assumed that below this

limitation the autoland system of the real A320 functions

properly. Beyond this crosswind limit no information was

found concerning the autoland system’s capabilities. It

should be noted that the magnitude of this limitation is also

similar for autoland systems of other aircraft. Thus, no

relative assessment can be performed in relation between

the crosswind landing capabilities of the original A320

autoland system and CLAS. However, it could be shown

that the autoland system used in the simulations was able to

cope with very large crosswind components in combination

with the steerable main landing gears and CLAS. As the

autoland system aligns the aircraft with the glideslope and

localiser of the ILS, while CLAS aligns the landing gear

with the runway direction, the aircraft touches down on the

Fig. 10 EFCS engineering simulator

Fig. 11 Autoland landing with heavy crosswind

On the use of a steerable main landing gear

123

runway at the desired position but still in crabbed motion.

Figure 11 shows a photo of an automatic landing during

final approach shortly prior to touchdown with heavy

crosswind.

One can clearly observe the large crab angle in Fig. 11.

In relation to the runway centreline, the aircraft’s nose

points to the right side at about 20�. The autoland system

maintains this crab angle until touchdown when CLAS

takes over ground control and aligns the aircraft with the

runway while forcing the aircraft to the centreline.

The autoland tests showed that CLAS was able to cope

with all tested crosswind conditions. Various wind speeds

were tested with crosswinds from either side. In order to

evaluate the system’s operational limitations even unreal-

istically large crosswind components up to 70 kts were

tested. In reality no aircraft would be operated under such

heavy wind conditions but for reasons of system testing it

was considered to be useful in order to investigate the

system’s performance and limits.

Under all tested wind conditions, even up to 70 kts,

CLAS was able to safely control the aircraft on ground and

to keep the aircraft on the runway centreline. Figure 12

exemplarily shows the ground track during autoland using

CLAS with 40 kts crosswind. The simulated approach was

on runway 25R in Frankfurt with wind from 339�, thus

from the right side from the aircraft’s perspective. The

figure shows the runway boundaries, the aircraft’s ground

track (red line), the touchdown point (red circle) as well as

the aircraft’s heading at specific positions during the

approach and landing (blue dashes).

One can observe in Fig. 12 that the crab angle is kept by

the autoland system until touchdown. After touchdown

CLAS aligns the aircraft with the runway, which can be

observed by the direction of the blue dashes, while the

aircraft is kept on the runway centreline (red line).

4.2 Piloted simulator tests

The main focus of the piloted simulator tests was to obtain

pilot rating in order to evaluate how crabbed landings

influences the pilots’ workload and safety. Another issue

was to compare the lateral accelerations and side forces

acting on the landing gear at touchdown and thus stressing

the aircraft’s structure with and without crabbed landings

under manual flight conditions and heavy crosswinds.

All together nine pilots from DLR as well as from dif-

ferent airlines, most of them being experienced with the

A320, participated the simulator campaign. The flying

experience of the pilots was well spread between a few

hundred and about 20,000 flight hours. Such good distri-

bution is important when dealing with a rather small

number of participants.

For the evaluation of pilot ratings a dedicated rating

scale was developed, which was derived from a rating scale

used for hazard assessment of atmospheric disturbances,

such as wake vortices [13]. In the past good experience was

made in various piloted studies in flight simulators or under

real flight conditions with the original rating scale.

Therefore, it was decided to adapt the rating scale for

specific task of crosswind landings. Figure 13 depicts the

rating scale used for the pilot-in-the-loop simulations.

Immediately after each landing the pilots had to rate the

three items ‘‘aircraft control’’, ‘‘demands on the pilot’’ and

‘‘safety’’ on a scale of 1–4, where a rating of 4 means that

the case is not acceptable. The ratings have to be given as

integer values, so no intermediate values are allowed. The

safety rating is to be treated independently from the other

two rating items, e.g. a poorly controllable and highly

demanding approach might lead to a go-around so that the

overall safety of the approach is not affected since the go-

around is a safe standard flight procedure. The pilot ratings

are purely subjective; hence they differ from pilot to pilot

due to their own safety perception.

Fig. 12 Autoland landing with 40 kts crosswind and CLAS

Fig. 13 Crosswind landing rating scale

D. Vechtel et al.

123

The evaluation of the pilot ratings showed a significant

reduction in all three rating items for the crabbed landings

assisted by CLAS (Fig. 14). Especially the demands on the

pilots were rated much better in this case compared to the

classical landing technique with decrab manoeuvre (with-

out assistance).

In all following figures ‘‘Without Assistance’’ means

those landings without steerable main landing gears, when

consequently a decrab manoeuvre had to be performed,

whereas ‘‘With Assistance’’ means those landings with

steerable gears and CLAS so that no decrab manoeuvre had

to be performed.

Regarding Fig. 14 and the differences in pilot ratings

between landing with and without decrab it must be kept in

mind that the lack of motion impression due to the fixed-

based simulator together with all other above-mentioned

issues degrading the degree of reality caused difficulties for

the pilots performing the decrab manoeuvre. As with the

steerable main landing gear the pilots do not need to per-

form a decrab manoeuvre; no difficulties due to the lack of

motion impression in the simulator occurred. For this

reason the difference between the ratings with and without

decrab might be smaller in real flight. Nevertheless, this

cannot counteract the fact that landings with CLAS

engaged were rated much better than those without CLAS.

Especially when the ratings are depicted as a function of

the crosswind component the benefit of crabbed landings is

clearly observable (Fig. 15).

Figure 15 shows that the ratings (averaged over all

three rating categories) with the main gear steering

functionality and CLAS are nearly independent of the

crosswind component. As expected the average pilot rat-

ings increase with increasing crosswind in case of land-

ings without assistance, whereas the ratings with the

steerable main landing gear and CLAS remain nearly

constant on a very low level. This implies that with

steerable gears and assistance by CLAS crosswinds are no

longer an issue for the pilots.

Another important benefit from steerable main landing

gears is the reduction of landing gear side forces especially

during touchdown. As CLAS aligns each landing gear strut

with the runway, lateral loads during touchdown can be

lowered significantly. Figure 16 illustrates an example of a

landing with a crosswind component of 40 kts from the

left. The figure shows time histories of the altitude, the

indicated airspeed, the lateral load factor and the side

forces of the nose gear (NG), the left (MGL) and the right

(MGR) main gear relative to touchdown (t = 0).

One can observe in Fig. 16 that the roll out on the

runway is significantly shorter in case of the crabbed

landing (about 18 s compared to almost than 25 s without

CLAS). In addition, the resulting lateral loads during roll

out due to steering inputs of the pilots in order to keep the

aircraft safely on the runway can be lowered significantly

due to the steering activity of CLAS. However, the biggest

benefit can be observed during touchdown. With fixed

main landing gears a large impact occurs during touch-

down especially on the right main gear. Obviously the pilot

rolled slightly to the right (leewards) during the decrab

manoeuvre, which caused the right main gear to touchdown

first. Furthermore, the pilot was not able to fully align the

aircraft with the runway, which caused lateral accelera-

tions. For this reason the right gear experienced large side

forces for a short period of time. These lateral loads on the

main gears vary significantly between the different pilots

and crosswind conditions.

Figure 17 shows the average maximum side forces as

well as the total maximum values which occurred during

the simulator campaign with and without steerable gears.

It is obvious that the maximum side force which

occurred with the steerable landing gear (during all land-

ings) is even below the average of all maximum side force

values which occurred with fixed main gears. Possibly,

such a reduction of the maximum lateral loads on the

landing gear can allow the design of lighter main landing

Fig. 15 Average pilot ratings as a function of crosswind component

Fig. 14 Averaged pilot ratings

On the use of a steerable main landing gear

123

gears. Indeed, in such case the reliability of CLAS needed

to be very high. However, any failure which would lead to

an inoperative CLAS would not be safety critical but just

tighten the crosswind limitations. In every case it must be

assured that the steering angle of each gear is correct, but

this is also the case for the nose gear steering of present

aircraft.

5 Future applications

Besides the aforementioned applications, steerable main

landing gears in combination with an automatic assistance

system like CLAS have the potential to improve not only

the landing but also the take-off capability of transport

aircraft and especially of (small) UAV which are heavily

affected by crosswind operations. For new aircraft config-

urations with very high wing spans or very low yaw control

effectiveness (e.g. strut braced wing concepts, blended-

wing bodies or flying wing) which have problems to carry

out an accurate decrab manoeuver, such combination of

steerable main gears and assistance systems can even

become an enabling technology to provide reliable cross-

wind operations.

Crosswind landing assistance system and steerable main

landing gears cannot solely be helpful under crosswind

conditions. Bad weather and runway conditions like rain or

snow contaminated runway can cause skidding of the air-

craft sideways across the runway upon touchdown. Skid-

ding can happen too, if the aircraft is taxiing. In every

situation when the aircraft is skidding sideways, the

steering of the main landing gears into the direction of

aircraft motion will help to regain friction forces. With the

steering motion the slip ratio decreases, the wheels begin to

roll again, and the cornering (and braking) forces will

increase. After the wheels have built up new friction forces,

the wheels can be steered back to the track the pilot has

directed.

Additionally, steering functionality of the main gears

could also help to improve ground manoeuvrability sig-

nificantly. In combination with electrically driven main

gears aircraft ground movement could be feasible without

external support like pushback trucks.

As mentioned above, aircraft with high aspect-ratio

wings, hence large wing spans can especially benefit from

Fig. 17 Main landing gear side forces during touchdown

Fig. 16 Crosswind landing with 40 kts crosswind from the left

D. Vechtel et al.

123

steerable main landing gears. Besides the aforementioned

applications such aircraft can also make use of the main

gear steering function at the gate. The ICAO gate classi-

fications can become a limiting factor for wing span. In

order to fit an aircraft with too large wing span into the

existing gate classifications, without the need of a complex

wing tip folding mechanism, steerable gears could also be

used for parking sideways.

6 Summary

Using the idea of steerable main landing gears, a crosswind

landing assistance system was developed. CLAS was

designed in a way that it can be used in manual flight as

well as with autopilot engaged. During approach all gear

struts are automatically aligned with the runway so that

under crosswind conditions the necessary crab angle does

not need to be reduced prior to touchdown. Therefore, the

aircraft can be landed in a crabbed way. After touchdown

CLAS stabilises the aircraft on the runway centreline and

slowly decreases the crab angle. All aerodynamic control

surfaces, differential braking as well as the steering of each

gear strut is used for controlling the aircraft on ground.

The steerable main landing gear and the developed

CLAS were implemented into a six-degrees-of-freedom

aircraft simulation of a typical mid-range transport aircraft.

A simulator campaign with pilots in the loop was con-

ducted in order to evaluate possible benefits from crabbed

landings. The pilots had to rate each landing with and

without steerable main landing gears under different

crosswind conditions by means of a dedicated subjective

rating scale. Generally speaking, the acceptance of the

pilots was good and the evaluation of the pilot ratings

revealed a significant reduction in pilot workload espe-

cially under heavy crosswind conditions. A general

increase in safety could be shown. Furthermore, the sim-

ulation results showed a significant reduction of the lateral

loads acting on the undercarriage. As the tyres are aligned

with the runway the lateral loads during touchdown and

roll out could be reduced significantly.

Concluding it can be stated that the study revealed

potential benefits under autopilot operations as well as

under manual control regarding operational crosswind

limitations and pilot workload. Besides possible safety

increases for existing aircraft layouts, the study also

showed that such a system could act as an enabling tech-

nology for possibly different future aircraft layouts like

aircraft with high aspect-ratio wings or blended-wing

bodies.

Acknowledgments The authors would like to thank all pilots who

participated on the simulator campaign for their cooperation. Special

thanks are given to the DLR-external airline pilots, who offered their

contribution voluntarily in their free time and always with great

commitment. Furthermore, the ambitious work of Bernhard Fernando

and Thomas Sassmann shall be mentioned, who performed most of

the system development as student assistants.

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On the use of a steerable main landing gear

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