cOCKPIT CLASSROOM 999

By Harold Holmes

SAFE TAXIING

Safe taxiing appears to be a fairlysimple maneuver compared with mostother aircraft maneuvers. However, ac-cident statistics bear out the fact thattaxi accidents are quite numerous. Col-lisions with other taxiing or parked air-craft happen all too often. Also, impro-per taxi procedures such as riding thebrakes or using improper braking whileturning is the cause of costly mainte-nance repairs to the mechanical linkageof the nose gear assembly, unevenwear of tires and brake repairs. The pur-pose of this article is to point out somesafe taxiing procedures which will re-duce accidents and maintenance ex-penses.

It requires much more power to startthe aircraft moving or to start a turn thanit does to keep it moving in any givendirection. For this reason, all controls,including the throttle, rudder pedals andbrakes, must be used properly. Checkyour brakes before moving more thanthe length of your aircraft. Particularcare must be taken in the use of thethrottle while taxiing to prevent over-heating and possible mechanical dam-age to the engine.

TAXIINGObjective: Taxiing is the controlled movement ofthe airplane under its own power, while on theground.

(Position of control while taxiing)•Black arrow indicates direction of wind.

Light airplanes are provided with nose-wheel steer-ing capability. This is done through a simple sys-tem of mechanical linkage connected to the rudderpedals. Push-pull rods connect the pedals to fit-tings located on the pivotal portion of the nose-wheel strut.

Taxiing with the steerable nosewheelnow common on tricycle-gear airplanesrequires little special piloting technique,but the habitual observance of all safetyprecautions is necessary, just as it iswith tailwheel type airplanes.

It is generally poor technique to useexcessively high power settings and at-tempt to control the airplane's directionand taxi speed with the brakes. Theflight controls are effective to a greateror lesser degree, depending upon thetaxi speed and velocity and relative di-rection of the wind.

Caution is required when taxiing lighttricycle type gear airplanes, especiallythe high wing type, in strong quarteringtailwinds. Such a wind can cause anairplane to flip over on its back unlessthe ailerons, rudder and elevators arepositioned properly.

To prevent the possibility of a flip-over when taxiing with a strong quarter-ing tailwind, the elevator should be in adown position, with the elevator controlforward. In addition, the aileron on theside from which the wind is blowingshould be in the down position, aileroncontrol moved in the direction oppositethan that from which the wind is blow-ing. Also, sudden bursts of the throttleand sharp braking should be avoidedwhen taxiing with strong quartering tail-winds.

Except while taxiing very slowly, it isbest to slow down before attempting a

turn, otherwise the turn is likely to betoo sharp or too prolonged, particularlywhen turning from a downwind headingto an upwind heading. It is best to slowdown before attempting to turn becauseof the tendency to weathervane. Exces-sive braking while turning can causenosewheel problems and unnecessarytire wear.

In turning from an upwind heading toa downwind heading, however, this pre-caution for slowing down is not as im-portant, since the same tendency willcause a deceleration of the turn.

One of the major considerations inthe taxiing of airplanes with the tail-wheel gear is its tendency to weather-vane while it is being taxied crosswind.This effect may be much less or evennegligible when taxiing airplanes with atricycle landing gear.

While the airplane is moving on theground it is considerably affected by thedirection and velocity of the wind. Whentaxiing into the wind the control effec-tiveness is increased by the speed ofthe wind as well as the tendency of allplanes to weathervane. The effect ofweathervaning makes the effective

OFFICIAL DESIGNATIONS OF STRENGTH OF WINDSStrength of winds are designated in three ways: 1) by .Beaufort ScaleNumber; 2) by National Weather Service designations; and 3) by velocityin miles per hour

BEAUFORTSCALE

012345

6

789

10

1112

NATIONALWEATHER SERVICE

DESIGNATIONS

CALM . . . . . . . . . . . . . . .LIGHT AIR . . . . . . . . . . . . .LIGHT BREEZE . . . . . . . . . . .GENTLE BREEZE . . . . . . . . .MODERATE BREEZE. . . . . . .FRESH BREEZE. . . . . . . . . .

STRONG BREEZE . . . . . . . .

MODERATE GALE. . . . . . . . .FRESH GALE . . . . . . . . . . .STRONG GALE . . . . . . . . . .WHOLE GALE . . . . . . . . . . .

STORM . . . . . . . . . . . . . . .HURRICANE . . . . . . . . . . . .

VELOCITYIN MILES

PER HOW

O t o 1I t o 34 to 78 to 12

13 to 1819 to 24

25 to 31

32 to 3839 to 4647 to 5455 to 63

64 to 73

34 AUGUST 1985

CROSSWIND

• Correct airplane's tendency to weathervane withrudder and brakes. (Brakes important.)

• Check Flight Manual for maximum crosswindconditions.

JET BLAST/JET INTAKE HAZARDSINGESTION DANGER AREA

25' SAFETY RADIUSCURRENT IET AIRCRAFTAT ALL THRUSTSETTINGS

iIDLE BUST DANCER AREA

2M' B-747 (IDLE)

s 1s

ISO' OTHER AIRCRAFT (THRUST)

375' B-747 (TAKE OFF)2N' OTHER AIRCRAFT (THRUST)

3 5O o

5 IT °•O »-

r—s z

wind velocity seem greater, and makestaxiing into the wind easier than taxiingin other directions.

The tendency for a plane to weather-vane is greatest while taxiing directlycrosswind, which makes this maneuverextremely difficult without brakes. It isalmost impossible to keep the planefrom turning into even a moderatebreeze (13 to 18 mph).

In taxiing downwind, the tendency toweathervane seems to be increased asthe velocity of the tailwind lessens theeffectiveness of the controls. When thewind velocity is greater than thegroundspeed, control surfaces are ex-posed to what is called a "reversedwind" condition, meaning that when anaileron is in an up position a tailwind willtend to lift that wing; whereas a windfrom the front of the aircraft with aileron"up" would force the wing down. Whentaxiing downwind with the speed of thewind greater than taxiing speed, useforward stick with elevators down. In theevent of a direct tailwind the aileronsshould be in a neutral position.

It is difficult to set any rule for a safetaxiing speed which is usually no fasterthan a person walking rapidly. What issafe under some conditions may behazardous under others (always taxicautiously). The primary requisite ofsafe taxiing is safe control; the ability to

stop or turn where and when desired.Movement of the plane should be de-pendent on the throttle. That is, thespeed should be slow enough to enableyou to stop the airplane promptly whenthe throttle is closed or the aircraft willstop on its own when the throttle isclosed.

One other thing to watch for in regardto taxiing speed is that an airplaneshould taxi no faster than would be safewithout brakes — just in case thebrakes do fail. Usually this means, asmentioned earlier, at a walking pace butit depends a lot on the circumstances .. . how many other aircraft are on ornear the taxiways, the length and widthof the taxiway, etc. If the clearance be-tween objects appears too narrow, itprobably is, so STOP — take a lookand use a guide to assist you.

Sharp turns and attempts to turn attoo great a speed should be avoidedsince both tend to exert excessivestrains on the airplane and are difficultto control once started.

In many airplanes with tricycle land-ing gears, the arc around which thenosewheel will pivot is restricted —damage done to the mechanical linkageof the nose gear is expensive. This maymake it impossible to pivot the airplaneon one wheel, which is poor practice inany event, and will require a larger area

for maneuvering. Generally, brakes andpower should not be used at the sametime — locking a brake and pivotingaround one wheel is not recommended.In starting to taxi a tricycle gearairplane, the airplane should be allowedto roll forward slowly to enable thenosewheel to center and to prevent thepossibility of a swerve into anotherairplane or nearby obstruction.

Pilots should realize that airplanesare not at home on the ground. Taxiingproperly requires a fine blending ofspeed control and throttle use, as wellas a well developed sense of anticipa-tion of the tendencies of the aircraft.

Remember, it requires much morepower to start the aircraft into motion(1100-1300 rpm) or to start a turn thanit does to keep it moving (800-1000rpm) in any given direction. However,care must be taken to avoid excessiveuse of the throttle and thus overheatingand causing mechanical damage to theengine.

Even/time the throttle is closed whengoing either downwind or crosswind,the forces causing the weathervaningtendencies immediately act to reduceeffective control and cause involuntaryturning. For the rudder to be effectivewhile taxiing it must have airflow pass-ing over it. In some cases a blast of airfrom the prop will aid in starting a turn

SPORT AVIATION 35

or making a correction while taxiing.Jet blast on an airport ramp is a defi-

nite hazard to a taxiing light aircraft. Jetblast can have a velocity of 590 mphand a temperature of 370 degrees at apoint 25 feet behind the engine. At adistance of 100 feet, blast velocity is 95mph and the temperature about 140 de-grees F. For safety reasons, lightplanes should not taxi closer than 1000feet (length of 3 football fields) behinda taxiing jet transport aircraft. Note thedifferences in the "idle blast" and 'lakeoff" blast danger areas. Remember thatprop blast from a heavier airplane canalso present a hazard to light airplanes.

In previous "Cockpit Classroom" arti-cles we have discussed flight in windyor gusty conditions. Taxiing duringwindy or gusty conditions should causepilots to use extreme care in decidingwhether or not the winds are too strongfor taxiing. A conservative rule whichcan be used to cover most lightairplanes is to compute 40% x the stallspeed of the plane. Let's say, for exam-ple, the stall speed is 50 knots — 40%x 50 = 20 knots. Taxiing should notbe attempted with winds over 23 mph.Strong crosswinds, above 20 mph, willpresent a problem of directional controlrequiring downwind brake to preventthe airplane from turning into the wind.If this happens, the brake should beperiodically released allowing the braketo cool.

Operating on the ground duringhigher than normal or gusty wind condi-tions or in close proximity to large andturbine-powered aircraft can be particu-larly hazardous for light airplanes.

The use of safe taxiing procedureson the ground is the sign of a good pilot.These procedures include the properuse of throttle and brakes, allowingenough room for turns without exces-sive braking, positioning of the flightcontrols properly during windy condi-tions, handling the weathervaning ten-dency and preventing a ground loopwhen landing. Braking action during thelanding roll should be considered to bea secondary function — use brakes onlyif necessary.

DOWNWIND

Taxi with yoke neutral or slightly forward (watchweather-vaning tendency because tail wind lessens effec-tiveness of controls).

The stronger the tail windthe more the elevators aredepressed.

Note—Do not taxi at speeds in excess of 15-20 knotsin wind of this same velocity.

USE OF THROTTLE — Hold throttlecontrol so that small changes (a fewmillimeters) can be made smoothly.

Use several fingers to act as a stop in contact with panel or quadrant.(Note: Do not hold throttle entirely by its knob or lever handle)

Check with your instructor for proper demonstration.

USE OF RUDDER — Rudder pedalsare the primary directional controlswhile taxiing.

Rest heels on the floor and balls offeet on bottom of rudder pedals.

Use firm movements with feet onpedals.

USE OF BRAKES — Move foot totop of rudder and depress or use hand-brake when it is necessary to stop.

Check braking action shortly after starting to taxi.Emergency stop — both brakes should be applied simultaneously. Apply

brakes smoothly, evenly and cautiously.BRAKES USED FOR: CHECK BRAKES FOR:

• Slowing • Holding 1. Sufficient force to stop aiprlane.• Stopping • Steering 2. Hold airplane during run-ups.

3. Permit steering while taxiing.

Remember that a surprising numberof accidents or incidents occur annuallyduring taxi operations. Is it becausepilots feel that taxiing is such anelementary operation that they have be-come complacent and inattentive tosafe taxi procedures and hazards onthe ground? Probably . . . all of us whofly need to concentrate more on keep-ing alert while taxiing.

> In strong headwinds move control wheel slightlyaft of neutral to control pitch attitude whiletaxiing.

UPWIND

In strong headwinds move the yoke slightly aft ofneutral.

36 AUGUST 1985

Evaluation

• Check brakes prior to each flightby braking plane to near stop duringtaxi.

• Use low power settings, 1000rpm.

• When brakes are used, closethrottle.

• Use brakes to assist in turning andstopping (after landing — use brakeswhen speed is under taxiing control).

• Keep speed under control at alltimes (fast walking speed). No fasterthan would be safe without brakes.

• Slow down — before attemptingturn — no sharp turns.

• Taxi slowly when attendant is as-sisting at the wing in strong winds.

• Avoid sudden burst of throttle andsharp braking especially in strong quar-tering tailwinds.

• Observe local taxi rules and towerinstructions.

• Adhere to taxi signals and clear-ances.

• Recognize and avoid hazards.• Avoid creating hazards to persons

or property.