Avoiding Control Flight Into Terrain

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Avoiding Controlled Flight into Terrain Copyright © 2013 by Sandel Avionics, Inc. All rights reserved 1 | P a g

Avoiding Controlled Flight into Terrain,

Obstacles and Wires

Authored by Sandel Avionics, Inc.

Sandel Avionics Inc., the terrain avoidanceleader, engineers and manufacturesadvanced avionics for rotorcraft and fixed-wing aircraft as well as provides embedded software for OEM applications. Its compactintegrated display systems, for the militaryand commercial markets, which include 3-ATI and 4-ATI retrofit primary displays,fixed-wing TAWS (Class A & B) and

HeliTAWS®--the helicopter industry’s onlycertified panel-mount HTAWS that alertsagainst wires, terrain and obstacles.

Founded in 1998 by inventor and aviationenthusiast Gerald Block, Sandel is managed by an independent board of directors, withthe stated business purpose of enhancing thecapabilities of pilots and their aircraft.

Vertically integrated, Sandel designs,manufactures, tests and markets its own products at its 16,000 square-foot

headquarters in Vista, CA, and provides product support through both theheadquarters plant and East Coast RegionalSupport center in Salisbury, NC.

Abstract

Based on the success of the fixed-wing TAWS, this paper briefly highlights the issues dealt within fixed-wing CFIT and the correlation of using a TAWS solution to address rotorcraft CFIT. It

also discusses the challenges and benefits of Helicopter Terrain Awareness and WarningSystems (HTAWS), and the operational reason and decisions made in the design of the leadingHTAWS today. Taken together, the improvements over a conventional HTAWS system showshow a commercial, off-the-shelf HTAWS can successfully be installed in tactical helicopterswith minimal modifications and reasonable costs to significantly reduce CFIT.




Introduction

In 2012 there were a total of 133 fatal

helicopter accidents worldwide whichinvolved 420 fatalities [Ref 20]. TheInternational Helicopter Safety Teamreports: Controlled Flight into Terrain(CFIT) ranks as the 13th most common of helicopter accident types and according to NTSB statistics, 60% of all CFIT accidentsare fatal. Despite its frequency, helicopter pilots may find it difficult to relate to theconcept of unknowingly flying their aircraftinto the ground. Many in-flight emergencies

can be detected and require a specific courseof corrective action. However, when CFIToccurs, the pilot usually learns of theemergency the same instant the helicopter impacts the terrain. Therefore it is importantfor pilots to understand what conditions and actions lead to CFIT so that they canrecognize and avoid these hazards to preventthe accident [Ref 21].

Fig.2 Relationship of CFIT Fatal vs. Non-fatal

Accidents [Ref 22]

Fig.2 CFIT to Environmental Conditions

[Ref 22]




How the Fixed-Wing Industry

Dealt With CFIT

A Short History of TAWS: Ground Proximity Warning Systems (GPWS)

Initial experience with Ground ProximityWarning Systems in fixed-wing airplanesfor commercial airline service providesuseful indications of the hurdles that must beovercome during the introduction of anynew warning system. Discussions about theconcept of terrain alerting met with ahealthy degree of skepticism and a concernthat this would be another noisy device in analready busy cockpit. The details of typical pilot comments varied but the central themewas “…most competent pilots know wherethey are and don’t really need such asystem.” Often management was similarlyskeptical, preferring additional training and appeals to pilot professionalism in lieu of anexpensive new system.

Those airlines who became early adopters of fixed-wing GPWS created simulator training

scenarios that ensured the terrain alerts werea surprise to the pilot and at the same timerealistic. This provided an element of realism that steadily drew converts to theGPWS. Once fixed-wing GPWSinstallations achieved a reasonable level of fleet penetration, there was a measurabledecrease in controlled flight into terrain(CFIT) accidents for equipped airplanes.Over time, the improvement in the accidentdata was simply too clear to ignore.

The original concern about false alerts wasnot as bad as some had feared butexperience showed that more sophisticationin the alerting algorithm was desirable. In particular, a forward-looking alert and afurther reduction in false alerts werestrongly desired. The issue of false alerts has

always been the more difficult aspect of TAWS. The original developers of GPWSrecognized that to be effective the warningmust command an immediate crew action.This made the alert conditions easier to

define and reduced the opportunities for false alerts. The second generation of GPWSwas developed in response to these needs,introducing database terrain and moresophisticated computations driving systemoperation. For equipped airplanes in airlineservice, CFIT accidents all but disappeared as a safety concern.

As the market developed & technologyadvanced, the leading TAWS providers took an innovative tack on their fixed-wingTAWS by developing a different algorithmfor generating a TAWS alert. By usingsimulation to test that algorithm against ILS(instrument landing system) approachesthroughout the United States, Canada,Europe, and Australia, they were able toimprove the alerting software.

The secret to success was development of analgorithm that answers the question, is it possible to get safely from the current

airplane condition to all possible runways?If the answer is yes, there is no alert. But if the answer is no, the terrain or obstacle that blocks a safe path becomes the cause of analert. TAWS does not know what path the pilot intends or even where the pilot isgoing. Instead it looks at every runway inlight of the continuously changing currentairplane position and velocity. Theoperational performance has beenoutstanding; by covering all the trajectory

options the TAWS appears, to many users,to read the pilot’s mind. The result is aneffective TAWS with a very low false alertrate. Note: Owner, operators, and OEMs thatselected a TAWS manufacturer using thislogic reaped higher success inimplementation, usability and safety.




Fig.3 IHST 80% Reduction in Helicopter Accidents Annual Goal Chart

How the Rotorcraft Industry

Can Address CFIT

Helicopter Industry Safety Initiatives and

Incidents

The International Helicopter Safety Team(IHST) was formed in late 2005 in responseto a consensus of government regulators,manufacturers, and helicopter operators thatthe rate of worldwide helicopter accidentswas unacceptably high and must be reduced.

“The model for IHST was the CommercialAviation Safety Team (CAST) that wassuccessful in motivating a reduction of the

large air carrier (United States Code 14 CFR

Part 121) fatal accident rate by 80% in 10years. The IHST accepted this accident-reduction mandate and formed industry and government teams to conduct a similar effortto reduce the worldwide helicopter accident

rate by 80% in 10 years (by 2016), from 9.4to 1.9 accidents /100,000 hours.” (Ref. 3)

The Helicopter Mission

Helicopters operate in a more demandingenvironment (than fixed-wing) where theopportunities for false alerts are significantlygreater. The vehicle differences are obvious:a helicopter can hover and change directionwithout any forward motion; it can climb or descend with or without any ground speed.Helicopters can take off and land from manydifferent locations, including those awayfrom an airport or heliport. They alsooperate routinely below 500 feet AGL(above ground level) with no intention to

land. Obstacles and especially power linesare a threat to helicopter operationsthroughout the mission, not just at takeoff and landing.




Fig.4 The FAA TSO C194 Minimum MOPS requirement only covers helicopter operations 500’

AGL or above.

The helicopter differences are significantenough to force a major reconsideration of the alerting requirement for helicopters bythe DOT/FAA. TSO-C194 Helicopter Terrain Awareness and Warning Systems

(HTAWS) and RTCA DO-309 MinimumOperational Performance Standards (MOPS)for HTAWS provide the initial compilationof requirements for an HTAWS– mainlyduring the cruise phase of flight [Ref 2].While these requirements are helpful they donot address many of the conditions routinelyencountered in helicopter operations. In particular, operations below 400 feet abovethe ground or in the vicinity of wires and other obstacles receive little attention in the

MOPS.

Why HTAWS

Helicopters demand disciplined operations because many missions operate close tochallenging terrain, involve off-airportlandings and takeoffs, and produce

distractions from pilot flying tasks. There isabundant exposure to hazards notencountered in fixed-wing flying. Amonghelicopter operations, medical rescue, lawenforcement, utility surveillance, oil rig

transportation, and firefighting, are by their nature non-routine and may terminate inlanding sites that have not been optimized or even analyzed for terrain, obstacle, and wireclearance considerations. Therefore theutility of on-board TAWS protection is veryhigh for helicopters. For these reasons,qualification for HTAWS MOPS [Ref 2],was created separate and distinct from fixed-wing TAWS.

Meeting the HTAWS requirements provides

reasonable protection for helicopter operations during cruise. In 2009, theleading TAWS manufacturer recognized thatthis is not sufficient for many helicopter operations and set out to create morecomprehensive protection that could be used




in all types of helicopter mission withoutconcern for false or nuisance alerts.

The Challenges and Benefits of

HTAWS

False Alerts/Nuisance Alerts

The FAA defines a False Alert as—Awarning or caution that occurs when thedesigned terrain or obstacle warning or caution threshold of the system is notexceeded, and a Nuisance Alert as—An alertthat occurs when there is no threat or isunnecessary for the intended operation. [Ref 23]

False alerts in any system are a problem, butfalse alerts in a system that requiresimmediate pilot action to preserve safety are particularly serious. (As a general rule of thumb a warning system should be at least10 times more reliable than the condition for which the warning is intended.) For immediate-action alerts, like terrain or wirewarnings, the notion of alert reliabilitycovers not just the hardware and software

but also the integrity of the basic warningalgorithm. Modern integrated circuits and fault tolerant designs have made hardwarefailures for digital systems rare events.Adherence to strict software design and coding rules can greatly reduce theopportunity for software errors. That leavesthe basic concept for the alerting algorithmas the dominant factor in determining thecorrect alert rate as well as the false alertrate for this type of warning system.

Together with the human factors design of the pilot interface, the ideal design willrespond to the mission conditions and all pilot actions, provide effective alerts whenappropriate, and have a near-zero incidenceof false or nuisance warnings.

Fig.5 A safety system that constantly sets off

nuisance alerts may be worse than no safety

system at all.

The experience gained in developing the lownuisance alerting concept for the fixed-wingTAWS was crucial in leading a fewmanufacturers to conclude that, in a morechallenging helicopter environment, a newalerting paradigm would be necessary. Theresult is a helicopter system with anextremely low false alert rate that works aswell for operation near the ground as it doesfor higher altitude cruise conditions. Anhigh performance HTAWS never makes the pilot prioritize multiple threats. While all

threats are shown visually, a warning is presented for the one threat that is mosturgent.

Because of the consequence of collisionwith terrain, obstacles or wires, it is particularly important to design an alertingalgorithm that adapts quickly to whatever the pilot is doing. For intentional low-altitude operations near obstacles or wires or for off-airport landings, having the pilot

designate their intention by selecting asuitable operating mode the system cantailor the alerting algorithm to better matchthe intended flight profile. An HTAWSshould support pilot mode selection for this purpose. However the system will functionwhether mode selection is accomplished or not.




For fixed-wing aircraft there are a large, butfinite, number of usable runways and operations at low altitude are restricted except near those runways. No such

limitations exist for helicopters. No amountof testing can prove that HTAWS recognizesevery combination of flight path, terrain, and obstacle a helicopter might encounter everywhere in the world. Instead the alertingalgorithm must be made to be as robust as possible and the system must use the “bestterrain information available” [Ref 2]. Anhigh performance HTAWS should do both.

Obstacles & Terrain

The size of the terrain grid determines howclose a terrain feature could be when ittriggers an alert. All terrain grid systemsassign a height to each grid area. This heightis the highest terrain in that grid area. A high point in one portion of the grid applies to theentire grid area. If the grid dimensions aremade smaller, the high point would propagate over a smaller area, allowing the

smaller grid to provide more accurate terrainalerting. [Ref 19] This means that ahelicopter maneuvering at an altitude wherethe grid high point could cause an alertwould get that alert anywhere in the largegrid but only in one or two of the smaller grid areas. To most pilots, testing showed that several of the terrain alerts on a largegrid system would be considered false alertssince the pilots considered the actual terrainclearance safe.

High performance HTAWS use high precision data which can reduce the grid sizeand thereby eliminate a significant number of false alerts without increasing the CFITrisk.

A similar situation exists for obstacles.Obstacles certainly have height but they aretypically small in diameter compared to atypical terrain grid. So even the smaller grid is not small enough to allow the use of

gridded obstacles. By treating the tower as a precisely located entity, the alert area is notaffected by the grid size so alerts will be precisely generated. The need for separateobstacle alerting is even more striking in thecase of systems using larger terrain grids.An advanced HTAWS makes allowance for guy wires by assuming that the alert areaaround an obstacle is proportional to theheight of the obstacle. So a 100’ tower would have an alerting area about half the

size of the already small terrain grid.

Fig 6. The diagram above shows how a superior

HTAWS uses a grid 25 times smaller utilizing 3

arc-second data vs. 6 arc-second data. Instead

of alerting on the object (X) as soon as it

appears within the grid of four (600’x600’), a

superior system will recognize the position of the aircraft relative to the object at a greater

degree and only alert on the object when it is

truly a threat to the aircraft. [Ref 19]

High precision terrain and separatedatabases for obstacles and wires are key

3 0 0 ’

300’ 6 0 0 ’

600’

X




Fig.7 Since helicopters fly, maneuver, and land in non-designated areas; pilots need a HTAWS that gives them pilot-selectable modes and alerting sensitivity. This allows the system

to provide warnings without nuisance alerts.

factors in reducing the opportunities for false alerts and improving pilot confidencein the utility of the most advanced HTAWS.

Non-Standard Landing Zones

An advanced HTAWS will store every published airport. As information becomeavailable, systems will also include all public helicopter landing pads. Buthelicopters often land at undesignated spots.A smart HTAWS supports this capability byrecognizing pilot intent when a landing is, or could be, imminent and assessing the safetyof the present path to a touchdown.

The “OFF AIRPORT” mode lets the pilotindicate that an off-airport landing isintentional. A similar assessment is madeduring takeoff from an undesignated field toensure that the path ahead is clear of terrain,

obstacle, or wire threats.




Transmission Lines

There are two fundamentally differenttechnologies for determining where wiresare located. One involves sensing the

presence of the electric field surroundingoperating power lines. [Ref 4] The second involves a database of wire locations. Thedatabase option provides a number of advantages, most important of which is thatthe alerting will operate whether the power line is energized or not. Emergencysituations such as fires can result in circuit breakers opening for affected power lines.With no power on the power line there is nofield to sense and systems that depend onsensing cannot determine the location of thestill risky wires. A database-driven systemswork equally well whether line power is onor off.

Fig.8 Sandel HeliTAWS with WireWatch®—a data

base driven wire warning system—shown here with

terrain and wire alerting on a 3 ATI display.

A concern with a database-driven system isestablishing a source for the data and validating the source is sufficientlycomprehensive to constitute a positivecontribution to safety. This was a difficult problem initially but detailed discussionswith power generation and distribution

companies have culminated in identificationof a reliable and accurate source for theinformation needed. As mapping dataimproves and increases, the system databaseis constantly being updated as the data

becomes available. At this point the database pinpoints major and secondarydistribution lines. Lower voltage distributionlines are partially covered. Note: The SandelHeliTAWS® with WireWatch® is the onlyHTAWS with a database and wire alertingoption available today. Soon the databasewill have the capacity to permit operators toincorporate additional wire informationdirectly into the system.




Conclusion

CFIT is a real and growing threat tohelicopters. As the accident rate rises, private and public entities must find a

solution to address the issue in an efficientand cost-effective way.

Meeting the minimum HTAWSrequirements provides reasonable protectionfor helicopter operations during cruise, butthis is not sufficient for many helicopter operations. What is necessary, is a morecomprehensive system that can be used inall types of helicopter missions withoutconcern for false or nuisance alerts. In

addition, for an HTAWS to be fullyeffective, it must give the pilot control of modes and sensitivity based on the missionthey are flying.

The avoidance of controlled flight intoterrain, obstacles, or wires is a major

objective for all helicopter operators. For thehelicopter pilot the right HTAWS willquickly become a valuable tool. For the fleetoperator a high performance HTAWS will provide assurance that their investment is

protected as well as mitigate the liability of not providing to most current safetyequipment in their helicopters. Helicopter owners and operators must have an HTAWSsystem designed to overcome the currentindustry shortcomings and a systemspecifically for helicopters that addresses allof the challenges associated with the widevariety of modern helicopter operations.HTAWS is not the only safety investment ahelicopter operator should make but it may

be the single most important safetyinvestment in the next few years.




References

1) Department of Transportation, Federal Aviation Administration, Aircraft CertificationService, Washington, D.C. “TSO-C194 Helicopter Terrain Awareness and WarningSystem (HTAWS)”. Effective date: 12/17/08

<http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgTSO.nsf/0/4E324B446BE11B2D8625752300762A36?OpenDocument>

2) RTCA, Inc. (Radio Technical Commission for Aeronautics) 1828 L Street, NW, Suite805 Washington, D.C. 20036, USA “DO-309 MOPS for HTAWS”. Effective Date: 13March, 2008 Controlled Document – http://www.rtca.org/store_product.asp?prodid=585

3)

4) International Helicopter Safety Team (IHST), “Calendar Year 2006 Report”, prepared by U. S. Joint Helicopter Safety Analysis Team; dated July 2010

<http://www.ihst.org/portals/54/jhsat/safety_reports/CY2006_USJHSAT_Report_09132010.pdf>

5) DOT/FAA/AR-08/25 “Safety Study of Wire Strike Devices Installed on Civil and Military Helicopters” September 2008.< http://www.tc.faa.gov/its/worldpac/techrpt/ar0825.pdf>

6) Department of Transportation, Federal Aviation Administration, FAA Safety BriefingMarch/April 2011 “Vertically Speaking” p.32 < http://www.faa.gov/news/safety_briefing/2011/media/MarApr2011.pdf>

7) Department of Transportation, Federal Aviation Administration, FAA Safety BriefingJuly/August 2011 “Vertically Speaking” p.32 <http://www.faa.gov/news/safety_briefing/2011/media/JulAug2011.pdf>

8) Federal Aviation Administration, Aircraft Certification Service, Washington, D.C.“TSO-C87 Airborne Low-Range Radio Altimeter”. Effective date: 2/1/66<http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgTSO.nsf/0/2C417369F3ACFDD886256DC70062A5C1?OpenDocument>

9) Department of Transportation, Federal Aviation Administration, Aircraft Certification

Service, Washington, D.C. “TSO-C113 Airborne Multipurpose Electronic Displays”.Effective date: 10/27/86<http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgTSO.nsf/0/7AF1C90D16DBDF9786256DAC0061E948?OpenDocument>

10) Department of Transportation, Federal Aviation Administration, Aircraft CertificationService, Washington, D.C. “TSO C-118 Traffic Alert and Collision Avoidance System(TCAS) Airborne Equipment, TCAS I”. Effective date: 8/5/88

http://www.rtca.org/store_product.asp?prodid=585






<http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgTSO.nsf/0/AE63DB5C4BFB280D86256DAC0061EA4D?OpenDocument>

11) Sandel Avionics “82046-PG-B1 ST3400H Pilots Guide” http://www.sandel.com/ST3400H_sup.php

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13) Sandel Avionics “ST3400H HeliTAWS® In-Flight Comparison Video” http://www.sandel.com/ST3400H.php

14) Sandel Avionics “HeliTAWS® In-Flight Demonstration Video”http://www.sandel.com/ST3400H.php

15) Department of Transportation, Federal Aviation Administration, FAA SAFO 10015“Flying in the Wire Environment”http://www.faa.gov/other_visit/aviation_industry/airline_operators/airline_safety/safo/all_safos/media/2010/SAFO10015.pdf

16) Helicopter Association International 1635 Prince Street, Alexandria, Virginia 22314-2818 Video “Surviving the Wires Environment”http://www.rotor.com/Publications/HAIVideosLibrary/SurvivingtheWiresEnvironment.aspx>

17) Robinson Helicopter Company 2901 Airport Drive, Torrance, CA 90505 USA Safety Notice: SN-16 “Power Lines Are Deadly”http://www.robinsonheli.com/srvclib/rchsn16.pdf

18) Robinson Helicopter Company 2901 Airport Drive, Torrance, CA 90505 USA Safety Notice: SN-26 “Night Flight plus Bad Weather Can Be Deadly”http://www.robinsonheli.com/srvclib/rchsn26.pdf

19) Curt Lewis, P.E., CSP Article: “Three in hospital after Air Evac helicopter hits power line” Flight Safety Information August 2, 2011 - No. 159 (newsletter) CURT LEWIS& ASSOCIATES, LLC (on file at Sandel)

20) The Sandel high precision grid display is much smaller and therefore more accurate

than is typical of fixed-wing TAWS and adaptations of those systems that have been previously marketed for helicopters.

21) HeliHub.com. 2012 Fatal helicopter accident statistics.http://helihub.com/2013/01/03/2012-fatal-helicopter-accident-statistics/

22) International Helicopter Safety Team. (n.d.) Training fact sheet - Controlled flight intoterrain: CFIT-how does it happen? http://www.ihst.org/Portals/54/insights/CFIT.pdf

http://www.faa.gov/other_visit/aviation_industry/airline_operators/airline_safety/safo/all_safos/media/2010/SAFO10015.pdf


http://www.rotor.com/Publications/HAIVideosLibrary/SurvivingtheWiresEnvironment.aspx%3e


http://www.robinsonheli.com/srvclib/rchsn16.pdf


http://helihub.com/2013/01/03/2012-fatal-helicopter-accident-statistics/

http://www.ihst.org/Portals/54/insights/CFIT.pdf



http://helihub.com/2013/01/03/2012-fatal-helicopter-accident-statistics/










23) Frazer, R. (1999). Air medical accidents: A 20 year search for information. AirMed

Journal, September/October, 34-39.

24) FAA Proposed Changes, AC-27-1A and AC-29-2B. Chapter 3 AirworthinessStandards Normal Category Rotorcraft, Miscellaneous Guide, d. Definitions. 2012

Contact Information

Sandel Avionics, Inc.2401 Dogwood Way, Vista CA [email protected]

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Avoiding Control Flight Into Terrain