TCAS, GPWS/EGPWS: Are there good reasons for ignoring warnings?

[Don Hudson]

Occasionally the debate arises regarding ignoring the warnings these two systems provide flight crews. TCAS warnings in particular are the subject of such discussions and I know that under circumstances of the pilot's visual contact with the ground/surrounding terrain one can reasonably ignore GPWS/EGPWS warnings.

Are there arguments or occasions for ignoring the warnings of either systems?

[Seeker]

It's happened to me that I got a TCAS warning, thought I had the aircraft in sight and it turned out that there was another aircraft I didn't see.  After this bit of "experience" I no longer assume my understanding is correct.  I do know a fellow pilot who had a bad TCAS RA that was trying to force an aggressive downward avoidance maneuver below 1500' AGL (while on closely spaced parallel runways with known, and visual traffic on the other runway) so I wouldn't say "never" but think it's almost always conservative and correct to follow the guidance.

As for EGPWS, I would say that almost every CFIT occurred with the crew thinking they knew where they were - it would be extremely foolish to disregard a EGWPS while IMC.

[blues deville]

Ignoring a TCAS warning might be the last official decision made by a pilot. However some GPWS alerts are false and depending on your airline's procedures, can be considered nuisance warnings under certain conditions. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[Vsplat]

Crew interface with automated warnings.  Always an interesting discussion.  Allow me to offer a couple of thoughts.

Research from safety sources indicates that TCAS RAs are almost always valid, that is to say the warning is issued when the algorithm says it should..  If the aircraft is on approach to a closely spaced parallel runway structure, the TCAS is still valid, the issue is that TA/RA mode is active when the approach is designed without it.  In other words, operators flying these approaches need to decide, in accordance with the local ATC resources,  if TA/RA should be disabled as an SOP.  Teaching pilots to ignore TCAS warnings  eventually results in disregarding a valid warning.

Disregarding a valid TCAS warning can result in a rapidly increased risk of collision, especially if the OTHER aircraft OBEYS.  The trouble is that one crew has no idea what the other crew will do.

GPWS warnings, similarly, are almost always valid, that is to say, as with TCAS, they trigger when the algorithm says they should.  As has been mentioned by others above, the vast majority of crews that experience a surprise GPWS are lacking a piece of information that the GPWS has available.  The question is, how confident is the crew that they know what they need to?   In IMC or night conditions, compliance should be a no-brainer.  In daytime conditions, seeing terrain is not necessarily assurance of safety, I have at least one case on record where a crew maneuvring visually, both pilots heads up, lost track of vertical speed with autothrottle off, thrust at idle and speed well below planned.  If not for the warning, they would have had nothing to fight back with by the time they realised the trouble they were in.

Similarly, routinely ignoring GPWS and getting away with it eventually results in ignoring it and NOT getting away with it.

Some GPWS installations are more sensitive and prone to warning than others.  At airports where the terrain is challenging and normal approaches on profile will trigger a warning, many manufacturers now offer customization.  But, there is a cost....

FWIW

Vs

[Super 80]

I have heard pilots in the US complain that the equipment in some military aircraft will trigger TCAS from far outside a reasonable threshold.

[Vsplat]

Perhaps.  I had not heard this.  I'm not sure what yardstick is being used as reasonable.  It might be that the thresholds are different due to the unusual handling or performance profiles of some military types, but that would be just a WAG.

Vs

[boestar]

I would think that the conflict resolution algorithm takes the speed of the aircraft into account when deciding if there is a conflict.  military jets tend to fly faster than civilian aircraft so of course if the closure rate is high, the RA will sound earlier.

Just a WAG for me as well.

 

[Specs]

5 hours ago, Don Hudson said:

Are there arguments or occasions for ignoring the warnings of either systems?

TA only was required for ILS PRM approaches out of a concern for pilots following conflicting RA solutions but I believe that's been cancelled now and TA/RA is allowed.

There are certain aircraft specific failures that could specify ignoring Mode 3 and 4 GPWS warnings - A320 L/G gravity Extension - disregard the TOO LOW GEAR aural.

Thinking about it now though if you have an air data issue, couldn't your TCAS unit be using incorrect data in making it's computations and advisories?  Redundancy must eliminate the invalid data?  

[IFG]

I remember in the early days of GPWS; let's just say it was a lot better when algorithms got refined. I can't imagine not following EGPWS immediately in IMC, and never experienced any spurious warnings myself in VMC (except for some rare guidance in the flare ). Hypothetically, one might imagine a situation where a spurious warning is anticipated, and hold plans for ignoring it, but I think Vsplat is on point about disabling it if unavoidably necessary rather than 'desensitizing' what should be an almost visceral reaction in normal ops.

Likewise on TCAS, delay is not good. I've read that the original designers did not want any visual display at all, on the assumption that pilots would waste time analyzing it & deciding whether they had a better idea . Superficially, I've seen this, but then it was TA's getting attention rather than actual RA's.

I am curious about something, tho'.. I didn't have the opportunity for sim training on TCAS, not enabled on 727 sims. I read up as much as I could, and we did try some instructor-simulated profiles, but that doesn't get you through to a credible completed resolution, just the trainer's best guess. I did have some software a cut or two above the MSFlightsim level (Aerowinx 744) with several TCAS profiles, which I think helped to get the 'feel' of it. Here's a question:

What are the current Sim-training modules like. I've read and heard anecdotally about incidents involving very aggressive manoeuvering, but my understanding of the TCAS system doesn't suggest that should be necessary, which to over-simplify, says the resolution seeks to change your anticipated altitude in 20 seconds by 500 ft. That shouldn't take more than about a 5ᴼ pitch change for 20 seconds, without multi-000 ft zooms (Lordy, what if you were in the stack at LGA?) or injured people. Lacking authentic sim experience,  I'm curious how the modules are run.

TIA & Cheers, IFG

PS: Seeker, curious if you have any more detail in that lo-level RA, and how TCAS resolves such conflicts.

[Vsplat]

Specs, you are correct, TA/RA is active now for PRM approaches.and incorporated in PRM training.

Where an abnormal condition in compatible with TCAS or GPWS, the associated procedure normally incorporates setting or disabling the related system - for example, most single engine cases will have the crew turn the TCAS to TA only, as they can't really respond properly to an RA.  Flap issues may affect GPWS modes, etc.

Air Data system failures can create a whole host of spurious warnings.

Vs

[Specs]

I notice that Airbus also now offer A/P F/D AUTO TCAS  - It takes the RA response out of the pilots hands.  The algorithms must be pretty robust to get that functionality certified.

[J.O.]

1 hour ago, Specs said:

I notice that Airbus also now offer A/P F/D AUTO TCAS  - It takes the RA response out of the pilots hands.  The algorithms must be pretty robust to get that functionality certified.

I've seen a simulator demonstration - it's quite impressive.

[DEFCON]

Have the new TCAS systems been configured yet to alert an aircraft on approach when another aircraft, or even a ground vehicle, is inadvertently positioned on the landing runway?

 

  

[Don Hudson]

From AW&ST, February 05, 2016,

Quote

 

An update to the Traffic Alert Collision Avoidance System (TCAS) software, Version 7.1, reverses and simplifies the commands automatically broadcast to pilots to prevent mid-air collisions. It does this if it senses that an intruding aircraft remains on a collision course despite evasive action.

In TCAS Version 7.0, a resolution advisory tells a pilot to descend if another aircraft enters its airspace. It does not, however, reverse the command if the intruding pilot, for whatever reason, also descends. Version 7.1 will reverse the command and advise a pilot to “level off” and thereby avoid the potential for collision.

TCAS Version 7.1 has been a European mandate for forward fit aircraft since March 2012, and will be required as a retrofit in December 2015. The International Civil Aviation Organization mandated it as a forward-fit in 2014. Hong Kong mandated the upgrade for every forward-fit aircraft in January 2014, and as a retrofit by January 2017. The FAA has yet to introduce a regulation covering the upgrade. In addition to the equipment necessary to meet these mandates, OEMs and operators can add software updates to existing equipment that improve operational safety both on the ground and in the air. One example is Honeywell’s SmartRunway® and SmartLanding system.

SmartRunway provides real-time positional advisories for planes during taxi and takeoff in order to avoid runway incursions and other errors that create dangerous situations. Experts note that while mid-air collisions are extremely rare, the number of runway incursions is increasing. SmartLanding monitors an aircraft for unstable approaches and long landings and advises pilots of problems known to be leading causes of runway excursions.

http://aviationweek.com/mandate-and-safety-compliance/improving-collision-avoidance

 

[DEFCON]

Thanks very much Don; I think the 'Smart Runway' technology enhancements will be as consequential to flight safety as the introduction of TCAS itself was.

 

 

[Guest]

The following from the NTSB may be of interest to those following this thread.

 

NTSB Issues Safety Alert to Pilots on Midair Collision Prevention; Releases Animations Showing Limitations of ‘See and Avoid’

November 16, 2016

WASHINGTON – The National Transportation Safety Board issued a Safety Alert Tuesday to pilots with suggestions on what they can do to reduce their chances of being involved in a midair collision.

In an effort to illustrate the limitations of the “see and avoid” concept of aircraft separation, the NTSB created a series of animations depicting the pilots’ visual field of view from each of the four airplanes involved in two midair collisions that were investigated by the NTSB in 2015.

The animations show how difficult it can be for pilots to spot converging aircraft that may present a midair collision risk in a dynamic visual environment.

Using 3-D laser equipment, investigators scanned the cockpit windows and surrounding airplane structure of four exemplar airplanes involved in the two midair collisions to create animations that, combined with radar data, provided an approximation of what each pilot likely saw before the crashes. Investigators also used radar data to reconstruct how in-cockpit technology that provides pilots with graphical and aural alerts of nearby traffic could have made the pilots aware of the approaching aircraft and possibly prevented the collisions.

“These accidents and the animations clearly demonstrate the safety benefit of augmenting pilots’ vision with technological safety nets,” said NTSB Chairman Christopher A. Hart. “Technologies in the cockpit that warn of traffic conflicts through displays or alerts can help pilots become aware of, and maintain separation from, nearby aircraft, even if they have difficulty seeing them.”

On July 7, 2015, a Cessna 150 that had just departed from Moncks Corner, South Carolina, and an F-16 Air Force fighter jet on a training mission collided. An air traffic controller advised the F-16 pilot that the Cessna was a potential traffic conflict. The F-16 pilot was not able to visually acquire the Cessna until it was too late to avoid the collision. The two occupants of the Cessna were killed; the F-16 pilot ejected and survived. The NTSB determined that the probable cause of the crash was the air traffic controller’s failure to provide an appropriate resolution to the traffic conflict.

On August 16, 2015, a North American Rockwell Sabreliner inbound for landing at Brown Field Municipal Airport in San Diego and a Cessna 172 that was practicing landings at the same airport collided. The four occupants of the Sabreliner and the sole occupant of the Cessna were killed. A cockpit visibility study revealed the fields of view of both pilots were limited and partially obscured at times. The NTSB determined that the probable cause of the accident was the air traffic controller’s failure to properly identify the aircraft in the pattern and to ensure control instructions were being performed.

The NTSB said that contributing to both accidents were the inherent limitations of the “see and avoid” concept of traffic separation. These limitations, combined with errors by the air traffic controllers, resulted in the pilots’ inability to take action to avoid the collisions.

The Safety Alert highlights the value of traffic avoidance technologies to pilots as an aid to detecting and avoiding other airplanes in flight. Such technologies also serve as another layer of safety in the case of air traffic control errors, such as those referenced in the two accidents above.

In addition to issuing the Safety Alert, the NTSB made recommendations to the Federal Aviation Administration and the three companies operating federal contract control towers in the U.S., asking them to brief air traffic controllers on the errors in the two midair collisions and to include these accidents as examples in initial and recurrent training.

The Safety Alert, Prevent Midair Collisions: Don’t Depend on Vision Alone, the animations, safety recommendation report, accident reports, and are all available at http://go.usa.gov/x8c5G.

###

CONTACT US: Media Relations Division | 490 L'Enfant Plaza, SW | Washington, DC 20594

(202) 314-6100 | NTSB Media Relations | www.ntsb.gov

The videos can be viewed at: https://www.youtube.com/playlist?list=PL5aVmmm4Qt9H9XDVaSTJawGLHP7ZVpdp6

 

 

[Kip Powick]

excellent videos..thanks

[Don Hudson]

Thanks Malcolm...I hope they're part of ab initio & commercial training.

[DEFCON]

Thanks Malcolm. Other than for a C - 177RG that I accumulated a few hours on, which was considerably better than other Cessna models, I never cared much for high wing aircraft because of the restricted visibility in turns.

I wonder why the F - 16 pilot wasn't using his look down shoot down radar to keep an eye on traffic?

 

 

 

[boestar]

It could be that the Radar system on the F-16 is restricted use in areas surrounding radar controled facilities.  The power of the radar could potentially overwhelm the facility radar.  Procedure would then tell them to switch off when under radar control.

 

[DEFCON]

Thanks Boestar, that's an intriguing explanation.