Airbus "TERRAIN…TERRAIN" & "TOO LOW, TERRAIN"

[Microburst2002]

Hi there

There is a line in the GPWS procedure (which is memory item in Airbus) that I don't fully understand.

In case of a "TERRAIN… TERRAIN" or a "TOO LOW, TERRAIN" alert from the GPWS, the procedure is:

"adjust the flight path or initiate a go around"

anyone can elaborate on this?

What is the procedure in BOEING?

cheers

[vilas]

MB
Something like TCAS TA and RA, in GPWS you get caution and warning. Cautions are " Don't sink, Terrain ahead, obstacle ahead and too low flaps. Warnings are executive commands like " Obstacle ahead pull up, Terrain, Terrain pull up and Terrain ahead pull up. With a caution you have 60 seconds to impact and with a warning it is 30 seconds to impact. With a caution it may be possible to silence the caution by corrective actions like too low flap means you have not selected flaps full. Either selecting full or correcting GPWS and MCDU to 3 will silence it. Similarly after take off or GA if you start descending, which you are not supposed to it will caution you "don't sink" which will stop when you stop descent. But when warning is issued it is too late and immediate GA with TOGA power and full back stick must be initiated.

[flyingchanges]

TOGA power and full back stick must be initiated.

I really hope not...

Oh, wait, I guess you can do this in an Airbus and it will only kill you sometimes.

[sonicbum]

As vilas said, if you get a warning you must apply TOGA and full back stick. In case of cautions there are several courses of actions according to the type of caution.
Furthermore if your airplane is equipped with the T2CAS function you might get some other warnings like the Avoid Terrain.

[vilas]

I was talking about Airbus and no it doesn't kill you unless you are in 737.

[Wizofoz]

... Or in Direct Law.....

[safetypee]

Airbus has optional fits for either EGPWS (Honeywell) or T2CAS (Thales). There should not be any major difference if both systems are certificated to the same standard (TSO-C151a).
The principle is that these systems provide boundaries for safe flight with an alerting (amber) awareness and a warning (red) commands. Amber level voice alerts are associated with amber lights, Red levels with a red light and where enabled, terrain-map pop-up depending on system type.

The red warning level requires executive action and is to pull-up wings level to establish the best rate of climb. Wings level has been subject to great debate, which in the early days of EGPWS involved all manufacturers, and the manoeuvre had to correspond with the terrain database/map accuracy. It is likely that as systems have developed, particularly with T2CAS, that in some circumstances a turn might be allowed, but always remember it is terrain (obstacles) that have to be avoided.

Amber cautionary alerting covers many scenarios and generally provides additional time for manoeuvre (but don’t depend on specific time values), - quickly evaluate the situation and adjust. Thus procedure wording can involve flight-path adjustment, climb, GA, turn, or change of configuration.

If a TERRAIN alert is given during an approach then a GA would be the better course of action as the EGPWS and procedures are designed such that alerts should not occur; if they do something is wrong and it is better to establish what at a safe altitude.
TERRAIN alerts at other times might be quickly resolved with flight path adjustment, but don't prejudge any situation assesment.
Other alerts during the approach such as Glide Slope or those involving Flap/Gear, might be subject to operator procedures, but in general the better option is to climb to provide time for evaluation.

Never try to second guess the situation; EGPWS reliability is now significantly better than the rate of human error – EGPWS is an error detector, yours or someone else’s.

[Reverb_SR71]

I know this is off topic but what does IMHE number mean ??

For eg in the QRH 1.8b-IMHE .

Thanks

[DozyWannabe]

... Or in Direct Law.....

I would imagine that a failure sufficient to latch Alternate or Direct Law would require a different approach procedure - you'd likely be into QRH territory rather than memory items.

[tubby linton]

Dozy you are incorrect on so many levels and I do not have sufficient available time on this planet to tell you why. Go and read the fcom
Plane Airbus A320 (http://www.smartcockpit.com/plane/AIRBUS/A320.html)

[DozyWannabe]

@tubby linton:

It'd help if you let me know which one I'm supposed to be looking at. If the procedure indeed states "full back stick", then depending on how long the stick is held back that will eventually have a very different effect depending on the FC law.

(And incidentally, I did state that I was in effect making a guess!)

[tubby linton]

I cannot see that you were guessing from your post. Read the Fcom (there is only one) and you will read how the control laws change and the appropriate procedures. The fctm will also be of interest.

[DozyWannabe]

I cannot see that you were guessing from your post.

My apologies, I was hoping the phrases "I would imagine" and "you'd likely be" would indicate that I wasn't certain.

Read the Fcom (there is only one) and you will read how the control laws change and the appropriate procedures. The fctm will also be of interest.

Right, but the link you gave me requires individual downloads of the sections, and it's quite laborious and time-consuming. I have a (kindly donated) complete set of FCOM docs kicking around on a CD-ROM somewhere, but it's still in a box from a recent move, and I don't have time to dig it out.

For a non-pilot, I'm fairly conversant in the Airbus FBW control laws - and the point I was trying to get at was some clarification regarding vilas and sonicbum's assertion that:

...if you get a warning you must apply TOGA and full back stick...

specifically regarding how long to hold the stick back for.

Because, if I understand it correctly, when the stick is held back in Normal Law with TOGA power applied, then the High AoA Protection will allow the stick to be held back indefinitely, whereas outside of Normal Law (i.e. Alternate/Direct/Abnormal Attitude) then it is possible to pull the aircraft into a stall if you hold the stick back for too long.

Therefore I assumed it likely that "you must apply TOGA and full back stick", if that is indeed what the manuals say, would have caveats in such conditions.

I didn't think such an assertion would be obviously incorrect...

[Microburst2002]

Hey guys, this was my thread! :p

Safetypee said: If a TERRAIN alert is given during an approach then a GA would be the better course of action as the EGPWS and procedures are designed such that alerts should not occur; if they do something is wrong and it is better to establish what at a safe altitude.

TERRAIN alerts at other times might be quickly resolved with flight path adjustment, but don't prejudge any situation assesment.

Other alerts during the approach such as Glide Slope or those involving Flap/Gear, might be subject to operator procedures, but in general the better option is to climb to provide time for evaluation.

Never try to second guess the situation; EGPWS reliability is now significantly better than the rate of human error – EGPWS is an error detector, yours or someone else’s.

That is more or less what I was thinking. I consider GPWS as a thing that saves you from yourself. A tool that gives you a chance to get away after you screwed it badly or some faulty or wrongly designed equipment was about to kill you. In approach, a those alerts require a go around, IMHO. As for the other situation other than approach… That is more complex.

Shouldn't "adjust flight path" mean always "climb away, you are way too low"?

I find the wording of this procedure lacking clarity

[*Lancer*]

The caveat in FCOM is that with protections lost (DIRECT LAW), you must respect the stall warning - i.e. reduce the AoA, and fly VSW.

In that sense the actual procedure you would apply is broadly the same as Boeing - i.e. pitch up to stick shaker onset if required, and fly VSW.

[framer]

In that sense the actual procedure you would apply is broadly the same as Boeing - i.e. pitch up to stick shaker onset if required, and fly VSW.
In the 737-800 my outfit doesn't pitch up to stick shaker, we pitch up to 20 degrees, the only time we would go to stick shaker is if terrain contact was imminent.
I think it is a good idea to decide now, in the comfort of your lounge ( or hotel room) , to carry out the escape procedure if you get the warning and are in IMC or at night. The reason I say this is that these warnings are disregarded for long periods of time, sometimes until terrain contact, because the pilots have lost SA.
You shouldn't make a judgement call about whether or not to carry out the escape procedure simply because the time you need it, your judgement is faulty. If your SA is fine and you get the warning and carry out the procedure, no worries, good practice and you are justified by the warning.
My two cents.

[vilas]

Reverb_SR71
It means applicable to IAE. Metric, Honeywell, Enhanced GPWS aircrafts.

[AerocatS2A]

Microburst, sometimes a "terrain terrain" warning can be fixed with a minor adjustment to the flight path. If so there is no need to do a full blown escape procedure. You need to use judgement.

[vilas]

MB
A caution such as too low flaps, glideslope, don't sink obviously don't need a GA. All you do is correct the configuration and GS. However about terrain etc or in doubt about SA you should execute GA. As I said before after a caution is sounded you have 60 seconds to impact.

[Microburst2002]

Well, yes, but that's exactly what I mean. I take the GPWS as something that will warn me that my situational awareness is not as good as I thought. Once I doubt my SA I have to go away from terrain.

By the way, TOO LOW FLAPS is a go around, according to the procedure. You are really low if you hear that and you still don't have the flaps…

The TOO LOW TERRAIN is what you get when you are not so low yet, but you are very low indeed. If you are too low you did something wrong, like busting a minimum altitude or something. Go away, I say. I guess there are scenarios where a TOO LOW TERRAIN or a TERRAIN TERRAIN do not require a go around or climb initiation, specially when in VMC daylight, but I bet most of the times the best course of action is go around or climb.

[safetypee]

Micro, :ok:

Aerocat et al. Alternative action for alerts might be plausible, but any situation where the alert or warning questions your judgement does not warrant further use of judgement.

A favourite reference Celebrating TAWS ‘Saves’: But lessons still to be learnt. (www.icao.int/fsix/_Library%5CTAWS%20Saves%20plus%20add.pdf)
Note all of the incidents, but particularly # 7, re the crew’s judgement of flying level or re-establishing a safe approach path.

framer, I recall a simulator trial at an operators conference where pilots were expecting a Pull Up warning; the purpose was to evaluate differences in procedure application and timing. The scenario was at min effective timing for the equipment – no pre alerting (yes it can happen, see incident #8)
Half of the pilots did not survive the encounter. The deviance was divided equally between a late/slow reaction or less attitude/speed trade than required/available.

[AerocatS2A]

Just because you get a callout, doesn't mean you have lost SA and shouldn't continue using judgment. If the problem is obvious and you can fix it, then fix it, if not, go around. A glide slope callout at 1200 AGL should get the response "correcting", not "going around". Not that the latter is wrong, just unnecessary on an otherwise stable approach. Likewise a glide slope callout in the flare when landing on a runway with a displaced threshold can be ignored under some circumstances.

Terrain callouts can also be momentary blips in an otherwise good approach. A visual approach into an airfield with high terrain can sometimes trigger EGPWS callouts that aren't "spurious" but are just indicators of minor transgressions.

[*Lancer*]

AerocatS2A, given that GPWS Mode 5 (Glideslope) 1- only works below 1000' AGL and 2- would indicate LOW on approach, both your examples demonstrate the importance of not assuming you haven't lost SA!

[safetypee]

Aerocat, I disagree with the view of knowing or not knowing the state of awareness. A significant aspect in many accidents is surprise; a situation which was not anticipated, or an outcome which does not match actions or intent. IMHO any EGPWS alert would more likely fall into this category; if you are surprised, then act.

There may be ‘obvious’ situations where a small correction will apply, providing the alert (the situation) was expected. Being able to anticipate an event is part of awareness; thus a callout – “we may get an alert because” imparts knowledge because the situation is sufficiently understood and to a degree is being managed; situation awareness – being able to project ahead.
Calls after the event have little meaning other than to cover embarrassment.

If anticipation is absent, then so might general situation awareness; what then is the value of judgement.

(GS alerts are also inhibited below xx ft according to system/instalation)

[Microburst2002]

The "AVOID TERRAIN" and the "PULL UP" alerts are straightforward: pull up for your life.

"SINK RATE" and "DON'T SINK" are straightforward, too: correct it if you don't want to get in trouble.

"TOO LOW GEAR" and "TOO LOW FLAPS" are straightforward as well: go around

"GLIDE SLOPE" is straighforward, too: correct, or ignore if intentional

"TERRAIN AHEAD" is relatively simple. Adjust flight path by stopping descent, starting a climb and/or turning as necessary to avoid terrain as shown on the ND.

But the ones that seems ambiguous to me are "TERRAIN-TERRAIN" (excessive terrain closure rate) and "TOO LOW TERRAIN" (unsafe terrain clearance when not in landing configuration) : Adjust the flight path, or initiate go around
May I choose? Based on what?

A After “TERRAIN” has sounded twice, the warning switches to “PULL UP”, and is continually repeated until the aircraft leaves the warning envelope. In addition, the PULL UP lights come on.

It sounds quite serious to me. If I am in IMC or in a dark night, I'll go around unless there is specific information of that airport giving false alerts.

A "TOO LOW TERRAIN" alert comes if you are flying at 250 kt or more and below 1,000 RA without being configured for landing (that's not good). If you are flying at 170 or so, it will come at about 300 RA and not configured yet (that's bad)
It is a very serious one, which I will regard as genuine unless I can see with my own eyes.

I totally agree with Safetypee. If I disregard a GPWS is because I can see terrain with my own eyes or if there is reports of nuisance alerts. I will never disregard a GPWS alert in IMC or night because I am convinced I have been doing everything OK so far and there is no reason for an alert.

[AerocatS2A]

Lancer, 1 was a bad example. 2 has happened to me when following an ILS down to a circling minima for a straight in approach to a runway with a displaced threshold. The final 400' or so of the approach (visual on temporary PAPIS) was way above the normal glide slope which should not normally cause a callout, however, in the flare the glide slope signal fluctuated a bit and the indicator briefly went from full fly down to full fly up and back. It caused a glide slope callout. Was it a surprise? Yes. Had we lost SA? No. Should we have gone around? No definitely not. In my opinion, if your immediate response to a callout is "why did it do that?" Then you've lost SA, on the other hand if it is immediately obvious what has happened then it is no different than a support call from the PNF.

[AerocatS2A]

Microburst, you never disregard a GPWS alert. It went off for a reason. If you know why it went off and you can fix it, then fix it. If you don't know why it went off or you can't fix it then then get out of there. Very simple.

[FE Hoppy]

M202
Is there no Terrain clearance floor in the system you are describing?

If there is then all bets are off at 700RA where you will get a "Too Low Terrain" or similar regardless of configuration unless you are approaching a runway.

[Microburst2002]

Yes, I haven't checked the heights and conditions for the TCF of the EGPWS, but If I recall correctly they are really "conservative" in the sense that if you are really at that height in that place you have made something really wrong.

Aerocat2sA said it well
Microburst, you never disregard a GPWS alert. It went off for a reason. If you know why it went off and you can fix it, then fix it. If you don't know why it went off or you can't fix it then then get out of there

That, for me, is a go around unless visually I can determine I'm OK every time I hear TERRAI TERRAIN or TOO LOW TERRAIN

[safetypee]

Micro ... unless visually I can determine I'm OK ... :=

See events #1, #7, and #8 in ‘TAWS Saves’ (#21)

[autoflight]

TOO LOW FLAP is not simply restricted to final approach with the final landing flap not yet set.

With only approach flap set but otherwise in the landing config, it is possible to get this callout with steeply rising terrain, more so with normally expected higher downwind groundspeed during circle to land. Less steeply rising terrain could still be a great danger, but there may be no TOO LOW FLAP callout due to reduced rad alt ROD. Most other GPWS callouts are inhibited because of the aircraft configuration.

Check ALL the conditions that can give this callout.

There are many cues in aviation for immediate action. There is a good case for immediate action in this case.

The most modern GPWS may have addressed this.

[Microburst2002]

Black hole noted, Safetypee!

I have to add, then, that I will only disregard one of those alerts if I am in daylight VMC CAVOK.

The PULL UP alerts are never ever to be disregarded, of course.

[safetypee]

Roger that Micro, but note this photo for further thought:-

http://i60.tinypic.com/30a730g.jpg

Retrieved from just below a hilltop (1,120ft), where previously a crew, flying day VMC above cloud, had confirmed their location by a geographic feature (having often flown the route), then descended into cloud. Mistakenly the locating hilltop identification was in error by 8nm ...
- and the QNH was 1013/2992.
That picture was on my office wall for many years; probably make a good screen-saver now.

Also lookup the CFIT accident at Sao Jorge (Azores) 11 Dec 1999 – misidentified an Island, turned and descended into cloud and into another island cliff face.

No Exceptions … If-then-Do.

[Microburst2002]

Hmmm, scary...

But if there are clouds it is not CAVOK. In a CAVOK daylight (not twilight or anything, plain daylight and you can see the beautiful landscape, the runway and its surroundings, and terrain features and obstacles, and PAPI showing on slope… And always referring to the TERRAIN TERRAIN or TOO LOW TERRAIN only (no pull up alerts) wouldn't you continue?

but I see those are a lot of conditions. In that case it would be worth at least leveling off, if there is no subsequent pull up alert, nor further alerts, and all those conditions are met (specially the on slope PAPI) you might continue. If the alerts continue, a go around if probably the best even if everything seems Ok, since I might be nicely approaching the wrong runway…

Damn, I see it is better just to always trust the voice!:ok:

[safetypee]

Micro, the discussions in your thread have been most valuable, if only to identify the complexities that the industry tends to introduce to apparently simple aspects of safety.

Operations pre GPWS, allocated significant attention to avoiding the threat of CFIT, but there were few ‘detector’ systems and no drills.

With GPWS there was a simple drill:- IF “Whoop Whoop Pull Up”, THEN pull up. This was a memory item which added to an increasing number of operating memory drills.

EGPWS introduced more alerting levels and a range of warning prefixes. Whilst it could be argued that the prefixes helped to understand the situation, the drill should always be the same, and should not be delayed by re-evaluation or context dependant modifiers, but perhaps the drill is not always published in a simple manner.
The prefixes might help understand the situation after the event, but this analysis should only be conducted when a safe operating situation has been assured – and checked.

EGPWS is by far the most significant contributor to flight safety in recent years; it evolved from a simple concept, requiring additional features to counter the ingenuity of human error – particularly thinking that the situation is understood.
The industry should take care in not to overcomplicate operational use, nor use systems in ways other than designed, but unfortunately emerging pressures from reduced training / experience and a litigious social environment require more checklists and published explanation, which unfortunately often become the rule, whereas the intent was to provide guidance because not every situation can be foreseen, particularly when it involves the human.