View Full Version : 777 unreliable airspeed in cruise

19th Dec 2016, 17:23
Consider the following: you are flying at a high cruising level in your 777 and experience an AF447 kind of unreliable airspeed situation. According to Boeings memory items you should set 4 degrees of pitch and 70% N1 (at least in the GE model that is) and then consult the NNC which will lead you to the inflight performance tables for accurate values. Personally it doesn't feel quite natural to me and on my last type I was taught that if it flew straight an level for the past hours than it will for the next minute or so until the tables are handy.

Whats your (or your training departments) take on this?

19th Dec 2016, 17:35
A great deal of discussion in the archives of AF447 exists on this very thing. Although the 777 and A330 are both heavy twins, trying to compare the two (computerized) approaches to UAS is problematic, due to fundamental design discrepancies?

Has a 777 ever reacted (automatically) to UAS by altering (degrading) flight control to a separate flight control system? Climbed rapidly absent pilot input? Trimmed automatically into the STALL?

anson harris
19th Dec 2016, 17:48
I'm told (different Boeing heavy type, but similar memory items) that we should follow the memory items regardless.
I too struggle with the concept of changing things if the aircraft is doing what it's supposed to be doing.

19th Dec 2016, 22:39
Going by the book,
Unreliable airspeed in cruise will invite the necessary memory items FIRST. As unrealistic the values look, before jumping to the tables, it has to be concluded whether the airplane is in trim (still or gotten out of trim)

If in trim still, tables can be refered to with a/c flown in primary flight control mode with pitch/power etc.

If not in trim, the recent step added is - disconnection of PRIMARY FLIGHT COMPUTERS. (This enables a direct mode of operation of stabilizer and other flight controls)
Probably then jumping to the tables would be better.

In short -
Inducing a memory item value of QRH to a a/c already in a steady state won't be
Desirable as the a/c would probably be maintaining very accurate pitch/power however if things get out of hand due to changing airspeeds (& it most certainly will) then restrictimg to the memory item values to begin with won't be a bad idea.

That's just my opinion :) corrections welcomed :)

19th Dec 2016, 22:44
A great deal of discussion in the archives of AF447 exists on this very thing. Although the 777 and A330 are both heavy twins, trying to compare the two (computerized) approaches to UAS is problematic, due to fundamental design discrepancies?

Has a 777 ever reacted (automatically) to UAS by altering (degrading) flight control to a separate flight control system? Climbed rapidly absent pilot input? Trimmed automatically into the STALL?

There was an event reported years ago on a b777 which abruptly intiated a massive climb by 4000-5000 ft in cruise as a result of a false ADIRU acceleration sensed by A/P. What I remember reading is the SAARU added an opposite effect to the whole climb event (not sensing 'rightfully' any acceleration) thereby restricting/limiting the event to just 4-5K altitude gain. So this was not technically a UAS, but False acceleration & input by ADIRU. google can help if you want to read on that :)

Monarch Man
19th Dec 2016, 23:02
Agg, rather than over complicating a rather simple process, how about this.
Lets examine why the memory power and pitch settings are stipulated, and what they give you.
Realistically all they are designed to do is too keep you away from the ground until you can identify and manage your airspeed via alternative methods, they are in essence a basis from where you can start and are just like half the other memory items on the 777 in that they are as much about liability as they are to provide safety.
I KNOW that in the cruise 2.5 degrees and 80-85% N1 will yield level or near level flight within the airspeed limits, 4 degrees and 70% N1 is pointless, likewise down low, lets say below 10000ft or so, the numbers make more sense because they are designed to keep you away from the ground.
As for training departments, well where I work there is likely to be both a common sense approach as well as a slavish adherence depending upon the ego in the seat behind.

The Banjo
20th Dec 2016, 06:08
There is a bulletin, subject: "avoidance of abrupt flight control input as a result of a sudden and unrealistic drop in indicated airspeed".

It reads as you wish in that IN THE ABSENCE OF EICAS MESSAGES Fly the airplane at normal pitch and power settings.

Hope this helps.


20th Dec 2016, 10:50
Although our company is B737 iso B777 I'll give you our training department's vision (same philosophy for all Boeing models).

The logic of the checklist is the following:
1. Fly a safe pitch & power setting
2. Fly a known pitch & power setting
3. Compare the ASI with the known pitch & power settings to identifiy which ones are still reliable.
4. Use the reliable ASI

(This is a change to the old checklist, where the ASI's were compared with the other ASI's)

That being said, the checklist is written for all scenario's. We are being taught that some common sense does apply ;)

If it is very obvious what is going on (like the cruise example above) you are actually already in step 2 so there is no use for the step 1 (memory items).

However, if there is any doubt or it is not immediately clear what is going on, buy yourself some time by applying the memory items. Then you can calmly investigate with the checklist.

The memory items are written in that way that at higher altitudes you will have a descent and at lower altitudes a shallow climb while always staying within the low & high speed limits.

20th Dec 2016, 12:24
mop, you assume a 'hypothetical' situation, overlooking step one. How do you know airspeed is unreliable.
If only one EFIS airspeed display is suspect then perhaps a comparator alert is given.
If a background system is the problem, such as airdata, again there could be an alert or failure warning.
A more complex problem could involve several air data systems like AF, which challenged the automatic 'two out of three' comparator logic, and then handed the situation to the pilots to solve, - where the complexity of situation could 'fool' the two out of three pilots cross check ( but there were only two).

The first step is to determine what the problem is; a single or multiple airspeed display, or airdata which could involve many other aircraft systems - flight control, altitude, vs, ...
I suspect the checklist or memory items (I'm not T7 qualified) relates to specific circumstances. These conditions must be identified before acting. Check any explanation given by the manufacturer, is the malfunction alerted, if so how, what is the drill title, and what alternative actions are there.
Your scenario assumes a situation, then jumps to an activity which might be inappropriate for the actual situation.

Step '0', understand the situation, avoid expectation (manage surprise);
then apply the drill according to your current understanding, consider what should or could happen,
then reassess the situation according to your projections and revised actual conditions,
then reassess the situation, ...
then, ...

20th Dec 2016, 18:05
Can't speak for anyone else, but at some stage (like right now) it has sunk in that this discussion hosts large jet operators who don't know for sure what to do when they lose an instrument.... Haven't mustered the courage yet to step aboard an A330, let alone fly AirFrance....

20th Dec 2016, 19:39

21st Dec 2016, 02:37
Concours, this thread scares me too.

The checklist is excellent. Just do it, and you will survive. The checklist will descend the aircraft. Which is what you want. Who wants to stay in coffin corner with unreliable airspeed? Not I, and not Boeing, that's for sure.

Ignoring a checklist in this circumstance could only ever be acceptable if following the checklist would further jeopardise safety. About the only example I could think of would be traffic below. If traffic below, simply turn off the airway and then follow the checklist.

If you were the F/O, and had an unreliable airspeed in the cruise, and the Captain said "I'm just going to set 2.5 degrees and 85%", what would you do?

Personally I would say, "No, there's a checklist for this", and "Maybe 2.5 degrees and 85% is correct, but will doing that improve safety? No? So let's follow the checklist.". If that didn't work it would be "Taking Over", and set the attitude and thrust from the QRH memory items.

Once the memory items are complete, we can reference the remainder of the checklist, and have a discussion about whether the Captain would like to resume being a Captain.

You see, when you start to vary SOP's, it can only be for a significant improvement in safety, and there are 2 pilots on that flight deck. They both need to be in the loop.

In both Air France and Air Asia, one pilot was doing the wrong thing, the other pilot knew it, but failed to take over properly.

21st Dec 2016, 04:05
The unreliable airspeed checklist is great for when people are disorientated or unable to control their aircraft. However if you are in level flight at cruise and familiar with the pitch and attitude requirements of your aircraft and are aware that you have unreliable airspeed it is actually safer to leave the autopilot engaged and flying the aircraft level than to disengage it and raise the nose to a pitch attitude and power setting that will initially give you a climb followed by a descent. That is of course provided the aircraft is doing a satisfactory job. We are not familiar with hand flying at altitude and even flying the pitch and power settings as per the checklist will lead to a can of worms while one person hand flies it and the other goes through the checklist. I can understand that some modern pilots with no understanding of the basics need to stick slavishly to the checklist especially when things go awry but to deliberately take an aircraft away from a straight and level situation that is under control is not desirable.

Checklists cannot always be appropriate for every situation. For instance the ground proximity checklist dictates that the wings must be level for the manoeuvre however if I am heading for the highest point of a mountain I will bank left or right if necessary to avoid it rather than fly into it. It is best if some areas are left gray so pilots can follow checklists if appropriate or ignore it if slavish adherence to them can lead to jeopardising safety.

21st Dec 2016, 07:12
Chances are slim to none your autopilot will remain engaged with an
unreliable airspeed situation.

21st Dec 2016, 09:31

You state that if you are familiar with the pitch attitude and thrust in cruise then it is safer to leave the autopilot engaged.

Well, I hate to break it to you but the autopilot is not going to give you a pitch attitude. It's going to pitch for whatever pitch it thinks is necessary to maintain what it thinks is your altitude. Your altitude comes from the same air data computer that is giving you unreliable airspeed.

So which is it? Are you going to set a known pitch attitude and thrust, or are you going to leave the autopilot engaged? You can't do both. This is the danger of ignoring the checklist, even if your "non-modern aviator" ego thinks you can do better than the checklist. Look, maybe you can, maybe you can't. As soon as you choose to ignore the checklist the "modern" pilot sitting next to you now has no idea what is going on. So now you are single-pilot IFR with unreliable instruments. Not a nice place to be.

Yes, there can be a time and a place to decide that the checklist is not the most appropriate response to a situation. Those situations are defined as where following the checklist may jeopardize safety. This is not one of them.

Just set 4 degrees and 70%. It's safe. So what if you climb then descend? It will be a shallow climb followed by a shallow descent. It's a big sky out there. Then you are both in a known situation and the checklist will help you resolve the issue.

21st Dec 2016, 11:35
I agree with Derfred.

Why is the first (recall) item on the checklist an instruction to disengage the autopilot? Because much of the automation has potentially become as unreliable as the airspeed. It follows up by removing misleading guidance and possible interference from the FBW systems.

There is significant danger involved in applying non-SOP methods due to instant diagnoses. There are many manifestations of pitot/static failures and part-failures and they can be even more confusing when they happen in turbulence and/or shear (which is unfortunate, as this is a more likely scenario). Best to just follow the QRH unless you’re actually going to hit something.

As an aside, I’ve often thought it would be useful to have a guarded button that removed all displays of airspeed from the cockpit and informed the automation to disregard it. Half of the annoyance is getting warnings and/or undesirable behaviour triggered by something you know is wrong.

Monarch Man
21st Dec 2016, 11:44
Deferred, Im going to assume for a moment that you have many thousands of hours of experience, and more to the point a good deal of that time on the 777.

As we are talking about unreliable airspeed 'in the cruise" I'm curious to your rational that blindly adopting a pitch attitude and power setting without considering your situation is a sensible course of action..it is in fact on the basis of what your experience may be, troubling to me at the very least.
Deferred if you are genuinely advocating this, then I'd suggest a quick review of your Boeing QRH checklist instructions..contained within.

While every attempt is made to supply needed non–normal checklists, it is not possible to develop checklists for all conceivable situations. In some smoke, fire, or fumes situations, the flight crew may need to move between the Smoke, Fire or Fumes checklist and the Smoke or Fumes Removal checklist

Seems to me that based on the document Boeing produces...they themselves provide some common sense guidance in this regard and even give an example to reiterate the point I believe.

Lets now assume for a moment you or your colleague aren't conscientious nor is your SA great and you find yourself close to max FMC alt when you encounter the top of a CB (at night..doesn't it always happen at night!) thanks to low radar reflectivity etc etc..and hey presto you get the stick shaker and the overspeed....are you going to reduce thrust to 70% and pitch up to 4 degrees? how long till you stall the aeroplane aerodynamically? how long till you overspeed it? how long till the other guy is useful again and gets past that startle effect?
I KNOW that 85% N1 and 2.5 degrees is safe in the cruise...and will keep me safe.
Don't believe that I'm not using a bit of common sense and experience to add resilience to the Boeing memory items and checklist?...then lets look past the memory items...and understand where the rest of the checklist leads you.

5 Chooseone:
Aircraft can be trimmed to desired pitch attitude:
Go to step 6
Aircraft cannot be trimmed to desired pitch
PRIMARY FLIGHT COMPUTERS disconnectswitch.............. DISC

Go to step 6
6 The following are reliable:
Ground speed Radio altitude

So there we have it...Pitch and N1 are reliable! who'd have thought it?

moving on...

Note: Stick shaker, overspeed warning, and AIRSPEED LOW alerts may sound erroneously or simultaneously.
Note: The flight path vector, and pitch limit indicator are unreliable.
Refer to the Flight with Unreliable Airspeed table in the Performance Inflight chapter and set the pitch attitude and thrust setting for the current airplane configuration and phase of flight.

And we get get to the meaty bit...because we've ALREADY established a safe pitch and power setting for the current phase of flight, it gives us more time to make a better assessment of the issue thanks to the aircraft never entering an undesired state.
Now we have time to complete the checklist having overcome that startle effect and dealt with the fear factor, after all its an aeroplane and basic physics apply, Attitude, Power, Trim.....just like a C150.

The intent of checklists and memory items are to AID to the pilot, they are and were never intended to replace your thought and reasoning process.

Lastly, failing all of that merely take a quick mental note of your steady state N1 and Attitude when it settles down in the cruise...you'd be amazed how that works.

Now if we are discussing unreliable airspeed in the climb or descent...thats another topic for discussion, and much more related and in tune to the checklist memory items for N1 and Attitude.

21st Dec 2016, 12:54
Personally I would say, "No, there's a checklist for this", and "Maybe 2.5 degrees and 85% is correct, but will doing that improve safety? No? So let's follow the checklist.". If that didn't work it would be "Taking Over", and set the attitude and thrust from the QRH memory items.

Once the memory items are complete, we can reference the remainder of the checklist, and have a discussion about whether the Captain would like to resume being a Captain.

OK, i'll give you a scenario:
- Daylight CAVOK
- FL370 for the last hour

Pitch and thrust have remained the same for the last hour. You get a warning: "Airspeed disagree". The aircraft does not change pitch & thrust.

You look and see that the ASI are:
- PF: 253kts
- SBY: 250kts
- PM: 45kts

After crew agreement, the crew realizes that it's pretty obvious what is going on and just remains at the known pitch settings while doing the checklist...

Now, if you would be my copilot on that day and you don't agree with this. Then sure, we'll do the memory items. That would fall under the "not immediately obvious case" (at least for 1 pilot). I agree with you that the last thing you want is a flight deck where 1 of the pilots is lost.

Anyway, scenario 2: During descent (IMC), you hear a bang. 10 seconds later you get "Airspeed unreliable". The A/P starts pitching down. You decide together that you do the memory items...

Amadis of Gaul
21st Dec 2016, 16:27
Can't speak for anyone else, but at some stage (like right now) it has sunk in that this discussion hosts large jet operators who don't know for sure what to do when they lose an instrument.... Haven't mustered the courage yet to step aboard an A330, let alone fly AirFrance....
I wouldn't put too much credence into anything you read around here. For one thing, there is no way to know who here really is a "large jet operator" and who is not. For another, topics seem to rapidly deteriorate into pissing contests on who knows their respective "book" better, while any common-sense airmanship concepts are quickly discarded.

It is what it is...

21st Dec 2016, 16:28
What does Boeing say? 4 NU and 70%, then checklist.

If you know the numbers and can instantly match what the checklist requires??? The checklist has 5,000' altitude blocks and scans 100,000 lbs. What's the checklist going to get you? After extrapolating from four boxes?? It's trying to get you the pitch and power you've been holding for the last 30 minutes. But if you don't know it don't start guessing.

Just checked what I see vs. what the checklist calls for. .8% difference. Any guesses on if a four box extrapolated answer, with heart pounding, will be within .8%???

21st Dec 2016, 18:47
Straight from the Boeing technical/test team..... The unreliable airspeed Pitch and Power settings are for one purpose and one purpose only. To stop you damaging the aeroplane.

An incredible amount of time and effort went into developing the memory item numbers as they must cover all weights and altitude scenarios. If you are high and heavy you will descend if you are light and low you will climb but you will never stall and never overspeed with these numbers. It is also assumed likely you will have begun to lose control of the aeroplane before an unreliable airspeed diagnosis hence the importance of the numbers. Airmanship always supercedes any Boeing QRH. There is no requirement to set these numbers if you are entirely satisfied with current and correct pitch and power after the failure and situational awareness is firmly intact.

21st Dec 2016, 19:06

You wrote

Slightly patronising tone but I will overlook it. If you are in a 777 simulator in cruise and a pitot or pitots block but the altimeters remain working normally the autopilot will remain engaged and will maintain the FL. That is the situation I was referrring to. If the autopilot disengages and the aircraft goes into a climb or descent with confusing instrument indications then of course I will do the unreliable airspeed checklist.

Good posts above from Monarchman and rblykyv and 8che who I believe have a better understanding of the spirit of the QRH.

21st Dec 2016, 23:40
(Airbus FBW)
Developed when flying across the Atlantic; I got into the habit when PF, of writing the current pitch and N1 onto the MCDU scratchpad every 30 mins or so during the cruise.

Usually it is 2.5 degrees up, but varied from 74% to 83% N1 depending on variant (320/321/330) and weight.

(Obviously we did not fly A320/321 across the Atlantic !)

22nd Dec 2016, 10:51
Airmanship always supercedes any Boeing QRH. There is no requirement to set these numbers if you are entirely satisfied with current and correct pitch and power after the failure and situational awareness is firmly intact.

Is that a personal opinion or a Boeing one? I'm not taking sides, but.....
I found it very frustrating in a TR course where the demo of unreliable airspeed scenario is always given during departure. The SFI has spent some energy beating into the cadets that 60%/6 is the standard for 220kts level flight as a starting datum. There you are on a SID in LHR TMA climbing 6000' when you notice there problem. You level 6000' and set parameters that have you climbing into the FL descending traffic. Now you have a problem and ATC has a problem. They need to get you out of the way, away from the SID/STARS and let you sort it out. Not always so easy as you are constantly climbing. In the sim it takes the guys 2-3000' to arrive at the QRH tables for level flight. Guess what? 60%/6.

There is talk on the CRJ crash thread about making a quick analysis of the 3 sets of instruments and make a choice which is correct. Sounds easy, but not allowed according to the strict adherence of QRH teaching in some airlines. No short cuts.
There is then the scenario of all 3 ASI's being wrong. What do you do then. A.N.C. The QRH takes quite a while to arrive at 'comparison' of ASI's and then the parameter tables. Total failure could bring confusion and in a panic perhaps the QRH memory figures are the best to keep you out of some trouble?

IMHO it would not help the matter if PF (captain) made up their own SOP in a critical situation and in doing so pushed the PM (young F/O) out of the loop. The PM would not know what was going on, most likely did not agree and you now have a discussion about what you're doing while you're doing it, with endless "yes, but...." coming from PM. Is that wise when the focus should be on solving the problem and saving the a/c? There could now be doubt in how much the PM could trust you.

There could also be a static problem. In the sim it is always a blocked pitot, yet in Peru it was blocked static, and that can be much more disorientating. I've never been given that scenario in 35 years. Waste of a simulator.
Murphy is alive and still out there.

Monarch Man
22nd Dec 2016, 11:13
RAT 5, 1st things first, honestly do you care about ATC and their problems? If you have an issue..deal with it, don't fly into a hill, then tell them (7700 in the transponder works a treat).

With respect to the CRJ accident, what you say may well be true, but we are discussing a 777 with unreliable airspeed, if you understand the checklist then you know that the attitude information is reliable, the ASI or ASI's giving erroneous information are largely to be ignored as the reliable sources are attitude, N1, groundspeed.

A little knowledge can be dangerous, or useful, it depends on how you apply it. There are a few things related to checklists on the 777 that I have committed to memory beyond what is "required" for the simple reason that I feel it gives me more capacity to deal with non-normals should they happen.

Lastly with respect to an SFI and a TR course, thats merely a basis from which to continue your education, use your frustration positively and improve your understanding. FWIW there are people where I work who can programme an ILS via the FMC blindfolded (useful in a cockpit full of smoke)..and yes the one I know does have a life outside of aviation.

22nd Dec 2016, 13:04

Its a Boeing opinion ! Also my opinion and the opinion of every aviation authority in the world.

Its called Commanders legal authority. Boeing supply the tools. Captains are legally bound to pick up or drop those tools in any way they see fit to assure safety. Don't be lead down rabbit holes. Checklists are used to help achieve an outcome. Airmanship dictates there use or not.

22nd Dec 2016, 14:24
I am with you 8ache. I know there are some operators who do not teach according to that philosophy; it is very rigid and trained monkey stuff. Perhaps they are fearing the legal comeback if it goes oops. Some think that following the rigidity of a check list gives a legal safety suit for any questions that might come later.
I expect this to be another ever spinning circular discussion with a never ending story. Always good to read opinions.

22nd Dec 2016, 14:58
Thank you for all your inputs, quite some interesting views here. My question was really aimed at a scenario where there is good situational awareness and a very obvious problem (e.g. a single ASI jumps to 0). Of course if there is confusion or an upset situation these memory item values could save your day and should be applied if ever in doubt. Pitching up though in tight RVSM airspace when there is no need gives me a headache...

23rd Dec 2016, 05:13
mop, your scenario (e.g. a single ASI jumps to 0) is not an unreliable airspeed scenario, it is an airspeed indicator failure. In this case (assuming the remaining two airspeed indicators are in agreement), I would not even commence the unreliable airspeed checklist.

However, you have mentioned the word "confusion" which is the main theme behind my opinion on this subject, which obviously differs from some other posters.

Many accidents occur due to confusion on the flight deck. Yes, there is always a sequence of events or errors that leads to the confusion (contributing factors), but it is often the confusion that is the final cause of the aircraft crashing.

Air France, Birgenair, AeroPeru and Air Asia all crashed with confusion on the flight deck. In particular, Air France and Air Asia both crashed with one pilot confused about what the other pilot was doing. In the case of Birgenair, it appeared that both co-pilots understood the situation but did not intervene out of deference to the Captain, who unfortunately was very confused.

Human factors are incredibly important in preventing non-normal situations from turning into aircraft accidents. Confusion can be a real threat - and the best way to resolve the confusion is good teamwork. Good teamwork requires a shared mental model.

In normal operations we create a shared mental model by using SOP's and briefings. However in non-normal operations, we haven't done a briefing (with the exception of engine-failure during takeoff), so the shared mental model comes from the QRH.

Now, with an unreliable airspeed, the potential for confusion is extremely high. Pilots really struggle to ignore instruments they have spent thousands of hours trusting. It is highly likely that contradictory indications are occurring (eg overspeed and underspeed indications or warnings). Pilot A will be looking at different information from Pilot B, and may have a different idea of what is wrong with the aircraft.

Now if Pilot A decides to use his airmanship (Boeing actually calls it "good judgement" in the QRH), and decides to pull an attitude and thrust out of his head instead of commencing the non-normal checklist, how is this going to affect Pilot B? How is this going to contribute to good teamwork? How is this going to resolve the confusion?

Pilot A has successfully added to the confusion because Pilot B does not know that Pilot A is an ace and has a photographic memory of all the attitudes and thrusts in the QRH performance in-flight section.

In short, it is not superior flying skill that is going to save this aircraft. It is superior teamwork.

Potential traffic in tight RVSM airspace needs to take a distance second in your consideration even if it does give you a headache. You now have an emergency. That means you now "own" the airspace. A quick mayday call to ATC telling them that you are unable to maintain altitude will suffice and they will get everyone else the hell out of your way.

I note all the opinions above, but I stand by mine. In my "good judgement", ignoring the checklist has no up-side, but a considerable down-side.

Monarch Man
23rd Dec 2016, 11:33

Very good post IMHO, I agree with pretty much everything you say and rather than pick it apart, hopefully I'm going to add to it.
The key as you note is the "shared" mental model, and in the instance of unreliable airspeed on the 777 this is most definitely a key to solving the problem.
The exact way its done is infinitely variable according to the crew, the circumstances et al, but ultimately its the key to both regaining SA.

Just as a note with respect to Birgenair 301, a 757 no less, if the Captain had a safe pitch and power setting in his bag of tricks (funnily enough on the 75 its 2.5 degrees and an N1 of 75% for level flight or 7.5 degrees and about 85% for a climb )...never trust the EPR setting if you don't trust the ASI on a RR powered 75.
They would have flown away fat dumb and happy and he would have had time to get over his confusion and recognise the issue...assuming of course he understood the AP logic and disconnected it as part of the memory items:ok:

24th Dec 2016, 09:41
Thanks, Monarch.

With respect to the Birgenair accident, at that time the Boeing checklist did not specify an immediate attitude and thrust in the memory items of the checklist as it does now. Boeing only reinvented that checklist in response to the Air France accident (as you would know). So the current Boeing checklist may have helped them back then.

Draglift, I apologise for the tone of my response, and thank you for letting it go. When scenarios come up in Tech Log, it is easy for various posters to interpret the scenario in different ways if the exact conditions of the questioned scenario is not very specific. I had worst-case scenarios in mind when I responded to your post.

Non-normal scenarios will always have a milion answers, and that's obviously why Boeing cannot provide a QRH answer for every scenario. In the same way, PPruNe cannot provide an answer for every scenario.

I have given my answer that will cover most scenarios. Other posters might have had different scenarios in mind when posting their answers. I finally provided my reasons for my answer (which I probably should have done earlier) but those reasons may also not cover all scenarios.

I will concede after reading all these posts that there may be situations where setting roughly known cruise thrust and attitude may be appropriate in some circumstances, so long as the shared mental model I explained previously is not compromised. That will depend on many factors, including how well you know the pilot sitting next to you, the conditions on the day, whether you have some idea of why the airspeed has become unreliable, and whether you are absolutely positive there is or will be no confusion prevalent on the flight deck.

Human factors cause a lot of aircraft accidents. They also save a lot of aircraft - you won't find me proposing autonomous airliners on those theads.

There aren't a lot of accidents put down to unreliable airspeed, so the data supporting appropriate corrective action is minimal.

My home port of Brisbane Australia had a wasp infestation a year ago resulting in at least 4 aircraft in one day suffering blocked pitots on takeoff. Fortunately, none of those incidents turned into accidents, they were all well handled, and I don't think they even made the local news. These wasps were so keen they could block a pitot in a 40 minute transit. (Only in Australia... I saw one of them devouring a bird-eating spider in the engine intake during my walkaround...:) ). We now install pitot covers every transit.

But as I said, the checklist changed considerably after the Air France accident. With a lack of data, any proposed corrective action outside of the "extensively studied" QRH actions are subjective or speculative.

I've certainly learned something from this thread...by thinking at length about what others have posted and re-evaluating my thoughts on the subject. If anyone else has learned anything from my posts then that makes me happy too.

24th Dec 2016, 11:51
Of course, what has not been discussed is one can think ahead and share one's mental model in advance. On a dark night in the tropics one can get the QRH out and review the items, or discuss with your colleague that if all goes squirrels or you get NAV AIR DATA SYS EICAS what pitch and thrust you are going to set after AP, AT and FD have all been switched off if you do not plan to follow the 4°/70%N1 in the QRH. This is fairly common practice at my airline.

24th Dec 2016, 12:23
Derfred, quite frankly I am also not a fan of the style of your earlier posts (not talking about the content itself). We all come here for an educational purpose and not some sort of pi**ingcontest. But as its christmas today I will also let it slip :}, and your last post was much more to my liking, summing it all up pretty nicely.

I posted here to get different perspectives on a scenario that in my opionion is covered rather vagely in the official documentation and found exactly what I was looking for. Next time though I will try to specify more in detail as I guess (as you rightly wrote) there are just to many variables to cover every possible scenario.

24th Dec 2016, 14:23
If the accident report was accurate the Birgenair was not strictly an airborne airspeed unreliable incident. It should never have got airborne but made an RTO at the 80kts call. Was the same true with the Aussie bees. Did there ASI's function correctly all the way through takeoff?
The other issue with Birgenair could be attached to the 'automatic dependency' threads. The captain engaged the autopilot to trouble shoot the problem. Problem was he engaged the CMD to the faulty system and the autopilot stalled the a/c. Dangerous to engage automatics if you don't know what they are going to do.

1st Jan 2017, 05:22
RAT - this is the most serious of the incidents referred to regarding the Wasps at YBBN.

Note it performed an RTO the first time and the speed difference was noted after V1 the second time.


1st Jan 2017, 10:02
Tx for the link. Very informative.

Broomstick Flier
7th Jan 2017, 17:01
From experience (in the sim) I realised that adjusting the power with Fuel Flow figures is much easier and straightforward than with N1.
For my type (767) I have some ballpark numbers in mind (like 2.5/3 pitch x 2.5t fuel flow) for different phases of flight. This is enough to keep things under control before resorting to the QRH.

8th Jan 2017, 21:49
I have never felt comfortable with the NNC. Because if nothing else, having required the setting of 4deg nu and 70% N1 - which I agree will keep you safe in the short term - at what point does the NNC - reading it explicitly - allow you to say : OK , now lets fly level for a bit (we are assuming Altitude is 'Reliable' here ) and establish, through the PI Tables, which of the 3 Airspeed Indications we have is 'reliable' - as one of them might be; and if one is, this makes things a lot more manageable. But any deviation from initial memory Pitch Attitude / N1 targets is not mentioned by the NNC. To read it would appear that the initial Pitch /Thrust values are held forever. Clearly this cannot be the intent.

It would be more robust - imho - if the NNC Memory Item was phrased as ' If Pitch Attitude /Thrust for phase of Flight is known ...Set required Pitch Attitude and N1 ; If not known , set 4 deg /70% Clean ( 10 deg / 85% if Flaps extended).

This would cover the Cruise case - last 3 hours at 2.5 deg nu and 85% N1 - as well as the TO case. We know - or should know - that the required Pitch Attitude /Thrust immediately after Takeoff is 15 deg nu, or thereabouts with Take Off Thrust , and the Thrust is already set. (A/T disarmed) . By reducing pitch and thrust , and in the worst case, you will not stall but you could compromise terrain clearance.

Perhaps it would be better to maintain 15 deg nu / TO Thrust ( and config) until you are above MSA , then fly level and find a thrust to give you the target pitch attitude from PI. You are then flying at manoeuvre speed for the flap ( for Flap 5, for instance, you are now at Vref30 +40. ) Cross check Airspeed indications. Again, one might be correct.

This is consistent with the NNC Objective which includes 'establish a reliable source of airspeed' . And the NAV AIR DATA SYS NNC ( which will be displayed if all 3 Airspeed inputs to the ADIRU are different) will even suggest you select an identified reliable source to the off-side of the flight deck through the AIR DATA / ATT ALTN switch