777 unreliable airspeed in cruise

[8che]

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.

[draglift]

Derfred

You wrote

[quote, 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.][/quote]

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.

[Uplinker]

(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 !)

[RAT 5]

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]

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.

[8che]

RAT 5

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.

[RAT 5]

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.

[möp]

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...

[Derfred]

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]

Derfred,

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

[Derfred]

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.

[Jwscud]

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.

[möp]

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.

[RAT 5]

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.

[overhere]

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.

https://www.atsb.gov.au/publications...r/ao-2013-212/

[RAT 5]

Tx for the link. Very informative.

[Broomstick Flier]

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.

[je.f]

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