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unreliable airspeed jet transport aircraft

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unreliable airspeed jet transport aircraft

Old 14th Apr 2015, 13:19
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In my opinion, the title of the procedure is self explanatory: Unreliable Speed.
¿Do you rely on your airspeed indications?
-YEP: continue normally
-NOPE: don't try to find out why, don't troubleshoot on your own, just do the manufacturer troubleshooting procedure to confirm/get some reliable speed indication, or fly with reference to tables or BUSS if there is none.

It suffices with not relying on the airspeed, even if you don't have a clue of what is going on. It may take some time to answer the question and come to the conclusion that you actually don't trust your instruments. A mixture of airmanship, experience, knowledge and system warnings will get you to that conclusion if something is not all right with the speed indications. With time there will be more system warnings available, for example some cautions to advise of a non reasonable angle of attack for a given speed and load factor, or something similar. Helpful stuff.

Every time there is an ADR fault, or flags, you should check the consistence of the remaining indications, specially with pitch, thrust and automation behavior just in case you have got one of those two wrong computers voting the good one out cases.

Most integrated ab-initio pilots do an MCC course after they get their frozen ATPL. It is intended to get familiar with airline type of operation, but it is also a good opportunity to practice hand flying in jets (sim, of course).

I did a lot of it, and I was able to fly an ILS without speed indication very easily. We flew pitch and thrust and we could perform the whole approach, from the hold till landing, just by using the pitch references we had for each configuration. When I practice Unreliable Speed in my company simulators, I just retrieve that from my hard disk and have a relaxed session.

I hate that they have removed the pitch references from the QRH on account of having the BUSS. Why do they do that? Is the BUSS immune to failure or what?
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Old 15th Apr 2015, 00:56
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Lightbulb Alternative to Pitot

Looking for a different option for determining IAS, what about a "reverse style" pitot that measures vacuum pressure. That way it will not get blocked. Anything in it will get sucked out. Something like a trumpet facing backward. If it needed clearing, just let some cabin air into it, say, every 30 seconds. Probably no need to have the ability to heat it either.

If you know altitude, outside air temp/pressure, and with some sensitive vacuum gauge/wizardry, it should be possible to determine IAS with greater reliability than using pitots.

Has anything like this ever been considered?
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Old 18th Apr 2015, 02:05
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Just an Idea

Maybe Aircraft manufacturers should design and incorporate an "unreliable airspeed switch" into their aircraft. And add it as one of the memory items in case of suspected unreliable airspeed.

the way it could work is simply to write an algorithm which takes some known accurate values such as current pitch, power, weight, configuration and altitude (gps or otherwise), etc, and calculate a 'predicted' speed value based on the above values remaining constant. The switch would cause the speed bug to be replaced by a predicted speed bug (different colour)

This would require training pilots in the interpretation of the predicted speed info and the limitations/inaccuracies associated with the system.
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Old 18th Apr 2015, 02:31
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Any jet with full trust and 10 degrees pitch up will climb away.

I had an unreliable airspeed in IMC take off in a A310;bit scary but came on top at 10,000', at which point the pitot tube cleared and things got normal. Just a whole bunch of warnings regarding slats and flaps.

Also I remember towing gliders for a whole day with airspeed indicating '0'.

At the end of the day maybe it is easier to learn to fly than to remember al the patches people come up with and try to mold into SOP's.
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Old 18th Apr 2015, 03:59
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Any jet with full trust and 10 degrees pitch up will climb away.
Try that with "Any jet" at cruise altitude near max altitude, or even at optimum cruise altitude...
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Old 18th Apr 2015, 07:05
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To intruder: at max altitude I have no intention to climb away, you?
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Old 18th Apr 2015, 08:21
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It's going to be very interesting the 1st time someone follows this advise and sets an attitude & power setting at a low level which causes a climb, then gets an RA, goes into a complete panic and the a/c goes out of control. I wonder if all the consequences have been thought through, or was this a quick knee jerk reaction to fix something that should have been fixed via another solution. I think of human intervention into nature problems. There is a pest so introduce its predator. Then sit back and watch the utter mayhem you've caused. Most things do not have isolated consequences. Could this be a slice of swiss with the hole no open? Would better training not be the better longterm solution? Hence my wonder if this is a quick fix.
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Old 18th Apr 2015, 20:01
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at max altitude I have no intention to climb away, you?
You made the general statement, not I...
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Old 19th Apr 2015, 01:58
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JT responds:
I used to get the cadet level first endorsement trainee progressively up to the point where s/he could fly a single pilot low vis circuit with all the PS instruments progressively failed (or covered) during the takeoff. The aim was to keep it under a reasonable semblance of control, find the ILS and execute a safe landing recovery in min vis conditions.
Seems to me Asiana could profit from this exercise, except VFR no ILS, in transition training for 777 captains.
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Old 19th Apr 2015, 04:49
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I might add to my previous statement, especially for the people only referring to SOP's, that common sense prevails. Despite the efforts to contain every action in boxes, history proves you cannot fly planes without knowing how to fly planes.
For intruder I might add that at or near max altitude, a pitch attitude of anything between 2 and 4 degrees pitch up might save you from falling out of the sky.
Wings level, of course.
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Old 20th Apr 2015, 12:55
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On the 767 I was taught and use as an immediate balkpark (before resorting to the QRH): 2.5 degrees pitch and 2.5t fuel flow (find FF easier to adjust than N1). That will keep me flying.

Next step would be pulling the aural warning CBs and call for the QRH, aviate, navigate and communicate, and a hot coffee, please.
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Old 20th Apr 2015, 14:26
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For intruder I might add that at or near max altitude, a pitch attitude of anything between 2 and 4 degrees pitch up might save you from falling out of the sky.
...which is a BIG difference from your initial 10 degree pitch recommendation...

As Broomstick noted, knowing a ballpark pitch and fuel flow for level flight is an even better idea. For the 744, the 2.5/2.5 rule works fine at lighter weights (I use 320T as a ballpark discriminator between "light" and "heavy"), and 3.0 on the fuel flow when heavy. If you can't get to the lower EICAS ENGINE page right away, start with 90% N1. Within 0.5 degree pitch and 0.5 T/hr fuel flow (likely within the "jitter" of manual flight when in extremis), that works at any altitude when clean. You may climb or descend a bit, but your airspeed and vertical speed will be controllable.

What too many people fail to realize is that if airspeed fails airborne, the "Don't-a touch-a nothin'!" rule will usually work for the initial assessment. If the FMS fails, bringing down the VNAV, LNAV, and Autothrottles, the last thrust setting will likely still be active; and if the autopilot was on, the reversion to pitch- and heading-hold modes should keep you flying for a long time.

VERY few "emergencies" need IMMEDIATE action to the extent that a few seconds of thought will be detrimental. Even when I was flying single-engine A-4s off the carrier, an engine failure off the catapult was about the only emergency that fit that definition. When I was an instructor in that airplane, we stressed to the new students that the FIRST action in any other emergency was to wind the clock. That gave a couple seconds pause to figure out what was REALLY wrong...
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Old 20th Apr 2015, 16:48
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All good common sense thoughts: which brings me back to an earlier question, why have the manufacturers, seemingly, jumped up with a quick knee jerk change to QRH? I sure there are some 'grey beards' in Seattle & Toulouse. Did the legal boys have louder voices? IMHO this issue is deeper and more complex than a 'quick fix for all occasions' type solution.
Just a thought: we've only mentioned Boeing & Airbus in this discussion; what do all the other manufacturers have to say about this unreliable airspeed scenario? This is possible in any a/c. What does Embraer, Canadair, Biz-jets, recommend?
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Old 20th Apr 2015, 18:01
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In my Bombardier bizjet, they don't even include a pitch and thrust table, or any memory items. It just goes for the old fashioned maintain control, compare values and select a reliable source.

Their QRH is generally pretty ropey though so it's no great surprise.
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Old 20th Apr 2015, 18:46
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Going back to the original question, which was asking about diagnosis as opposed to actions.

I think it’s a really good one. We all know (or should know) the memory drills for unreliable airspeed in our various types. When to apply them is more of a problem area, as the aircraft will almost certainly be deviating from its previous flightpath when you do, with varying levels of excitement depending on how busy the environment is.

It’s all very well sitting in the the simulator doing a UA exercise but there are too many levels of unreality here, IMHO. First, you know it’s coming and second, there are only so many ways to get a sim to behave.

In real life, there are many situations where the pitch/power relationship can be well away from normal but all the aircraft systems are functioning correctly. Some are dynamic and some can last a considerable time. If you encounter horizontal shear or temperature gradients, or climb/descend through a vertical shear, you will get pitch/power/airspeed combinations very reminiscent of a UA scenario.

How to tell the difference? Well, if you’ve got a conflict of opinion between airspeed sources, then it’s a genuine problem. If you haven’t, it’s still possible that they have all been affected at around the same time. When I had ice crystal icing a few years back, it happened to both engines within seconds of each other, which I suppose is not that incredible seeing as they’re virtually identical.

If the attitude is not what you expect, it could be from external forces or driven by false data interacting with the auto flight systems. FBW and non-FBW aircraft behave differently AP-out as well. I’m not at all surprised that it takes some time to register that not all is right as the symptoms of flying through changing wind and/or temperature are very similar to those you might get with certain types of UA.

In summary, I don’t think it is that obvious initially in many cases that you have a UA problem, given that during almost every flight there are instances of UA-like symptoms which are simply due to environmental changes.
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Old 21st Apr 2015, 04:44
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That is right!

For that reason, the moment you don't quite trust your indications you should trigger the procedure application. You might be well in an overspeed, or approaching stall, so you have to do it promptly and do the trouble shooting by the book. No assumptions nor own troubleshooting. By the book. And performed properly because it will not work if you don't do exactly what it is laid out.
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Old 21st Apr 2015, 19:54
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"By the book. And performed properly because it will not work if you don't do exactly what it is laid out."

One of the points of this debate is to ask why 'the book' has been changed. People had done it by the book in its previous life and it worked. I would like to think pilot are thinking animals and not just trained monkeys. Procedures are not cast in stone; they evolve & develop. Understanding why is part of our education. Here we are asking why; is it an improvement? Just following a new version, blindly, without thinking will not enhance one's education of the aviating environment.

Unreliable airspeed commences with a memory checklist. Thus it is not complicated to include simple memory parameters. B737 new QRH has 'flaps up' and 'flaps extended' pitch/att numbers. There is debate about whether they cause their own consequential problems. Flaps extended implies UP speed or slower. We all know 6 & 60% works. Flaps up implies 230-250kts and higher. <10,000' 3 degrees 65% works, 25,000 3 degrees 75% works; 35,000 2 degrees 85% works. Surely 2-3 degrees is easy to remember & 10/20/30,000' & 65/75/85% ball park figures are also easy. Highly trained, well educated, intelligent pilots should be able to handle that small memory task.

If you set the QRH figures at low levels you will climb. What will you do with an RA telling you to stop climbing or reduce V/S? Why not TA only? Why is it that B737 QRH is so reluctant to include TA only in more manoeuvres? Surely anytime you are a wounded bird and are restricted in any way the TA only should be selected, ASAP, to warn other a/c that you can not manoeuvre freely?
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Old 22nd Apr 2015, 07:22
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...the moment you don't quite trust your indications you should trigger the procedure application.
Agreed. But what actually defines “the moment"? I think the OP was looking for definite trends/indications/events that might alert experienced pilots to a developing UA scenario.

I was trying to make the point in my previous post that we experience environmentally-driven UA-like symptoms far more often than we do actual UA itself, therefore naturally have trouble diagnosing real UA when it happens as we are exposed to “fake” UA on almost every sector.

One of the points of this debate is to ask why 'the book' has been changed.
Possibly because of incident/accident trends and a de-skilling of the ‘traditional’ way of flying? Or that it was recognised that in a high-stress scenario, having something simple and widely applicable might lead to a better overall performance from an average crew?

In older aircraft, you just took the AP out (if it was in in the first place) and carried on flying, pretty much. With FBW, autothrottles, envelope protection, etc. that may not be enough as the protection systems could be trying to kill you and you have to positively assert control over them.

Regarding TCAS vs. UA drills, I think the other aircraft involved will get their RAs modified in the light of you not following yours. Also, as TCAS uses pressure altitude, any static problems will render it fairly useless.
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Old 22nd Apr 2015, 11:12
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'I think the other aircraft involved will get their RAs modified in the light of you not following yours.'

I'd like a definitive 100% black & white answer to this question. So far in trying I've been unsuccessful. I think the time bubble that envelops/protects the attacked a/c would not give TCAS RA the time to change its mind and issue commands to the 'innocent' a/c to run away and give the crew time to react.
The 'note' in the Engine Failure QRH when selecting TA Only mentions only that it is .........to avoid climb commands.......There are far more scenarios where this should be selected. In this case I'm amazed that it is No. 9 in the sequence. IMHO if your performance is compromised TA only should be ASAP.

And; in a Volcanic Ash scenario where both pitots are effected by ash + engine flame out and you are in 'windmill start' envelope so you need speed (even worse in a total flame out where speed is vital & thrust is zero) and now you have an 'unreliable speed' event; I expect the ensuing confusion as to which memory items to follow accompanied by a massive CRM breakdown would be entertaining. The 'old grey beards' might come into their own.
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