unreliable airspeed jet transport aircraft

[sudden twang]

Seems to me that this is a real problem area has anyone got any top tips on how to teach the recognition of this? Specifically the recognition as opposed to the recovery.

Many Thanks

[cosmo kramer]

737 QRH:
Condition: The pitch attitude is not consistent with the phase of flight, altitude, thrust and weight, or noise or low frequency buffeting is experienced.

And some snips from the 737 FCTM:

When the ram air inlet to the pitot head is blocked, pressure in the probe is released through the drain holes and the airspeed slowly drops to zero. If the ram air inlet and the probe drain holes are both blocked, pressure trapped within the system reacts unpredictably. The pressure may increase through expansion, decrease through contraction, or remain constant. In all cases, the airspeed indications would be abnormal. This could mean increasing indicated airspeed in climb, decreasing indicated airspeed in descent, or unpredictable indicated airspeed in cruise.

Hence, the two above leads to the following:

Early recognition of erroneous airspeed indications requires familiarity with the interrelationship of attitude, thrust setting, and airspeed.

..and this recommendation:

The flight crew should be familiar with the approximate pitch attitude for each flight manoeuvre... ...For example, climb performance is based on maintaining a particular airspeed or Mach number. This results in a specific body attitude that varies little with gross weight and altitude. Any significant change from the body attitude required to maintain a desired airspeed should alert the flight crew to a potential problem.

So know the relevant pitch/thrust values. That way it should be easy to recognise something askew.

[sudden twang]

Thanks CK,
Im not on the 737 but that is consistent with the other 5 types of Boeing that Im familiar with.

I have found that with most crews during refreshers that it takes some time for them to recognise that something is awry. This is backed up with investigations following events. There seems also to be an inordinate amount of time before the recommended pitch and power settings from the memory drill are adopted.

Just wondered if anyone had a top fix for this.

[toffeez]

sudden twang

"Seems to me that this is a real problem area has anyone got any top tips on how to teach the recognition of this? Specifically the recognition as opposed to the recovery"

Not sure about that. The crew of AF 447 recognised the unreliable airspeed, but messed up everything that followed. They didn't follow the established procedures and flew a perfectly flyable plane into a stall - and the Atlantic.

[RAT 5]

A classic problem with this recognition is the "follow the flight director" drilling during training. Rather than that I teach "look through the flight director". Learning the basics is not now part of the teaching. The basic balanced triangle of 'IAS + ATT + Power' = performance. Is that equation correct? If the autopilot is in CMD then normally the FD is centred. This is a sucker. I teach stalls with FD is use: guess what, the cross of the FD is perfectly centred but at the wrong attitude for the normal phase of flight. It's an eye opener. I teach a lot of flying A/P NPA approaches without FD's to demonstrate they tell you very little. An NPA needs a good scan of the basic performance displays. Teach guys the basics and they will detect the problems. Sadly, not enough guys are drilled with this discipline.

[sudden twang]

I didn't want to bring AF447 into this it's been done here.
Thanks RAT 5 v helpful post and food for thought.

[RVF750]

Very good post RAT5.

[flyingchanges]

Also good to know approximate thrust lever angles (sorry Airbus pilots) and trim settings for cruise.

[DozyWannabe]

Originally Posted by flyingchanges

...good to know approximate thrust lever angles (sorry Airbus pilots)...

I'm a little confused by that - As I understand it, if autothrust is disengaged the FBW Airbus thrust levers work the "old-fashioned" way - I think the only difference between TLs on FBW Airbii and other types is that the Airbus A/THR doesn't physically move the TLs to match its settings, and other types' autothrottles do.

@sudden twang:
(Caveat : I'm a software engineer, not a professional pilot - but I've been aviation-mad from the crib, and have been around this forum long enough to pick up a few things. Apologies if this becomes a mini-essay, but I hope you'll find it interesting... )

So - Unreliable airspeed (or UAS). If I've understood things correctly from what I've read and been told over the years it's undoubtedly a potentially hazardous condition, but if handled correctly it's more of an annoyance than a major threat. If it occurs in-flight, the piloting mantra "Aviate, Navigate, Communicate" (in that order) is key - before doing anything else (including diagnosing the problem), you have to make sure your flightpath is relatively stable. In the case of the Airbus FBW types, the flight data computers will detect a discrepancy in airspeed readings and automatically disconnect autopilot.

If the flightpath was straight, level and more-or-less stable (e.g. in the cruise phase) at the time, then there's a good chance that little or no manual intervention will be necessary. However if a pilot is not sure, one thing you'll see repeatedly through the AF447 thread is a reference to the term "Pitch and Power". What this means is that in the event of losing airspeed information, the manufacturers provide a lookup table in the reference handbook which says that if the pilot sets a certain pitch attitude and thrust/power setting, then the aircraft will tend to maintain altitude and sufficient airspeed. The settings from the A330 handbook are below (note that the higher you go, the lesser the required pitch attitude):



Once the aircraft is as straight, level and as stable as possible (external factors like turbulence notwithstanding), then the crew can take the time to troubleshoot the issue. Most modern airliners will have flight data computers which are capable of determining which of the airspeed data sources is incorrect, otherwise it's a case of the crew comparing the airspeed instrument readings on the Captain's side, the FO's side and the standby and rejecting the one which is reading differently from the other two. In the event of ice crystal blockage, the problem will usually go away on its own in a short space of time.

The technical background to the issues leading up to AF447 was that the A330 was originally built and certified using pitot tubes (airspeed sensors) made by Goodrich, and in fact the standard fit was still using the Goodrich models at the time of AF447. I think it was in the late 1990s that Airbus gave customers the option of having pitot tubes made by Thales fitted instead, and AF was one of the customers who took up that option. While the Thales "AA" sensors met all the certification requirements, it became apparent on the line that they were more susceptible to ice crystal blockage than the Goodrich sensors. In fact it became apparent that in certain conditions it was possible for two, or even all three of the sensors to ice over, which was a very serious issue, as both automatic and manual UAS diagnostic procedures start with the assumption that only one reading is faulty. When this came to light, Thales revised the design and produced a new "AB" model sensor - and Airbus began a service programme intended to replace all AA models. I believe that the accident aircraft was due to have the fix performed around a month after the crash.

In any case, what's important to remember is that the UAS problem itself was not a major causal factor in the crash of AF447. In fact there were a little over 30 UAS incidents involving the A330 between the introduction of the Thales option and AF447 and all of them were resolved safely.

The above relates solely to a UAS condition once the aircraft is already in-flight. If you watch an flight deck video of an airliner taking off, you'll hear the non-handling pilot refer to the airspeed indicator coming alive, and making airspeed callouts as the aircraft accelerates. As I understand it, these callouts are cues for the handling pilot to glance down from the windscreen to check their own airspeed indicator and see if it matches. If it doesn't, then the takeoff should be rejected and the aircraft won't leave the ground.

That *should* have been the case with Birgenair Flight 301, a B757 chartered at short notice to replace an aircraft with technical problems - but for whatever reason, the Captain (as handling pilot) elected to continue the take-off and become airborne. Being a slightly earlier generation of aircraft, the B757's air data setup does not automatically compare the three inputs. The autopilot was configured to take air data readings from the Captain's side, and unfortunately it turned out to be the side suffering from faulty readings. By the time the crew realised that the autopilot was behaving strangely, the confusion and workload had increased to a point where they were at an unusually steep pitch attitude, and they made the incorrect call that neither of the airspeed indicators were working, and they eventually lost control and crashed.

The reason I bring that accident up is to illustrate what I said at the beginning - namely that UAS in and of itself isn't necessarily dangerous. In most cases that I know of, it is the response to the UAS - and/or not paying attention to the precautions intended to prevent going into the air with unreliable airspeed readings which are the real danger. However, it's worth bearing in mind that UAS incidents are pretty rare to start with, and improper responses even rarer.

[flyingchanges]

As I understand it, if autothrust is disengaged the FBW Airbus thrust levers work the "old-fashioned" way

If they do, how many pilots do you think would know where they need to be physically positioned to provide cruise thrust? Since they spend all of their normal operation in a fixed position, I find the aforementioned pretty unlikely.

I know exactly where the thrust levers need to be with the trim between 5 and 6 to provide a safe starting point if things go really really wrong.

[DozyWannabe]

@flyingchanges:
That's a different thing, really. The Airbus FBW architecture means that you can use the detents for pitch and power in the UAS scenario (see the image in my previous post). Autotrim is still active in all sub-modes of Alternate Law, so if you use the sidestick to set the pitch attitude, it'll stay there.

Regarding use of manual thrust in FBW Airbii, I dug up a thread from a couple of years ago which suggests that it happens more than you seem to believe:
http://www.pprune.org/tech-log/51486...al-thrust.html

I guess what I was trying to say is that there's nothing inherent to the Airbus FBW technology that prevents pilots from doing what you describe.

[vilas]

flyingchanges
If you are an Airbus pilot you would know that a very small movement of the thrust levers causes a large variation in N1. You don't need to know thrust lever angle but you should remember the N1 itself. If you set 2.5 degrees of pitch and 80%N1 till you get the QRH out you will be fine. The main problem is recognition of UAS condition. Because AP and ATHR may not disconnect but misbehave initially. Basic principle in airbus FBW is no instinctive aggressive large side stick inputs.

[Goldenrivett]

Hi msbbarratt,

Crazy idea? Unnecessary idea?

I agree.

If the main pitots have failed due icing problems, why would a sprung loaded vane not be covered in ice?

Tiger Moth Air Speed Indicator

[vilas]

Unless technology comes out with some radically different means of speed measurement. making another conventional instrument is not going solve the problem because it may suffer from same inadequecies.

[DozyWannabe]

Originally Posted by msbbarratt

To an outside observer this all seems like pilots sometimes have to infer important information (e.g. the pitots are iced up) indirectly and somewhat subjectively from other data.

Not really - the point is that as long as procedures (e.g. automatics off, stabilise flightpath) are followed, it's a fairly simple and straightforward process of elimination. More to the point, if it is an ice crystal blockage, then there's a good chance it will have cleared by the time the aircraft is stable. Apropos of nothing, I'm inclined to think that inferring important information from other data (e.g through the instruments) is part and parcel of what a pilot is expected to do as a matter of course!

Additionally, I'm pretty sure that as far as the A330 is concerned, the only incidents where two or more pitot sensors froze over involved the Thales AA probe - which was being taken out of service even before AF447 happened. After AF447, I believe the Thales AB model was also withdrawn from service and all aircraft retrofitted with the Goodrich probe.

[Pitot-mounted cameras] - Crazy idea? Unnecessary idea?

Not so much that as a case of the "using a sledgehammer to crack a walnut" analogy. Being a techie myself, I'm well aware of a natural inclination on our part to want to find technological solutions for a problem right out of the gate - as such, one of the things drilled into us by our Software Engineering teachers was to recognise that tendency and give the matter serious thought, because additional complexity means additional components capable of going wrong.

In this case, the rarity of even one of three pitot tubes icing over is a strong indication that adding complexity for its own sake would be the wrong thing to do. You also have to bear other things in mind - for example that the conditions in which icing occurs increase the likelihood of obscuring a camera's view, or that UAS can also be caused by a fault in the air data computer module.

[RAT 5]

Pilot training and airmanship application needs improving. I just saw The "Air Crash Investigation" of an ATR crash due to icing on the upper surface of the wings. This lead to much in flight testing of the capabilities of the de-icing boots and general operation techniques in icing conditions. What I found very disturbing was the crew behaviour to an unusual attitude of the a/c. I assume the tv presentation was accurate. It was reported that they were holding/cruising in icing conditions. The a/c was flying a little more nose up than normal. The pilots perceived this as an uncomfortable body angle for the cabin crews who were dong a trolley service. Rather than think/wonder and investigate why the a/c should not be behaving normally they elected to extend flaps to 'lower the nose' and make it more comfortable. The consequence of this was to increase the flow back of ice on the wings. The program suggested that was the end of the thinking/wondering why process. That blasé attitude is what I find disturbing; it led to a minor incident becoming a fatal crash. There are serious other such scenarios around the world. Time to wake up guys. It once again brings up the never ending circular discussion about whether 3-4 years experience and being an SOP guru is enough for todays captains. It also brings up the discussion about why it is that Turbo-Prop regional captains, who operate a lower powered a/c in the more challenging weather conditions and challenging airports, are mostly less experienced and much less remunerated than their glitzy jet peers. A captain/crew with 30/50/75/150/200 pax lives at stake should be treated more equally in qualification and rewards. Sorry for thread creep but I think we need more thoughtful pilots that often seems to be the case.

[parabellum]

Not sure if it is still there, but in the back of the Vol 3. (I think), of the Boeings I have flown, (B737,747-400,757 and 767) there was a table of settings for flight with unreliable airspeed, (one of the few pages rarely amended as it was a copy of a print out from all the certification test flying).
Most guys had this copied, laminated and stuck in the back of their clip board. I would be surprised if Airbus didn't have a similar table. Given that there are three ASIs, a failure of all three is very unlikely but not impossible.


During a training session in the SIM we would sometimes practice a failure of the handling pilots ASI at the top of descent right down to landing, with all the configuration changes involved etc. Two, working as a team, non handling pilot with the table in their hand, calling the settings and it was invariably a successful landing. It is wrong, in a big way, to suggest that a pilot should have memorised all the variables of N1/2, EPR, attitude for all configurations and speeds from cruise to landing.


Personally I think it should be practised more often, it builds confidence in the event it should actually happen. Bit of a shock if the first time you have to do it is for real!

[FE Hoppy]

inertial + Cl should be enough to derive a reasonable airspeed. I'm pretty sure this is already in use.

[john_tullamarine]

As some of the greybeards have suggested, a ballpark knowledge of the gauge numbers is enough to get the crew out of trouble for the majority of failures.

Main problem I see is that, unless the pilot either handflies enough (or, at least, follows what the automatics are doing routinely in an intelligent monitoring way) s/he is not going to have that facility when the gremlins set the scene for excitement as they do from time to time.

It's not difficult .. only requires some repetitious directed exposure/practice.

Similarly to Parabellum's sim example, 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.

Not looking for any great finesse .. just a workmanlike, successful result. Main aim of the exercise was a confidence builder but it reinforced to the trainee that one actually can recover the aircraft with a lot of stuff not working .. just takes a bit of thinking and keeping calm while one puts the acquired basic flying skillset into practice.

In the real world, naturally, one doesn't do that sort of thing, other than in extremis, and the two crew recovery with the aid of the QRH is the rule - usually for a simple failure situation. One uses everything available to assist resolution of the problem.

However, it is useful to the pilot to know that it can be done if the wx conditions aren't too severe.

I really can't see why a simple failure such as loss of the ASI should be cause for major panic for a well-trained, intelligent pilot. Even if the instrument is giving obviously duff gen, it is a simple matter to cover it so that the output doesn't confuse/complicate the pilot's cognition process.

[RAT 5]

"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."

Oh that it was still possible in this day of self-funded type ratings, rigid SOP's and tick the boxes necessary to reach an acceptable standard to pass LST. Bring back real training to a high standard with real depth & breadth, rather than coaching to reach an acceptable level to squirm through an LST. Even the weeks of Line Training do not fill in the empty holes in the fundamental education of how to fly the a/c..