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737 airspeed unreliable QRH

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737 airspeed unreliable QRH

Old 18th Mar 2015, 22:20
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737 airspeed unreliable QRH

A few days back I received a new procedure in my 737 QRH, for airspeed unreliable. We now have a set of thrust settings in combination with a specific pitch attitude, as memory items.
Config flaps up: 4 degrees pitch up, and 75% N1
Config flaps extended: 10 degrees pitch up and 80% N1

This afternoon we were happily flying along at FL400 at M.79, pitch was 2.5 degrees and N1 was 89%.
My point is this: if our airspeed was going haywire at that point, the new procedure would require us to increase pitch to 4 degrees, while retarding thrust to 75%!!! At FL 400????
No way.

Where did this procedure come from? My airmanship is going to prevent me from following these new memory items. I refuse.
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Old 18th Mar 2015, 23:23
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Boeing has introduced this for most if not all of their types. I'll simply let Boeing answer this for you:

These pitch and power settings provide a simple reference setting for the crew to use for a short period of time while the initial steps of the checklist are accomplished. These settings do not ensure a level flight or constant airspeed at any particular altitude/airspeed/weight combination. However, they do ensure that at any and all weight/altitude combinations, the aircraft will accelerate from low speeds, and slow from a high speeds, as we cannot assume the aircraft is in stable flight when the NNC is run. It is more likely that the crew or the autopilot has destabilized the airplane as a result of erroneous airspeed indications prior to identification of the need to run the NNC. These memory item pitch and power settings often result in a significant climb (from lower altitudes) and can also result in a gradual descent (from higher altitudes).

Remember unreliable airspeed is rarely obvious as in your example. Things don't go haywire it can often be subtle to begin with. Try blocked Pitots on takeoff in the sim and see how many crews lose control of the aeroplane before they have a clue whats happened. Boeing assumes you may lose control before any confirmed diagnosis hence this procedure to keep you alive along enough to try and work it out.
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Old 18th Mar 2015, 23:28
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8, can I trouble you for a link to that piece please?
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Old 18th Mar 2015, 23:33
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Foxniner,

Listen to your aircraft manufacturer they know better than all of us and have genuinely produced a procedure to follow which covers all situations for all types, truely incredible.

What will happen at FL400 is that you will decelerate and begin a descent. During which you then complete the rest of the QRH and eventually get to setting pitch and thrust from the PI section.

Lord spandex, if you have a Boeing FCTM it should have been updated at the same time and is in there.
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Old 19th Mar 2015, 07:46
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If you do this manoeuvre during SID, e.g. 6000' level flight on a busy TMA, you are heading for trouble. ATC want you in level flight. You set these parameters and climb and accelerate. While attempting to follow the check list you get an RA. ATC is shouting at you to maintain 6000' due traffic. By sensible airmanship you have noticed your Power/Attitude/Speed does not achieve the required performance and you do an immediate cross check of all 3 ASI's. You decide quickly who has the functioning side and they become PF. The PF calls for QRH. What is wrong with that process? If the PF now sets the QRH memory parameters you disturb a controlled situation and make t worse. Is that good airmanship?
I'm curious why you would want to do something you know will not work for you. Indeed, IF you are confused, then perhaps follow the new QRH procedure. It should say:
CHOOSE ONE: Confused ......... 4 degrees 75% .....SET.

Not confused........Fly the a/c as required.
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Old 19th Mar 2015, 07:56
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Thing is if you follow the procedure you have TIME. AC will find the speed that these settings will deliver and then will stay in stable flight while you troubleshoot. Yes you may descend or climb if you already are slow and if you are near traffic declare a pan and do what you like while you sort it.
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Old 19th Mar 2015, 08:41
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I believe that AF 447 is behind this change. It is a catchall but would have saved that aircraft.
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Old 19th Mar 2015, 08:45
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Cross checking your ASI has been found to be a poor way of establishing a reliable source. What if you have two that agree but are incorrect or a strange failure like an ADIRU fault etc. what are you comparing them to and how do you determine correct indications?

All these scenarios are covered in the QRH.

Declare PAN/Mayday and fly the aircraft.
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Old 19th Mar 2015, 09:39
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Originally Posted by RAT 5
If you do this manoeuvre during SID, e.g. 6000' level flight on a busy TMA, you are heading for trouble. ATC want you in level flight. You set these parameters and climb and accelerate. While attempting to follow the check list you get an RA. ATC is shouting at you to maintain 6000' due traffic.
RAT5, remember the reason of the unreliable airspeed could also have an effect on incorrect altimeter readings, including those providing input to transponder for altitude alerting. Thus you may think you are in level flight, or indeed ATC might, when in fact you are not. TCAS is only as good as its inputs, which in this case can be wrong. The list of items which may be unreliable are included at the bottom of the 737NG checklist by Boeing, as is the list which is relievable.
The following are reliable:
•Attitude
•N1
•Ground speed
•Radio altitude
One or more of the following may be evidence of unreliable airspeed or Mach indications:
•Speed/altitude information not consistent with pitch attitude and thrust setting
•SPD failure flag
•SPD LIM failure flag
•IAS DISAGREE alert
•Blank or fluctuating airspeed display
•Variation between captain and first officer airspeed displays
•Radome damage or loss
•Overspeed warning
•Simultaneous overspeed and stall warnings.
Until you identify failure (possibly after running the checklist) you cannot guarantee that any RA is valid either as the selected altitude information (item 1 on above list) may not be accurate.
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Old 19th Mar 2015, 10:21
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RAT 5


Listen to Skyjob....Listen to Boeing and then read about AEROPERU 603. (there are other examples too)

Last edited by 8che; 19th Mar 2015 at 10:40.
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Old 19th Mar 2015, 10:51
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The 4°/75% is ballpark figure which keeps the aircraft flying, when you have either no airspeed indication or no idea which airspeed indication is correct.

It is not meant as a "I can still fly RVSM to destination" figure. Its sole purpose is to keep enough airflow over the wings so the aircraft continues to fly (but not so fast as to have it fall apart), while you bring out the PI figures, as suggested.
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Old 19th Mar 2015, 11:27
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Why is there so much opposition to a manufacturers procedure??
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Old 19th Mar 2015, 12:40
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Why is there so much opposition to a manufacturers procedure??

I'm not sure that is quite correct. It's a very sweeping statement. It is a considerable change from the 'manufacturer's previous procedure'. This is not the first time the manufacturer has made significant changes. If it was OK before what has caused such a large change? I've asked Boeing pilots about other changes they've made previously to their QRH procedures and received very vague wishy-washy answers.
It's not an opposition it's a cry for information and understanding. Nowt wrong with that; it's what pilots do.
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Old 19th Mar 2015, 13:01
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The OP has stated he is going to ignore it as have many pilots I have talked to. It's frustrating when so much has been done to come up with the changes for the better
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Old 19th Mar 2015, 13:22
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Try blocked Pitots on takeoff in the sim and see how many crews lose control of the aeroplane before they have a clue whats happened.
I don't know about blocked pitot heads because that should normally be picked up during the take off roll during the 80 knots (Boeing) or 100 knots(Airbus) IAS check, thus permitting an early abort if required.

The really nasty gotcha (in the 737 Classic anyway) is a blocked captain's static vent. During the take off roll until airborne, flight instrument indications are normal. After lift off the captain's altimeter does not move; nor does his VSI. This is where the captain has to be careful when the first officer calls "Positive rate of climb" on his side that the captain doesn't automatically reply "Gear Up". Because he does not have the usual indications of positive rate of climb on his side since his static vent is blocked.

Soon after in the initial climb (about 30 seconds after airborne) the captains ASI needle starts to slowly go backwards and indicate a speed loss due to its blocked static vent. As the airspeed needle continues to fall back, it triggers a spurious wind-shear warning and a few seconds later, off goes the stick shaker as well. The stick shaker of course is giving a false warning and the first tendency as a startle factor (seen in the simulator, anyway) is to lower the nose.

The fix is to disregard the spurious warnings and maintain the correct climb attitude for the configuration, check the correct N1. A quick check of the ground speed read-out will give the pilot a confirmation which ASI is untrustworthy. That is one reason why a glance at the expected ground speed read-out at the 80 knot or 100 knot call is a useful back-up against an unexpected erroneous airspeed event.

The blocked static vent can cause real confusion initially; which is why the pilot needs to fly expected body angle and N1 for the state of flight until trouble shooting sorts things out. During the sorting out process in the climb, be careful not to overspeed the flaps as it is all too easy to concentrate on various aural and visual warnings and ASI readings and forget that the aircraft is still accelerating fast.

Last edited by Judd; 19th Mar 2015 at 13:37.
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Old 19th Mar 2015, 14:13
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Originally Posted by Judd
The blocked static vent can cause real confusion initially; which is why the pilot needs to fly expected body angle and N1 for the state of flight until trouble shooting sorts things out. During the sorting out process in the climb, be careful not to overspeed the flaps as it is all too easy to concentrate on various aural and visual warnings and ASI readings and forget that the aircraft is still accelerating fast.
And if transponder is also set at 1 then incorrect TCAS information is available to crew and ATC until rectified...

Agreed with Boeing: FLY the aircraft, set the pitch and power and troubleshoot.
Flaps out, usually thus low level, give a climb rate.
Flaps up, usually higher flight levels/altitude, give a descent rate.
Both above scenarios are safe, sure there are exceptions to the norm, but it is safer then any other option.
Do not disregards the Boeing advice, follow it instead.
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Old 19th Mar 2015, 14:21
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Lets see...Boeing has tested, demonstrated, and validated these procedures in the E cab, Level D simulator and the actual aircraft. Go ahead and use your own procedures in lieu. Let us know how they work out for?
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Old 19th Mar 2015, 19:14
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If this procedure had been around (and flown) by Birgenair 301 and Air France 447, then they and their passengers might be still around to talk to. Air Asia 8501 is looking similar.

When I did the training, it was explained much like the quote from Boeing near the beginning of the thread. It’s a two-sizes-fits-all get-you-out-of-the-doo-doo procedure for when the aircraft is misbehaving and you have picked it up late or are uncertain as to exactly what’s happening.

Having an insidious pitot/static issue in IMC and turbulence is very different to a sudden loss of airspeed indication when you’re fully stable on short finals. However, if you make a mess (however small) of the rest of the approach and landing with your own version of “airspeed unreliable”, the first question will be: “Why didn’t you follow the QRH drill and sort it out later?”

I know this seems like “dumbing down” and I myself make sure I know enough performance attitudes to cover most phases of flight without having to look them up but from a point-of-view of overall safety, this procedure will as near as possible 100% keep you flying and under control. As professionals we are paid to follow (within reason) the safest path, not the most personally satisfying or neat one...
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Old 19th Mar 2015, 20:07
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It is not only the 737. On the 747 we now have two different pitch and power combinations for unreliable airspeed. One for flaps up and one for flaps extended.

For flaps up, we set 80% N1 and 4 degrees nose up. Yesterday in cruise at a heavy weight a bit above optimum altitude was typical at 2.5-3 degrees nose up and 92% power.

It seems foolish to me to increase pitch and reduce thrust if we notice that our airspeed is unreliable. In fact it might be downright dangerous.

Last edited by JammedStab; 19th Mar 2015 at 20:24.
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Old 19th Mar 2015, 21:00
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YES, I'M IN FINE TRIM THANK YOU!

So far none of the aircraft manufacturer(s), Mr A nor Mr B nor Mr E, appear to mention checking that the Stab Trim is somewhere near where it should be for the configurations.


Or is the profession of the opinion that it's not really important, or maybe such a check will overload the highly trained and competent Bonobo Chimps in the average airliner flight deck?


It certainly seemed to play a large part in some of the recent LOC/stalling accidents and incidents, yet has been omitted from the latest revised procedures.


Braced for incoming..........
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