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B737 Airspeed Unreliable?

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B737 Airspeed Unreliable?

Old 8th Oct 2019, 03:34
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B737 Airspeed Unreliable?

At risk of the usual cadre of crackpot loons answering

10/80% flaps out or 4/75% flaps up targets in the event of Airspeed Unreliable?

Boeing are peddling this as a go-to solution to Airspeed Unreliable, it looks reasonable in a takeoff scenario at moderate weights however highly dubious in high altitude scenario with ice related Airspeed issues etc

what do you think?
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Old 8th Oct 2019, 05:53
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It’s not designed for level flight, it’s designed to keep the aircraft flying. It will put you in a descent, and into a safe place to get yourself into the QRH and find the QRH pitch and power setting for your phase of flight.

A more interesting one to think about is a take off from a performance limited field, high flap setting, at MTOW with airspeed disagree at V1, I don’t know many who’ll rush to set 10/80 until well clear of the ground.
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Old 8th Oct 2019, 06:46
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Originally Posted by gravityf1ghter View Post
It’s not designed for level flight, it’s designed to keep the aircraft flying. It will put you in a descent, and into a safe place to get yourself into the QRH and find the QRH pitch and power setting for your phase of flight.

A more interesting one to think about is a take off from a performance limited field, high flap setting, at MTOW with airspeed disagree at V1, I don’t know many who’ll rush to set 10/80 until well clear of the ground.
who would change thrust after V1? The A/T is in thrust hold
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Old 8th Oct 2019, 13:33
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FFS if only people bothered to open the forbidden manual called FCTM.

Memory items for target pitch and thrust must be accomplished as soon as it is
suspected that airspeed indications are incorrect. The intent of having memorized
pitch and thrust settings is to quickly put the airplane in a safe regime until the
Airspeed Unreliable checklist can be referenced. The following assumptions and
requirements were used in developing these memory items:
• The memorized settings are calculated to work for all model/engine
combinations, at all weights and at all altitudes.
• The flaps up settings will be sufficient such that the actual airspeed
remains above stick shaker and below overspeed.
• The flaps extended settings will be sufficient such that the actual
airspeed remains above stick shaker and below the flap placard limit.
• The settings are biased toward a higher airspeed as it is better to be at a
high energy state than a low energy state.
• These memorized settings are to allow time to stabilize the airplane,
remain within the flight envelope without overspeed or stall, and then
continue with reference to the checklist.
• Settings are provided for flight with and without flaps extended. The
crew should use the setting for the condition they are in to keep the
airplane safe while accessing the checklist.

The memorized pitch and thrust setting for the current configuration (flaps
extended/flaps up) should be applied immediately with the following
considerations:
• The flaps extended pitch and thrust settings will result in a climb.
• The flaps up pitch and thrust settings will result in a slight climb at light
weights and low altitudes, and a slight descent at heavy weights and
high altitudes.
• At light weight and low altitude, the true airspeed will be higher than
normal, but within the flight envelope. At heavy weight and high
altitude, the same settings will result in airspeed lower than normal
cruise but within the flight envelope.
• The goal of these pitch and thrust settings is to maintain the airplane
safely within the flight envelope, not to maintain a specific climb or
level flight.
• The current flap position should be maintained until the memory pitch
and thrust settings have been set and the airplane stabilized. If further
flap extension/flap retraction is required refer to PI-QRH Airspeed
Unreliable table.
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Old 8th Oct 2019, 13:47
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High handed follower?

The issue is principally higher altitude and higher weights, in turbulence

Do the Boeing Go-To figures propose stand-up to scrutiny? For example 4/75% on the MAX is considerably more thrust

Who is Lazy Pilots or Boeing?
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Old 8th Oct 2019, 15:58
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Originally Posted by FlyingStone View Post
FFS if only people bothered to open the forbidden manual called FCTM.
Great reference, but not all airlines use the generic Boeing FCTM. My airline has a tailored manual that does not provide these bullet notes on the Airspeed Unreliable NNC. I’m curious if either Ethiopian or Lion Air used this manual because one of the ongoing debates in the 737 MAX thread on the R&N side is whether the crews should have retracted the flaps given that they were initially dealing with an unreliable airspeed issue.

As far as the takeoff scenario, throttling back to 80% is no worse than losing an engine. That being said, in this situation one must be mindful of obstacle clearance, so the pilots should consider following any engine-out routing that might apply for the particular airport.
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Old 8th Oct 2019, 18:47
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Originally Posted by sekmeth View Post

who would change thrust after V1? The A/T is in thrust hold
Memory item Number 2- Autothrottle- Disengage. Followed by number 4, set pitch and power.
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Old 8th Oct 2019, 19:45
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Right after take off? Isn't it better to set 15° up and TO thrust until above MSA.
I am asking as a relatively inexperienced FO, with airspeed unreliable being an item in my next simulator check in a couple of weeks.
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Old 8th Oct 2019, 20:37
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Originally Posted by Banana Joe View Post
Right after take off? Isn't it better to set 15° up and TO thrust until above MSA.
I am asking as a relatively inexperienced FO, with airspeed unreliable being an item in my next simulator check in a couple of weeks.
I tend to agree with you, but Boeing’s guidance seems to suggest sooner. There is no good reason to do anything with the thrust setting until at least 400 AFE. See what your instructor has to say.
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Old 9th Oct 2019, 05:51
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Originally Posted by Banana Joe View Post
Right after take off? Isn't it better to set 15° up and TO thrust until above MSA.
I am asking as a relatively inexperienced FO, with airspeed unreliable being an item in my next simulator check in a couple of weeks.
I’m inclined to agree with you as well, however, that is not what Boeing says. No memory items below 400ft, so pitch for 15 and leave the thrust, but then at 400ft, if you are already aware of the unreliable airspeed, accomplish memory items, including setting pitch and power.

10 degrees and 80% isn’t ideal in this situation, but it is safe, which is what Boeing have designed it to be.

Back to to my original post, the time to consider not doing this, is a hot and heavy take off from a short field with terrain around, maybe a high flap setting, as I said, I don’t know many about who will rush to set 80% in that situation.
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Old 9th Oct 2019, 08:46
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Originally Posted by Tomaski View Post
Great reference, but not all airlines use the generic Boeing FCTM. My airline has a tailored manual that does not provide these bullet notes on the Airspeed Unreliable NNC.
What's the objective in doing that? I understand the need for tailored manuals, such as company procedures or profiles. But omitting very relevant safety information (especially after the MAX accidents)...?

For example 4/75% on the MAX is considerably more thrust
I haven't flown the MAX, but why would 75% N1 on the MAX be considerably more thrust than on the NG?
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Old 9th Oct 2019, 09:50
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Originally Posted by gravityf1ghter View Post


10 degrees and 80% isn’t ideal in this situation, but it is safe, which is what Boeing have designed it to be.
I am not sure about this statement if departing from a Swiss airport or Ljubljana. These parameters keep the aircraft flying in a safely manner, but they don't cater for obstacle clearance in all cases.
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Old 9th Oct 2019, 10:20
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Originally Posted by Banana Joe View Post
Right after take off? Isn't it better to set 15° up and TO thrust until above MSA.
I am asking as a relatively inexperienced FO, with airspeed unreliable being an item in my next simulator check in a couple of weeks.

Bear in mind the information given in the FCTM, quoted in #4 above: The memorized settings are calculated to work for all model/engine combinations, at all weights and at all altitudes.

And: The goal of these pitch and thrust settings is to maintain the airplane
safely within the flight envelope, not to maintain a specific climb or
level flight.


MSA could be 7,000 feet or more, and your company will have emergency turn procedures at airports with MSA issues.

Pilots should not invent their own procedures - (not suggesting you were).

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Old 9th Oct 2019, 14:04
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Originally Posted by Banana Joe View Post
Right after take off? Isn't it better to set 15° up and TO thrust until above MSA.
I am asking as a relatively inexperienced FO, with airspeed unreliable being an item in my next simulator check in a couple of weeks.
15° up and TO thrust are what you are going to fly (approximately) until you reduce to climb thrust at 1,500ft. That is a relatively busy period, especially if something like an ASI has gone wrong and the associated 'startle factor'. Best stick to that rather than fiddle with thrust and attitude changes close to the ground and you might find during that time that you have two ASIs that agree. If not, I would say that above 1,500ft you go for you flaps thrust/attitude settings. Don't try to do too much close to the ground, just fly the aeroplane.
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Old 9th Oct 2019, 14:15
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Originally Posted by Trossie View Post
15° up and TO thrust are what you are going to fly (approximately) until you reduce to climb thrust at 1,500ft. That is a relatively busy period, especially if something like an ASI has gone wrong and the associated 'startle factor'. Best stick to that rather than fiddle with thrust and attitude changes close to the ground and you might find during that time that you have two ASIs that agree. If not, I would say that above 1,500ft you go for you flaps thrust/attitude settings. Don't try to do too much close to the ground, just fly the aeroplane.
See the above post from the 737 FCTM, that is not what Boeing says to do. As soon as you are aware of the problem (with the caveat of no memory items below 400ft) accomplish the memory items.

1500ft? MSA? Is that from your potentially failed pitot/static?

Stick to the procedures.
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Old 9th Oct 2019, 16:56
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It would be nice if Boeing would provide a suggestion in the FCTM of the recommended course of action in the event of Airspeed Unreliable during or after takeoff, but they don’t.

This leaves thinking pilots trying to work it out for themselves, hence this thread (and it isn’t the first, this has been discussed on this forum previously).

I’ve noticed that it creates two schools of thought: those who would maintain TOGA thrust and TOGA attitude until a safe altitude, and those who would commence the memory items at 400 feet.

To my thinking, both options are “safe”. As to what option your airline or your simulator instructor is looking for is probably best discussed before you do it.

My airline exposed me to it in a simulator a couple of years ago at 32C, 79.0T (ie MTOW) Flaps 5 at sea level. He made us observe the rate of climb once we got to 10/80% - it was in excess of 1000fpm. Way in excess of an engine out scenario. That was good to experience, because now I know. I didn’t know prior to that and may have had trouble believing it.

Reducing thrust to 80% and lowering the nose 5 degrees after taking off at MTOW in a hot climate is not something that would be considered naturally safe unless you had seen it before.

One objection to delaying the memory items is that it delays turning F/Ds off. We are accustomed to follow the F/D pitch bar on takeoff, and with Airspeed Unreliable, the F/D might take us somewhere we shouldn’t be.

Last edited by Derfred; 9th Oct 2019 at 17:19.
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Old 10th Oct 2019, 00:29
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Originally Posted by gravityf1ghter View Post
A more interesting one to think about is a take off from a performance limited field, high flap setting, at MTOW with airspeed disagree at V1, I don’t know many who’ll rush to set 10/80 until well clear of the ground.
Why in the world would you continue a takeoff if you had an airspeed disagree before V1?
There have been very bad outcomes when pilots have done just that...

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Old 10th Oct 2019, 02:16
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Originally Posted by tdracer View Post
Why in the world would you continue a takeoff if you had an airspeed disagree before V1?
There have been very bad outcomes when pilots have done just that...
It depends on how soon before V1.

At my airline, we procedurally do an airspeed crosscheck at 80 KIAS, and if they agree we continue. From that point we are in the "high speed" regime and - procedurally - we only reject the takeoff for a narrow set of circumstances, and airspeed disagreement is not one of them. That being said, if I'm lightweight and rolling down a 12,000 foot or more runway, I still might make the decision to reject past the 80 knot point. However, I fully expect to have that decision questioned by both my Chief Pilot and the FAA.
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Old 10th Oct 2019, 05:52
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Originally Posted by tdracer View Post
Why in the world would you continue a takeoff if you had an airspeed disagree before V1?
There have been very bad outcomes when pilots have done just that...
The reason to reject above 80kts, and before V1, are very narrow. The only one that unreliable airspeed would potentially fall under is ‘Unsafe or unable to fly’- Is the aircraft actually unsafe to fly? Now your next decision, what is actually your V1? You don’t have a reliable airspeed indication, so V1 is not accurate.

At this point, do not be lulled into trying to diagnose the reliable airspeed, as per the Boeing checklist and FCTM, you are not in a position to be doing this until you are sat stable and in trim, at a pre determined pitch and power for the phase of flight from the QRH.
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Old 11th Oct 2019, 10:17
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As Boeing say there are 4 reliable indications:
- attitude
- N1
- groundspeed
- radio altimeter.

If I have a suspected Airspeed Unreliable why would you do anything except maintain standard takeoff pitch - 15 degrees - and maybe tweak power back to MCT, sit fat dumb and happy until MSA THEN start screwing around with things?

You can do anything you want - as long as you can justify it.
To me achieving MSA then doing the Airspeed Unreliable checklist increases the safety of the aircraft, that's what I'd explain to those lined up against me at the enquiry - just remember the warm and fuzzy feelings Sully and the F/O received! - for them to chew on.

Ultimately the passengers are safe, the aircraft's secure on the ground and I've written/am writing the report(s)....what's your f**king problem??

Cheers.
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