B737NG Airspeed unreliable after TO

[Bobermo]

Hi there,

Following our recent recurrent simulator sessions there is some discussing among the trainers how to interpreted the recent change in the FCTM regarding unreliable airspeed after takeoff.
FCTM B737NG page 8.32 states:
Takeoff If an unreliable airspeed event occurs during takeoff at or after V1, maintain takeoff thrust, set 15° pitch attitude, retract the landing gear, and maintain flap configuration. Climb at 15° pitch attitude until reaching acceleration height. At acceleration height, do the Airspeed Unreliable NNC memory items and checklist.

In my company the acceleration height, when flying a N-1 or NADP 2 departure, is 800ft AGL. However Boeing stated the following on page 3.32:
The minimum altitude for flap retraction is 400 feet. The altitude selected for acceleration and flap retraction may be specified for each airport. Safety, obstruction clearance, airplane performance or noise abatement requirements are usually the determining factors. Some operators have adopted a standard climb profile for all of their operations based on the airport which requires the greatest height for level off to clear a close-in obstacle with an engine failure. During training flights, 1,000 feet AFE is normally used as the acceleration height to initiate thrust reduction and flap retraction. For noise abatement considerations during line operations, thrust reduction typically occurs at approximately 1,500 feet AFE and acceleration typically occurs between 1,500 and 3,000 feet AFE, or as specified by individual airport noise abatement procedures.

So the question is, what is the acceleration height Boeing is referring to on page 8.32?
Curious about your thoughts!

[FlyingStone]

The engine out acceleration height.

Applying Airspeed Unreliable memory items could severely reduce your climb performance, and it’s good to at least clear the 2nd segment beforehand.

[Bobermo]

The engine out acceleration height.

Applying Airspeed Unreliable memory items could severely reduce your climb performance, and it’s good to at least clear the 2nd segment beforehand.

I agree, but sometimes that engine out acceleration height could be more than 2000ft AGL (if in mountainous area). 15 degree of pitch seems quite aggressive in that case when doing a balanced field takeoff with ATM and derate.. Going to try this out next week in the sim to see what happens.

[FlyingStone]

Well, aggressive or not, you want to be clear of any terrain issues before reducing thrust.

While you’re in the sim, see how well the aircraft climbs at MTOW, 10deg pitch, 80% N1 and takeoff flaps (bonus points for F25), and decide whether you think it’s worth being slightly more “aggressive”.

[Bobermo]

Well, aggressive or not, you want to be clear of any terrain issues before reducing thrust.

While you’re in the sim, see how well the aircraft climbs at MTOW, 10deg pitch, 80% N1 and takeoff flaps (bonus points for F25), and decide whether you think it’s worth being slightly more “aggressive”.

Yes, definitely agree with the objective of clearing terrain first.

Thanks for your input! I was the one in our group reasoning the same way. Other performed the memory items at 400ft (the lowest altitude we normally start with memory items).

[ACMS]

What’s to stop you climbing above MSA before you do the memory items? Nothing except the 5 mins at TO thrust limit…
Just don’t hit a hill fluffing around, there’s no rush.

[Flying Wild]

What’s to stop you climbing above MSA before you do the memory items? Nothing except the 5 mins at TO thrust limit…
Just don’t hit a hill fluffing around, there’s no rush.

Or 10 mins. Depends on how much your company has paid to CFM for the paperwork change.

[tdracer]

Or 10 mins. Depends on how much your company has paid to CFM for the paperwork change.
10 minutes is (normally) only approved for engine out conditions. Five minutes has no restrictions.

[john_tullamarine]

Nothing except the 5 mins at TO thrust limit…
Or 10 mins.
10 minutes is (normally) only approved for engine out conditions. Five minutes has no restrictions.

In an emergency (and I think a flight instruments problem qualifies, albeit with a reasonably routine fix), I don't think that anyone is going to be too critical if one holds onto a high thrust setting for a while longer than usual ?

[tdracer]

In an emergency (and I think a flight instruments problem qualifies, albeit with a reasonably routine fix), I don't think that anyone is going to be too critical if one holds onto a high thrust setting for a while longer than usual ?

Good point - the primary advantage of 10 minute takeoff is for planning - i.e., higher max TO weight if you need to account for a more distant terrain clearance.

If it's an emergency, use whatever thrust you have available (one of the early criticisms of FADEC was that it would normally prevent overboost in an emergency such as windshear when that extra thrust might make the difference). Of course, that rather assumes you have reasonably healthy engines. There was a Kalitta 747F crash back in the early 2000's where they lost an engine right at rotation - the pilot then pushed the throttles and overboosted the other three, one of which also failed due to the overboost (well used JT9Ds).

[john_tullamarine]

higher max TO weight if you need to account for a more distant terrain clearance.

That extra few minutes at T/O thrust can mean a significant increase in RTOW, usually because we can drive the second segment up quite a bit higher before having to roll over into the third (acceleration) segment. But you have to pony up the dollars to the OEM to get the Supplement in the AFM.

one of which also failed due to the overboost (well used JT9Ds).

Be aware that it can work two ways. I did an accident investigation into a twin fatal in Haiti, years ago. The aircraft engines were spec'd right up to the hilt and ran real well. We figured that what the crew thought was a failure, wasn't (but that's quite another story) and, when they pushed the throttles up, the engine producing thrust produced quite a bit more than expected, leading to a Vmc departure. While quite a few on the aircraft were killed, a very much higher death toll was avoided by virtue of where the aircraft came to rest (again, another story).

[BraceBrace]

The engine out acceleration height.

Applying Airspeed Unreliable memory items could severely reduce your climb performance, and it’s good to at least clear the 2nd segment beforehand.

Why mix up two different scenario's?

The engine out acceleration height is related to a worst case engine failure at V1. In those cases the acceleration height is related to an engine out procedure. SIDs and engine out tracks are frequently not the same. An SID comes with a "normal" acceleration height.

The acceleration height is not a fixed value. If you're capable of flying the normal SID, the normal acceleration height should work fine. If you doubt your climb performance there is nothing wrong with adapting the acceleration height to the MSA. If you have an engine failure together with airspeed unreliable and you're able to fly the engine out track, the acceleration height is the engine out acceleration height.

But this is interesting, because the memory items for unreliable airspeed are based on fully functional engines. Airspeed unreliable memory items with an engine failure will make you crash. You need 12° and TOGA. Nothing more, nothing less. 15° will make you lose airspeed and crash in many cases, 10° will make you hover. So don't mix engine out procedures with basic unreliable airspeed situations.

[FullWings]

Interesting discussion. I think it’s one of those situations (UAS + terrain) where it’s wise to consider what the greater threat is: flying into the ground because you applied the UAS drill or LOC into the ground because you didn’t.

I would be very surprised if 10degs and 80% (not on the 737 now but that seems about right?) gave you a shallower climb gradient than an engine failure at V1 on a full derate, so the getting-away-from-the-ground bit should be pretty much assured. The danger is that if you delay the UAS drill for too long, you may be in a position where it’s not going to work as effectively (or not at all) - I don’t think Boeing guarantee a successful outcome right out at the corners of the flight envelope, which is why prompt diagnosis and action is essential.

I have no insider knowledge but maybe there was concern that applying the drill at or during rotation wasn’t optimal and might delay gear retraction, thereby rendering the default pitch and power for UAS unsuitable. Better to get it away from the ground then go for it once the gear is away and a climb established? That would normally be not far away from AA, so standardise on that?

[FlyingStone]

Why mix up two different scenario's?

The engine out acceleration height is related to a worst case engine failure at V1. In those cases the acceleration height is related to an engine out procedure. SIDs and engine out tracks are frequently not the same. An SID comes with a "normal" acceleration height.

It’s not a mix of scenarios, but I was trying to emphasize that aircraft’s climb performance might be significantly degraded once memory items are applied (particularly when heavy and with higher flap settings). If you’ve made it to EO acceleration height, then the next segment enables a level flight (or 1.2% climb gradient without acceleration), so at least you’ve got some obstacle clearance in hand before you start reducing thrust and pitch.

Nothing wrong with flying to MSA first, though, but in some places of the world that would take quite a while (MSA in high teens or even higher), even on two engines. Worth mentioning is that EOSIDs can also be flown any time you’ve got a performance issue, they aren’t exclusively there for engine out situations - e.g. with gear that won’t retract when flying an SID with a very high climb gradient required.

[john_tullamarine]

aircraft’s climb performance might be significantly degraded once memory items are applied

Which is why us oldies memorised the relevant pitch and thrust settings for this and that ... also helps a LOT if you do some manual raw data flying for practice.

they aren’t exclusively there for engine out situations

But be wary that you need to match the OEI speed program to match the turn radius in tiger country. The escape path is not much use if you aren't on it ...

[megan]

I did an accident investigation into a twin fatal in HaitiCare to give info JT of aircraft ident?

[john_tullamarine]

Flugunfall 12 FEB 1996 einer GAF Nomad N.24A N224E - Port-au-Prince-Mais Gate Airport (PAP) (aviation-safety.net) (https://aviation-safety.net/database/record.php?id=19960212-0&lang=de)

[megan]

Many thanks John, the NTSB list the accident but there is no report attached.

[john_tullamarine]

An interesting story, good sir. However, I was working with the OEM at the time so it would not be appropriate to comment publicly. Perhaps when next we catch up for an ale or two ?

The NTSB was scheduled to come along but, at the last moment pleaded workload, so we two OEM chaps (me and my offsider FSR) were it. Where the report was promulgated past the OEM I have no idea ... I completed it as a contract task after I had left the organisation.

[BraceBrace]

I would be very surprised if 10degs and 80% (not on the 737 now but that seems about right?) gave you a shallower climb gradient than an engine failure at V1 on a full derate

The 10° pitch up will not make you climb, you need 12°. Try it in the simulator, even after pos-rate/gear up. Set 10° and you climb.

[FullWings]

The 10° pitch up will not make you climb, you need 12°. Try it in the simulator, even after pos-rate/gear up. Set 10° and you climb.
I don’t have access to a 737 sim any more but I suspect that if you left it at 10deg for a bit it would as the airspeed increased. My understanding is that the recall pitch/power settings are designed to put you somewhere in the middle of the speed range, well away from stalling. At high altitudes you’ll get a descent and low altitudes a climb, once you’re in a steady state. Maybe that’s why they changed the drill comparatively recently to use AA, in that it leaves room for this to happen? I’m on the 777 and this technique only made it into the FCTM in the last year or two.