Posts about: "V1" [Posts: 122 Pages: 7]

From tdracer
However, TCMA is only active on the ground (unfamiliar with the 787/GEnx TCMA air/ground logic - on the 747-8 we used 5 sources of air/ground - three Radio Altimeters and two Weight on Wheels - at least one of each had to indicate ground to enable TCMA). TCMA will shutdown the engine via the N2 overspeed protection - nearly instantaneous. For this to be TCMA, it would require at least two major failures - improper air ground indication or logic, and improper TCMA activation logic (completely separate software paths in the FADEC). Like I said, very, very unlikely.

Philosophically, it is primarily pre-V1. It prevents uncommanded high thrust when you are trying to chop the thrust on a RTO. Overthrust can generally get you off the ground post V1, as can (stating the obvious on a pilot forum) a single-engine loss post V1. So OK to shut down post V1 if still on ground.

might need a rethink ... And a reread.

The engine driven fuel pump is literally driven off the engine gearbox (driven by a mechanical connection to the N2 shaft) - if the engine's running, the gearbox is turning (baring a major mechanical fault). The engine driven fuel pump is a two-stage pump - a centrifugal pump that draws the fuel into the pump (i.e. 'suction feed'), and a gear pump which provides the high-pressure fuel to the engine and as muscle pressure to drive things like the Stator Vane and Bleed Valve actuators. It takes a minimum of ~300 PSI to run the engine - the HPSOV is spring loaded closed and it takes approximately 300 psi to overcome that spring.
Engine driven fuel pump failures are very rare, but have happened (usually with some 'precursor' symptoms that were ignored or mis-diagnosed by maintenance). It would be unheard of for engine driven fuel pumps to fail on both engines on the same flight.

As I've repeatedly posted, even a 100% aircraft power failure would not explain both engines quitting, at least without several other existing faults. Again, never say never, but you can only combine so many 10-9 events before it becomes ridiculous...

TCMA doesn't know what V1 is - it's active whenever the air/ground logic says the aircraft is on-ground.

It's been troubling a few of us for some time. To translate all that for the layperson, a momentary electrical event triggered at V1 (still on the ground) and restored after rotation before gear up is selected.(in flight)

I've read your cite four times and could not find anything implying that " a momentary electrical event triggered at V1 (still on the ground) and restored after rotation before gear up is selected." I guess you need to explain it to me like I was eight.

Sorry about that. The reply was aimed at the OP. from a software writers perspective, as you rightly point out it does not imply an electrical event actually occurred. As you raised that query writing software routines (programming) is no different and that's what we're looking for.

Something rather relevant has not yet been shown on this thread: a performance calculation.

Could somebody with access to a 787 performance calculator please show realistic figures for this type out of Ahmedabads runway 23?

Weather at the time of the accident:

VAAH 120830Z 24003KT 6000 NSC 37/17 Q1000 NOSIG=
VAAH 120800Z 25007KT 6000 NSC 37/16 Q1001 NOSIG=

With 240 souls on board, a payload of 20-22 tons might be a ballpark figure, and a fuel load of about 50 tons was claimed as realistic in earlier posts.

An appropriate flap setting, a flex temperature, V1, Vr, V2, flap retraction and green dot speed (or its Boeing equivalent), a climb gradient after departure, and some distances might be of interest, I\xb4d think.

If TCMA cut fuel flow while still on the runway the aircraft would have been decelerating from the moment it lifted off, which is not what the ADS-B data indicates. The kinetic energy in the rotating parts of the engine wouldn't add much speed to the aircraft as the engines run down with no more energy being added via fuel.

This is the best theory of TCMA activation so far, but it\x92s still very remote. In this theory there is no need for a TCMA anomaly, it needs only(!) two simultaneous engine anonalies (and sn improper crew action) that would trigger the TCMA as designed. If there are safeguards preventing this happening, they are not mentioned in this thread.

It also doesn\x92t explaine the ADS-B data showing acceleration after lift off.

If there was a loss of power, it happened after V1. Normally, at V1, all hands should be removed from the thrust levers until, type dependent, selection of climb thrust, which normally takes place not below 1000\xb4 above aerodrome level. An altitude the flight never managed to achieve. If some flight crew members seat was to slide back unintentionally, there are handles in the flight deck that one will instinctively reach for, no need to grab the thrust levers of all things.

Even if the levers may have been inadvertently pulled to idle, it would have taken one single energic adjustment to slam them to maximum thrust position (whatever it is called on the 787), for which there was some time during the brief flight. And two idling engines should not result in anything triggering the RAT release or APU auto start.

Of course, stranger things have happened, but I\xb4d consider this scenario not plausible.

To simplify the chicken/egg:

Tdracer earlier confirmed that an airplane electrical power loss would allow engines to keep running , because 1) engines are fully-capable of suction feed operation in takeoff envelope (if boost pumps lost), and 2) the EECs are powered by their own PMAs when running and to substantially below idle (I recall roughly 10% N2). Airplane powers the EEC for ground starts, prior to PMA coming online, and as backup to the PMA after that.

Related:

Engine igniters are powered by the aircraft. So theoretical full loss of aircraft power would disable Autorelight upon a flameout. Ignitors typically don't make the cut for most-essential battery-only loads because it would also take an engine flameout, and the airplane past V1 in ground roll can fly fine on one engine that can achieve takeoff thrust.

Autorelight is relevant - if there was a single-engine failure post V1, autorelight will attempt to relight the engine, so there is no need for a pilot to try to cycle the fuel switch to reset the EEC (potentially grabbing the wrong one), or to otherwise intervene. In such a circumstance, they need to trust their training. I've heard accounts that the most likely pilot instinct in such a situation would be to push the throttle(s) forward.

Finally - there was talk earlier about accidentally cutting the fuel switches - and it was duly noted that they have to be pulled out over the detent, so very unlikely. The same cutoff effect could be achieved with the engine fire handle(s), right behind the fuel switches on the pedestal - though they are an upward pull, so also not subject to inadvertent or accidental engagement.

Can any 787 drivers confirm if TCMA would/could activate if a pilot were to inadvertently cause unusual and/or asymmetric thrust with the levers after V1 and through VR (before being airborne)?

I.e. if there was some hesitation in aborting TO, could a few seconds at high power with somewhat asymmetric levers have caused TCMA to activate?

OK, I promised some informed speculation when I got back, so here goes:
Disclaimer: never worked the 787, so my detailed knowledge is a bit lacking.

First off, this is perplexing - especially if the RAT was deployed. There is no 'simple' explanation that I can come up with.

GEnx-1B engines have been exceptionally reliable, and the GE carbon composite fan blades are very robust and resistant to bird strike damage (about 15 years after the GE90 entry into service, I remember a GE boast that no GE90 (carbon composite) fan blades had needed to be scrapped due to damage (birdstrike, FOD, etc. - now that was roughly another 15 years ago, so is probably no longer true, but it shows just how robust the carbon composite blades are - far better than the more conventional titanium fan blades).

Not saying it wasn't somehow birdstrike related, just that is very unlikely (then again, all the other explanations I can come up with are also very unlikely ).

Using improper temp when calculating TO performance - after some near misses, Boeing added logic that cross-compares multiple total temp probes - aircraft TAT (I think the 787 uses a single, dual element probe for aircraft TAT, but stand to be corrected) and the temp measured by the engine inlet probes - and puts up a message if they disagree by more than a few degree tolerance - so very, very unlikely.

N1 power setting is somewhat less prone to measurement and power setting errors than EPR (N1 is a much simpler measurement than Rolls EPR) - although even with EPR, problems on both engines at the same time is almost unheard of.

The Auto Thrust (autothrottle) function 'falls asleep' at 60 knots - and doesn't unlock until one of several things happens - 250 knots, a set altitude AGL is exceeded (I'm thinking 3,000 ft. but the memory is fuzzy), thrust levers are moved more than a couple of degrees, or the mode select is changed (memory says that last one is inhibited below 400 ft. AGL). So an Auto Thrust malfunction is also extremely unlikely. Further, a premature thrust lever retard would not explain a RAT deployment.

TO does seem to be very late in the takeoff role - even with a big derate, you still must accelerate fast enough to reach V1 with enough runway to stop - so there is still considerable margin if both engines are operating normally. That makes me wonder if they had the correct TO power setting - but I'm at a loss to explain how they could have fouled that up with all the protections that the 787 puts on that.

If one engine did fail after V1, it's conceivable that they shut down the wrong engine - but since this happened literally seconds after takeoff, it begs the question why they would be in a big hurry to shut down the engine. Short of an engine fire, there is nothing about an engine failure that requires quick action to shut it down - no evidence of an engine fire, and even with an engine fire, you normally have minutes to take action - not seconds.

The one thing I keep thinking about is someone placing both fuel switches to cutoff immediately after TO. Yes, it's happened before (twice - 767s in the early 1980s), but the root causes of that mistake are understood and have been corrected. Hard to explain how it could happen (unless, God forbid, it was intentional).

The engine driven fuel pump is literally driven off the engine gearbox (driven by a mechanical connection to the N2 shaft) - if the engine's running, the gearbox is turning (baring a major mechanical fault). The engine driven fuel pump is a two-stage pump - a centrifugal pump that draws the fuel into the pump (i.e. 'suction feed'), and a gear pump which provides the high-pressure fuel to the engine and as muscle pressure to drive things like the Stator Vane and Bleed Valve actuators. It takes a minimum of ~300 PSI to run the engine - the HPSOV is spring loaded closed and it takes approximately 300 psi to overcome that spring.
Engine driven fuel pump failures are very rare, but have happened (usually with some 'precursor' symptoms that were ignored or mis-diagnosed by maintenance). It would be unheard of for engine driven fuel pumps to fail on both engines on the same flight.

As I've repeatedly posted, even a 100% aircraft power failure would not explain both engines quitting, at least without several other existing faults. Again, never say never, but you can only combine so many 10-9 events before it becomes ridiculous...

TCMA doesn't know what V1 is - it's active whenever the air/ground logic says the aircraft is on-ground.

No, no, no and again no.

This is a transport category aircraft. It will happily climb on one engine to a safe altitude where procedures are followed.

There is zero evidence of any engine "failures" or shutdowns by the crew.

However the gear is still down at around 500' agl.

Speaking as a B787 Captain..... There is so much rubbish and stupid suggestion being written here.

This aircraft was airborne for a grand total of 22 seconds, half of which was climbing to no more than 150' aal.

- No Flaps? Due to the setup of the ECL it is physically impossible to go down the runway without some sort of take-off flap set. The T/o config warning would have been singing it's head off. Despite assertions to the contrary I have seen no video clear enough to detect a lack of flaps.

- RAT out? Almost impossible, I have seen no quality footage that definitively witnesses the RAT being out. Those who think they car hear a RAT type noise might be listening to a motorcycle passing or similar. It takes a triple hydraulic failure or a double engine failure to trigger RAT deploment. They happily went through V1 without a hint of rejected take off so as they rotated the aircraft was serviceable. These are big engines, they take a long time to wind down when you shut them down. I have never tried it however engine failure detection takes 30s or for the aircraft to react and they were not even airborne that long.
- Flaps up instead of gear? The B787 flaps are slow both in and out. Given that the 'Positive rate' call is not made the second the wheels leave the ground, a mis-selection of flaps up would not cause any loss of lift for at least 20 seconds, by which time they had already crashed. I believe the gear remained down not because of mis-selection but because of a major distraction on rotate.

Discounting the impossible, two hypotheses remain:

1. Invalid derate set through incorrect cross-checking. Trundling down the runway takes very little power to reach Vr. It is only when you rotate that you create more drag and discover that you do not have sufficient thrust vs. drag to sustain a climb. Or....
2. Put 200' as the altitude target in the FCU. Immediate ALT capture and all the power comes off. PF is still hand flying trying to increase pitch but is already way behind the aircraft.

It could be after this that Boeing are forced to review the B787 practice of exploring the very edges of the performance envelope.

I have to agree with everything here except your assertion about engine shutdown.
Even though these are big engines with plenty of inertia, when you select engine shut off they spool down very quickly if on load. IE, The generators, two per engine and hydraulic pumps, etc, being driven by the (relatively) small mass of the N2 rotor will drag the speed down very quickly, the gennies will trip offine in seconds, the pumps will quickly reduce flow and pressure.
As for what went wrong.
If the engines have stopped working there has to be a common failure mode, fuel is one but as has been said, no other aircraft has had a problem, as far as we know. FOD? It would have to be something major to shut down two GeNX engines and there would be debris all over the runway, we would know by now.
I have no idea if the RAT has deployed, I can't see it in the video and the noise could be something else.
We shall see.
There is compelling evidence that flaps are set correctly and not retracted inadvertently.
I await further evidence.
Edit to add. LAE 40 years, type rated on 737 to 787 with lots of others in between.

I agree with the post above (edited for brevity) - and fear the thread is getting repetitive in the absence of new information.

The only thing I would add is the limited ADS-B data I have seen shows the a/c decelerating rapidly from the first data point onwards. It is possible the shutdown occurred when the a/c was on the ground (e.g. after V1). This may seem unlikely given the distance flown, but you can do the maths - a fast a/c has a lot of stored kinetic energy.

Do you stand by your previous assertion :-