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Musician
June 20, 2025, 10:56:00 GMT permalink Post: 11906831 |
At that point, the total energy of the system would have comprised of the kinetic energy of the aircraft travelling at Vr, the rotational inertia of the engines and the potential energy of whatever fuel is beyond the cutoff valves.
Q5: Would this total energy have been sufficient to get the aircraft 100ft into the air? |
sSquares
June 20, 2025, 10:59:00 GMT permalink Post: 11906833 |
II.
Fuel-related
1. Loss of electric fuel pumps
Suction feed would have provided sufficient fuel pressure.
2. Fuel contamination
No other aircraft affected, no measures taken at airport. Simultaneous flameout due to contaminated fuel very unlikely.
3. Vapour lock
Unlikely to occur in this scenario. Even if (momentarily) no sufficient fuel pressure from the center tank, the engines would have been fed by the wing tanks.
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Innaflap
June 20, 2025, 11:02:00 GMT permalink Post: 11906835 |
Engineer not a pilot. Experience in analog front ends, A2D and R2D conversion and embedded systems generally but no specific knowledge of the 787 or GEnx.
I like everyone else have no evidence that TMCA played a role but given that it is one of the few systems with the ability to cut fuel to the engines, here are some thoughts on how signal processing could have extended the window of when TMCA could bite. In particular, I'm looking at the time immediately after the nose lifts up when something may have physically shifted onboard. I'll phrase it as a number of questions but realise that the few people who can answer may not be able to for now. Thanks to tdracer's explanation on TMCA (albeit 747 not 787), we know that TMCA is a logic block within the FADEC whose only external inputs are a logic signal fron the aircraft that indicates whether it is on the ground or not and throttle position as determined by two independent resolvers per throttle side. The logic would seem to be something of the form If (G AND (N2>A OR N2>B)) Then CutOffFuel() where G is true when the aircraft is on the ground, A is an envelope defined by throttle resolver channel A and B is an envelope defined by throttle resolver channel B Q1: Am I correct in that assumption that when on the ground, overspeed with respect to EITHER resolver A OR resolver B can trigger TMCA? We have been told that the logic (ie true or false) signal G is determined from the Weight-on-wheels sensors and the RadALT. It is reasonable to suppose that the designers still wanted TMCA to function after a hard landing where some landing gear components had failed. Q2: When the nosewheel lifts off but the MLG is still on the ground and RadALT is close to ground, will G still be true? Next, it is common when data fusing multiple inputs that there is a desire to clean up a signal before it is sampled digitally. This can remove effects such as switch bounce. The inclusion of low pass filters or hysteresis will generally add a propogation delay. Q3: Is there a slow filter (Tc>=1s) in the ground/air logic which could have caused a slight delay before G became false after takeoff further extending the opportunity of TMCA to activate? Q4: Does TMCA act almost instantly or does it wait for the fault condition to stay asserted for a period of time before acting? At that point, the total energy of the system would have comprised of the kinetic energy of the aircraft travelling at Vr, the rotational inertia of the engines and the potential energy of whatever fuel is beyond the cutoff valves. Q5: Would this total energy have been sufficient to get the aircraft 100ft into the air? It would still need a mechanism for at least one throttle input to each FADEC to misbehave at the same time. Resolvers are fed with an excitation signal to the rotor and take back two orthogonal signals (Cos and Sin) from stator windings. Usually, the excitation comes directly from the resolver-to-digital (R2D) circuit but sometimes an external signal source is used. I would hope that in an aircraft system, each channel would be kept independent of everything else. Q6: Does the excitation signal for the 4 throttle resolvers (2 per side) come from 4 independent (internal) sources? My last thought for a single point of failure between both throttles would be a short between two wires or connection points carrying resolver signals, one from each side. Whether this could be caused by swarf wearing within a wiring loom, a foreign object moving about, crushed wires or even stretching of adjacent wires, I have absolutely no idea. Q7: Do resolver signals from left or right, either channel A or B, run next to each other in a loom at any point? What happens when the 2 disparate processes that form TCMA disagree? |
Raffael with FF
June 20, 2025, 11:04:00 GMT permalink Post: 11906838 |
Let me try to answer the questions about which I have some knowledge, as an aerospace engineer:
(I am not sufficiently informed to answer Q4,6 and 7, at the moment)
Q1: Am I correct in that assumption that when on the ground, overspeed with respect to EITHER resolver A OR resolver B can trigger TMCA?
We have been told that the logic (ie true or false) signal G is determined from the Weight-on-wheels sensors and the RadALT. It is reasonable to suppose that the designers still wanted TMCA to function after a hard landing where some landing gear components had failed.
At that point, the total energy of the system would have comprised of the kinetic energy of the aircraft travelling at Vr, the rotational inertia of the engines and the potential energy of whatever fuel is beyond the cutoff valves.
Q5: Would this total energy have been sufficient to get the aircraft 100ft into the air? Kinetic energy with a weight of 200,000kg, at Vr = 150kn = 77m/s: E_kin = 600MJ Rotational energy of a GEnX engine is hard to calculate as I don't find reliable values for the rotary inertia. I found some for a GE90 and could roughly estimate 100MJ of rotational energy for each engine. However, I seriously doubt that this energy could be effectively used to gain thrust, as the thrust will drop very quicjkly after the fuel is cut off. the required potential energy for a 100ft climb of a 200,000kg 787 is around 70MJ. This ignores aerodynamic drag, still, 100 ft of climb remains energetically feasible. However, it as been pointed out several times that the actual climb was higher than 100ft. Already for 200ft I would doubt the validity of my statement above. |
Mechta
June 20, 2025, 11:09:00 GMT permalink Post: 11906844 |
There are numerous pictures ot the outside of B787 centre tanks on the net. Does anyone one have any internal pictures, showing the tank floor and fuel pump pick ups?
We know the engines lost power in the initial climb, shortly after rotation. If there was water sitting between the tank lower skin stringers, the rotation would have been the point that the water could tumble over the stringers that were previously preventing its movement. accumulate at the back of the tank and enter both pumps more or less simultaneously. For background, I worked at Smiths Industries wet fuel testing the B777 gauging system on ground rigs, and at Airbus building and testing fuel tank inerting rigs. I've seen inside Airbus tanks, but not Boeing. |
OPENDOOR
June 20, 2025, 11:17:00 GMT permalink Post: 11906850 |
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Squawk7700
June 20, 2025, 11:20:00 GMT permalink Post: 11906855 |
There are numerous pictures ot the outside of B787 centre tanks on the net. Does anyone one have any internal pictures, showing the tank floor and fuel pump pick ups?
We know the engines lost power in the initial climb, shortly after rotation. If there was water sitting between the tank lower skin stringers, the rotation would have been the point that the water could tumble over the stringers that were previously preventing its movement. accumulate at the back of the tank and enter both pumps more or less simultaneously. In a well designed boat, you'd have each engine feeding from a different tank for the utmost in redundancy, but seemingly not so in all aircraft. |
MaybeItIs
June 20, 2025, 11:21:00 GMT permalink Post: 11906857 |
The ADS-B datagrams sent by the aircraft show a much diminished climb rate with decaying speed, betraying insufficient thrust in that phase of the flight. That somewhat contradicts your assertions.
I also do not have faith in anyone's ability to watch the cctv video and confidently determine through mere eyeballing that the climb rate did not decay by 15% within the first 100 feet or so.
(The ADS-B data suggests the speed diminished 7% for ~50 ft of climb.)
And why all the wrong figures for the height attained, quoted in previous thread? Can't all be the atmospheric conditions.
Other than your stone, even a glider can convert speed to altitude.
![]() To be honest, i believe that taking a lot of the evidence into consideration, it is possible to arrive at a limited number of scenarios for what is most likely to have happened. One fact that alters things substantially is whether the survivor's impression is correct that possibly the engines started to spool up again just before impact. If that's the case then what does that do to the possibility or otherwise that the TMCA system caused a dual engine shutdown? To me, since the world seems to be watching this forum, and we are getting no feedback from the authorities, what is posted here might be useful in helping the investigators look at things they might not have considered. Besides, as Icarus2001 has kindof suggested, it's probably a very good thing that there are clearly lots of keen eyes on this. Last edited by MaybeItIs; 20th June 2025 at 11:29 . Reason: Missing [/QUOTE] |
violator
June 20, 2025, 11:29:00 GMT permalink Post: 11906865 |
I had been wondering the same until I read that there is a forward and a rear pickup within the tank. Each pump in the centre tank draws from it's own pickup and is piped to the spar valves and then onto the engines.
In a well designed boat, you'd have each engine feeding from a different tank for the utmost in redundancy, but seemingly not so in all aircraft. |
Squawk7700
June 20, 2025, 11:34:00 GMT permalink Post: 11906868 |
Interestingly enough on Airbus aircraft even when there\x92s fuel in the centre tank the centre tank fuel pumps are switched off automatically after the flaps are extended for takeoff and each engine is fed by its respective wing tank for takeoff. Surprised it\x92s not the case for Boeings
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Lead Balloon
June 20, 2025, 11:38:00 GMT permalink Post: 11906873 |
We have an authoritative answer to that question, but only if the TCMA implemented in the FADEC used on the 787 engines functions in the way described in conceptual documents: If one of the two TCMA 'channels' for an engine 'thinks' the shut off criteria are satisfied but the other channel doesn't, the channel which 'thinks' the shut off criteria are satisfied 'wins' and the fuel shut off valve for that engine is therefore given a shut off signal.
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Mechta
June 20, 2025, 11:46:00 GMT permalink Post: 11906879 |
Interestingly enough on Airbus aircraft even when there\x92s fuel in the centre tank the centre tank fuel pumps are switched off automatically after the flaps are extended for takeoff and each engine is fed by its respective wing tank for takeoff. Surprised it\x92s not the case for Boeings
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Luc Lion
June 20, 2025, 11:51:00 GMT permalink Post: 11906889 |
I perfectly understand that there is much talking about TCMA here.
There is no direct evidence of what caused the crash but several indirect evidences point towards a near simultaneous shutdown of both engines without any visual clue of a catastrophic mechanical mishap. This leads to suspecting near simultaneous fuel starvation of both engines. As the purpose of TCMA is shutting down the High Pressure Shut-Off Valve (HPSOV) and thus the fuel feed of an engine, it's normal to collect information on TCMA, on how it works, and on what data feeds it. However, I hardly understand why there is no similar discussion about the spar valves and the systems that control their opening and closure. I understand that the B787 spar valves are located in the MLG well, or at least are maintained from within that well. If the engine shutdown happened when the gear retraction was commanded, that's a location commonality (although it's very unlikely that a mechanical problem happened in both wells at the same time). Also I understand that there are several systems that command the opening or closing of the spar valves: - opening: "Engine control panel switch" set to "START", or "Fuel control switch" set to "RUN" - closing: "Engine fire handle" pulled out. (I wonder if "Fuel control switch" set to "CUTOFF" also closes the spar valve). Are there direct wires running from these controls to the valves or is there a pair of control units receiving these signals and controlling the valve actuators? If the latter is true, where are these control units? I guess that the likely location is the aft EE bay. Are they beside each other? Last edited by Luc Lion; 20th June 2025 at 12:57 . |
Innaflap
June 20, 2025, 12:08:00 GMT permalink Post: 11906904 |
We have an authoritative answer to that question, but only if the TCMA implemented in the FADEC used on the 787 engines functions in the way described in conceptual documents: If one of the two TCMA 'channels' for an engine 'thinks' the shut off criteria are satisfied but the other channel doesn't, the channel which 'thinks' the shut off criteria are satisfied 'wins' and the fuel shut off valve for that engine is therefore given a shut off signal.
Are these values recorded in the FDR? Are values from the FADEC recorded? |
Musician
June 20, 2025, 12:13:00 GMT permalink Post: 11906909 |
Flightradar24 and ADS-B
Thank you for your reply! There's a lot we agree on; unfortunately, I'll be cutting that from my response here.
![]()
Sure, actual data is usually more accurate than eyeballed stuff. But not always. In fact, it's often the eye that determines that something measured or calculated is "Off". How accurate is ADS-B data? I've seen FR24 tracks go way off course then suddenly get corrected / interpolated, frequently. The erroneous data seems to be "removed" by their algorithm, but where are the errors arising? Why this inaccuracy, and therefore, how accurate are the datagrams referred to? I know there were no datagrams received during the backtrack that I accept actually occurred, but that's completely different from receiving erroneous ADS-B data.
However, the datagrams that FR24 actually received were correct. They contain the GPS position of AI171 and its unadjusted barometric altitude, as determined by its onboard instruments. This data is as reliable as the instruments themselves are. (An example here is that the NTSB wasn't sure that the altimeter on the Blackhawk that crashed at Washington-Reagan was accurate; if that is the case, the ADS-B data would also be affected.) On their blog post at https://www.flightradar24.com/blog/f...rom-ahmedabad/ , FR24 have published the data that they actually received.
Sure, the CCTV footage I've seen is very poor, a video, moved about and zoomed, of the CCTV screen. Not easy to judge, but still useful and could be analysed frame-by-frame to compensate for all the extraneous input. Anyway, it's obvious to me that the rate of climb dropped abruptly just before the flight attained its apex, as if thrust was suddenly cut off.
Knowing the momentum to altitude conversion, it might be possible to estimate whether that's true or not. The evident RAT deployment supports engine shutdown, not just engines to Idle, doesn't it? In that case, it would be useful to know at what altitude the engine shutdown took place.
(The ADS-B data suggests the speed diminished 7% for ~50 ft of climb.)
And why all the wrong figures for the height attained, quoted in previous thread? Can't all be the atmospheric conditions.
2) people adjusting for airport elevation (189 ft), but not for pressure: 437 ft 3) people adjusting for pressure, some adjusting for temperature, get 71 to ~100 feet for the last recorded altitude. But while ADS-B reception was lost then (or the transmitter lost power), the aircraft continued climbing; examine the cctv video, knowing the wingspan is ~200 feet, we see that the aircraft reached 200 feet but not much more.
One fact that alters things substantially is whether the survivor's impression is correct that possibly the engines started to spool up again just before impact.
If that's the case then what does that do to the possibility or otherwise that the TMCA system caused a dual engine shutdown?
[Now I just hope your post is still there as I post this. ![]() Last edited by Musician; 20th June 2025 at 12:26 . |
oyaji-fr
June 20, 2025, 13:49:00 GMT permalink Post: 11906988 |
Hi
Looking at ATA26 the engine fire control panel is energized by the hot battery bus (HOT BB). Is it credible that a failure of the hot battery bus (for example due to damage or liquid ingress in the P300 panel ) could lead to this situation? Last edited by oyaji-fr; 20th June 2025 at 14:07 . |
Capn Bloggs
June 20, 2025, 15:49:00 GMT permalink Post: 11907075 |
Disclaimer: the numbers I mention are from publicly available sources, namely Wiki (for the ZFW weight calculation) and a Boeing FCOM dated 2010, and my own estimations.
Strange, as I would have estimated this quite differently based on layman's intuition. If one assumes average values, then the approximate flight profile of AI171 according to layman's guidance certainly fits a situation in which the engines failed at or even very shortly before rotation.
​​​​​​​
Is VR about 20 to 30 knots above the landing speed?
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Would these 20 to 30 knots of additional energy be sufficient to lift the aircraft to a good 200 ft during and after rotation?
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If the angle of attack is then successively reduced, wouldn't the airplane still have enough lift to glide for a few seconds before losing all or nearly all lift?
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Wouldn't it be the case that if the thrust had only ceased five seconds after rotation, the aircraft would then have reached a good 250 ft with the engines still running and then another good 200 ft in normal conditions before the speed was used up to about 150 kn?
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AI171 probably didn't reach an altitude of 400 to 500 ft above ground (in relation to the airport), did it?
@Brace , I think you're exaggerating the residual thrust effect at lower RPMs. Of course 70% would get you round the pattern but you're at a much lower drag config and you're going much faster, again less drag. And are improved-climb takeoffs in the 787-8 even a thing? I can't see a two-stage rotation. I've made up a YT combo video: |
EXDAC
June 20, 2025, 17:12:00 GMT permalink Post: 11907144 |
tdracer posted - "
Commanded engine cutoff - the aisle stand fuel switch sends electrical signals to the spar valve and the "High Pressure Shutoff Valve" (HPSOV) in the Fuel Metering Unit, commanding them to open/close using aircraft power. The HPSOV is solenoid controlled, and near instantaneous. The solenoid is of a 'locking' type that needs to be powered both ways (for obvious reasons, you wouldn't want a loss of electrical power to shut down the engine). The fire handle does the same thing, via different electrical paths (i.e. separate wiring)."
Search this thread for "HPSOV" if you need confirmation of the quote. Note there are two shut off fuel valves per engine - the HPSOV and the Spar valve. Both stay where they are if power is lost. |
EDML
June 20, 2025, 17:18:00 GMT permalink Post: 11907146 |
tdracer posted - "
Commanded engine cutoff - the aisle stand fuel switch sends electrical signals to the spar valve and the "High Pressure Shutoff Valve" (HPSOV) in the Fuel Metering Unit, commanding them to open/close using aircraft power. The HPSOV is solenoid controlled, and near instantaneous. The solenoid is of a 'locking' type that needs to be powered both ways (for obvious reasons, you wouldn't want a loss of electrical power to shut down the engine). The fire handle does the same thing, via different electrical paths (i.e. separate wiring)."
Search this thread for "HPSOV" if you need confirmation of the quote. Note there are two shut off fuel valves per engine - the HPSOV and the Spar valve. Both stay where they are if power is lost. |
Crossky
June 20, 2025, 21:16:00 GMT permalink Post: 11907327 |
Hello, this is my first post on pprune; as a 787 pilot I’m also puzzled by this accident. All seem to agree that for some reason there was a complete electrical failure and RAT deployment. With a complete electrical failure all six main fuel pumps fail. Each engine also has two mechanically driven fuel pumps. On takeoff, if there is fuel in the center tank, it will be used first, pumped by the two center tank pumps.
My airline’s manuals don’t go into much detail, but I read on another site that if both the center tank pumps fail, the engine driven pumps aren’t able to suction feed well enough from the center tanks to sustain engine operation. If there was fuel in the center tanks, a complete electrical failure would soon lead to center tank fuel pumps failure (all fuel pumps failure as stated previously) and fuel starvation of both engines. A rescue from this situation would be an immediate selection of both center tank fuel pumps OFF (not if my airline’s non-normal checklists) and waiting for successful suction feed from the L and R main tanks to occur, this would take a number of seconds. |
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