Posts by user "Innaflap" [Posts: 6 Total up-votes: 2 Pages: 1]

Originally Posted by soarbum

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?

Originally Posted by Lead Balloon

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.

Originally Posted by Europa01

The excellent #724 post by user989 really should be seen as the defining statement on what is currently known.

I\x92d like to add a complimentary test to user989\x92s logic on TCMA faults.

Regardless of whether the \x91aircraft on ground\x92 signal was incorrect after rotation it would have been correct during the takeoff roll. IF there was an unrevealed fault in a thrust lever position signal THEN why didn\x92t TCMA activate during taxiing or the takeoff roll?

Such a fault occurring spontaneously in just the few seconds after rotation is way way down the probability table. Such a fault occurring spontaneously on both separate (think ETOPS) engine control systems is surely vanishingly unlikely.

They may be out there but you\x92d have to ask if TCMA is implicated where are the lower consequence precursor events in the 787 fleet? These might be spurious TCMA action on one engine or faults with \x91aircraft on ground\x92 found during maintenance or engines not responding to thrust lever position and so on.

Change Analysis would ask what happened differently in the few seconds after rotation on this flight that separates it from all other 787 takeoffs and why at that particular time ?The interim report will provide some answers until then please let\x92s confine this thread to fact based technical discussion and debate.

Re-reading this I did briefly consider suggesting engine overshoot of thrust lever positions and FADEC shut down on N1 overspeed but that leaves a lot of WHY and WHY both engines questions so I dismissed it.

Originally Posted by V1... Ooops

I think that it is understandable that the Indian accident investigation team leaders are being very cautious about when and what information to release. This is probably the first "really big" accident investigation they have done that has achieved so much international attention. I am sure that they want to be certain that when they do release information it is accurate and carefully presented, simply to avoid any embarrassment that might arise from a hasty or less than carefully thought out disclosure.

For that reason, I think we ought to give the Indian investigators the benefit of the doubt and assume that they simply want to make sure that when they do make a statement it is 'bulletproof' and reflects well on their skill, professionalism, and the processes that they have followed.

Originally Posted by Sailvi767

I suspect both recorders will contain the same data. Given the radio transmission after the loss of thrust the aircraft still had at least the emergency electrical bus powered. This should have kept both recorders online. It is however possible given the 10 minute battery backup that Boeing chose to put the the recorders on another bus but that\x92s not the norm.

Originally Posted by EXDAC

I am not aware of any requirement for a DFDAU (or equivalent) to store any data. I say "or equivalent" because in B717 the DFDAU is not an LRU. It is a functional partition of the VIA.

It's not clear to me that 787 EAFR even requires an external DFDAU. The GE EAFR does not -

"Provides Flight Data Acquisition function of ARINC 664 p7 data parameters \x96 No need for a Digital Flight Data Acquisition Unit (DFDAU)."

ref https://www.geaerospace.com/sites/de...rder-3254F.pdf