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Contact Approach
July 13, 2025, 20:48:00 GMT permalink Post: 11921640 |
Thanks for confirming - the point I'm trying to make (apparently badly) is twofold:
1. There's a discussion here about the delay of 4 seconds between switching engine 1 and engine 2 to RUN. The fact is that this is irrelevant in terms of outcome given the lack of APU at the start of this process. Moreover if the lack of APU was known to the pilots, then it would potentially explain this delay anyway (what was the point in flicking the other switch yet)? 2. The lack of APU until at least 12-13 seconds after loss of engines, lack of electrical power from the engines, and at best limited electrical power from the RAT (and for a time, none), would have affected other systems used by the pilots during this short flight, at least for part of it. This hasn't been discussed basically at all, despite a lot of discussion around the ten second delay to start the engines restart, even though that power loss would have been in that window. Ok so: 1) There may be a technical reason outside of my scope for what you describe. From an operational point of view, for every EFATO I\x92ve ever done in the sim we are taught ANC. Initially our primary focus is flying the aircraft, hence why it\x92s being suggested this was the work of the PM, the only one with sufficient capacity capable of such at that moment. The fact the APU hasn\x92t come online whilst under such workload whilst falling out of the sky at 300ft wouldn\x92t be much of a consideration from the PF 2) Without power, altitude or airspeed nothing else really matters at that stage, does it? The APU takes longer to come online than this flight lasted! |
Musician
July 13, 2025, 21:06:00 GMT permalink Post: 11921654 |
Thank you for your reply! I'm aware you know more about ADS-B than I do, and I'm happy to learn.
As you know, I'm referring to the map view and data on https://www.flightradar24.com/blog/f...rom-ahmedabad/ . 08:08:50.87 last full ADS-B message (172 kts) 08:08:54 APU inlet door starts to open, per the report. My problem here is that the report also says that the maximum speed was 180 knots IAS at about 08:08:42. And we have 2025-06-12T08:08:46.550875Z,8005ec,23.069138,72.625871,575 associated with 184 kts as the first ADS-B message in the sequence. Given that the clocks of ADS-B receivers are sometimes off, I think this might actually represent the moment of maximum speed; but then the AFS-B clock would be 4 seconds fast. And that's why I adjusted my times. A way to cross-check would be to track the aircraft position over time from the point of rotation, or to use the new photo in the report that shows the RAT over the runway to triangulate where that was.
Beware of comparing the speeds from ADS-B, which are groundspeeds, with those from the report, which are IAS.
Would low pressure and high temperature make IAS lower than ground speed, even with the 7 knot headwind? Either way, if they were decelerating at approximately 3 knots per second, then they would've ended up at about 150 knots IAS, which is less than Vr. When I look at the CCTV video, 13 seconds into the flight, the aircraft is just "over the hump" and starts descending, so that tracks. At this point, I don't know what having the engines spool up instead of being dead weight would've done to that flight path; and how successfully the aircraft could've been recovered from that. Maybe they had only 4 seconds to flip those switches, instead of 5 seconds? Still, the best bet is to not ask questions, but flip these switches back ASAP, no? Last edited by Musician; 13th July 2025 at 21:36 . |
Mrshed
July 13, 2025, 21:15:00 GMT permalink Post: 11921667 |
Thank you for your reply! I'm aware you know more about ADS-B than I do, and I'm happy to learn.
As you know, I'm referring to the map view and data on https://www.flightradar24.com/blog/f...rom-ahmedabad/ . That's what I thought at first, based on the time stamps. 08:08:50.87 last full ADS-B message (172 kts) 08:08:54 APU inlet door starts to open, per the report. My problem here is that the report also says that the maximum speed was 180 knots IAS at about 808:08:42. And we have 2025-06-12T08:08:46.550875Z,8005ec,23.069138,72.625871,575 associated with 184 kts as the first ADS-B message in the sequence. Given that the clocks of ADS-B receivers are sometimes off, I think this might actually represent the moment of maximum speed; but then the AFS-B clock would be 4 seconds fast. And that's why I adjusted my times. A way to cross-check would be to track the aircraft position over time from the point of rotation, or to use the new photo in the report that shows the RAT over the runway to triangulate where that was. Yes. Would low pressure and high temperature make IAS lower than ground speed, even with the 7 knot headwind? Either way, if they were decelerating at approximately 3 knots per second, then they would've ended up at about 150 knots IAS, which is less than Vr. When I look at the CCTV video, 13 seconds into the flight, the aircraft is just "over the hump" and starts descending, so that tracks. At this point, I don't know what having the engines spool up instead of being dead weight would've done to that flight path; and how successfully the aircraft could've been recovered from that. Maybe they had only 4 seconds to flip those switches, instead of 5 seconds? Still, the best bet is to not ask questions, but flip these switches back ASAP, no? |
Tailspin Turtle
July 13, 2025, 23:43:00 GMT permalink Post: 11921782 |
Thank you for your reply! I'm aware you know more about ADS-B than I do, and I'm happy to learn.
As you know, I'm referring to the map view and data on https://www.flightradar24.com/blog/f...rom-ahmedabad/ . That's what I thought at first, based on the time stamps. 08:08:50.87 last full ADS-B message (172 kts) 08:08:54 APU inlet door starts to open, per the report. My problem here is that the report also says that the maximum speed was 180 knots IAS at about 08:08:42. And we have 2025-06-12T08:08:46.550875Z,8005ec,23.069138,72.625871,575 associated with 184 kts as the first ADS-B message in the sequence. Given that the clocks of ADS-B receivers are sometimes off, I think this might actually represent the moment of maximum speed; but then the AFS-B clock would be 4 seconds fast. And that's why I adjusted my times. A way to cross-check would be to track the aircraft position over time from the point of rotation, or to use the new photo in the report that shows the RAT over the runway to triangulate where that was. Yes. Would low pressure and high temperature make IAS lower than ground speed, even with the 7 knot headwind? Either way, if they were decelerating at approximately 3 knots per second, then they would've ended up at about 150 knots IAS, which is less than Vr. When I look at the CCTV video, 13 seconds into the flight, the aircraft is just "over the hump" and starts descending, so that tracks. At this point, I don't know what having the engines spool up instead of being dead weight would've done to that flight path; and how successfully the aircraft could've been recovered from that. Maybe they had only 4 seconds to flip those switches, instead of 5 seconds? Still, the best bet is to not ask questions, but flip these switches back ASAP, no? |
DavidncRobson
July 14, 2025, 03:32:00 GMT permalink Post: 11921837 |
Thrust Recovery Time from Idle Condition
Originally Posted by
Mrshed
The question is what impact was had on attempts to recover the aircraft by the (presumed) lack of most or all electrical power. Response from BrogulT: From the point that the fuel cutoffs were switched back to run, the primary (perhaps only) relevant issues were time and altitude. The left engine was relit and recovering on it's own but the right would probably have needed cranking as it appears to have had a hung start because it was spooled down too far and the airspeed was too low. However, I'm pretty sure that even if the APU had already been running at takeoff, they still would have needed at least another 30 seconds if not longer for full thrust. Also, there was nowhere near enough time for the APU to start and come online here. Your claim that at least 30 seconds were needed to achieve full thrust prompted me to look up reports on the Airbus A320 crash at Habsheim in France in 1988 where the aircraft had been flown at low altitude at the minimum allowable speed and was unable to achieve sufficient thrust to ascend above a copse of trees. The extract below shows that from Idle to 83% N1 took 5 seconds after the initiation of a TOGA, so I think your estimate of 30 seconds is probably too high. However, after reading that report, I am of the opinion that AI 171 had no chance of recovering. If the FCS switches had been set to Run within 5 seconds of being set to Cut Off, there might just have been a chance, but it is not at all certain. (From Aviation Safety Network Sunday 26 June 1988) The aircraft took off from nearby Basle-Mulhouse Airport at 14:41 and climbed to 1000 feet agl. The crew started the descent three minutes later and Habsheim was in sight at 450 feet agl. The Pilot Monitoring informed the Pilot Flying that the aircraft was reaching 100 feet at 14:45:14. The descent continued to 50 feet 8 seconds later and further to 30-35 feet. Go-around power was added at 14:45:35. The A320 continued and touched trees approximately 60 meters from the end of runway 34R at 14:45:40 with a 14 degree pitch attitude and engines at 83% N1. The plane sank slowly into the forest and a fire broke out. PROBABLE CAUSES: "The Commission believes that the accident resulted from the combination of the following conditions: 1) very low flyover height, lower than surrounding obstacles; 2) speed very slow and reducing to reach maximum possible angle of attack; 3) engine speed at flight idle; 4) late application of go-around power. This combination led to impact of the aircraft with the trees. The Commission believes that if the descent below 100 feet was not deliberate, it may have resulted from failure to take proper account of the visual and aural information intended to give the height of the aircraft." |
BrogulT
July 14, 2025, 04:36:00 GMT permalink Post: 11921851 |
The extract below shows that from Idle to 83% N1 took 5 seconds after the initiation of a TOGA, so I think your estimate of 30 seconds is probably too high. However, after reading that report, I am of the opinion that AI 171 had no chance of recovering. If the FCS switches had been set to Run within 5 seconds of being set to Cut Off, there might just have been a chance, but it is not at all certain.
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Someone Somewhere
July 14, 2025, 08:01:00 GMT permalink Post: 11921930 |
Engine restart (i.e. "Quick Windmill Relight"): Even if the igniters were firing, at high power they won't actually spark (the electrical resistance or the air at several hundred psi prevents a spark) - so they won't spark until you get down somewhere near idle if you're close to sea level. Once the engine has dropped below the min idle, it takes a
long
time for it accelerate back to even an idle condition. At takeoff power, the compressor components get very hot - do a power cut the air coming in the compressor gets heated by the residual heat in the compressor. This in turn limits how fast you can add fuel in the burner without excessively back pressuring the compressor and causing a compressor stall. So it actually takes
longer
for the engine to accel to idle that it would during a normal (cold engine) start. The 5 seconds to 95% accel requirement referenced earlier is from a stable 'high' idle (we typically call it 'approach idle' since it's automatically selected when landing flaps are selected). Approach idle is ~10% N2 higher than the in-flight minimum idle, so that takes several more seconds. Bottom line, after initiating the Quick Windmill Relight, you're not going to have usable thrust for
at least
30 seconds - probably closer to 60 seconds..
If the restriction to getting the engine relit earlier (well above idle N2) is only the spark gap, I am somewhat surprised that beefier igniters, perhaps with high/low voltage settings (for emergency/normal use), are not used. Compressed air is a reasonable insulator, but it's nowhere the oil, SF6, or vacuum that HV operators use in tight spaces. Bigger igniters might mean you can spark the fuel at ~70% N2 at which point you're presumably seconds away from having thrust again, and don't do the significant engine damage associated with I assume EGT exceedances from scheduling high fuel to ramp N2 rapidly with already-hot parts.
My final thoughts for this event.
Misinformation, most of us including myself was of the view that Gear Up had been selected because we saw an image believed to be the accident aircraft with the bogeys stowed for retraction.The report image shows that they were not and the Gear Selector was down. A crucial element in the sequence of events.
I think there is a bit of confusion running in this thread about how the auto restart function works.
Normal start uses a lot of electrical power to drive the two starters. In a situation with only the RAT supplying electrical power there won't be anywhere near enough power to turn even one starter. Restart relies on windmilling only. Igniters don't need a huge amount of power, fuel will be gravity fed to the engine driven pumps. The APU autostart function will use power from the dedicated APU battery only. Edit to add, it can take an age for the APU to start off the battery. Well over a minute. You can spin up the engines in three ways: starter motor (electric or pneumatic, depending on type), windmill (but at low speeds, the RPM given by a windmill won't be enough), or the inertia of the already spinning engine. Quick relight I believe is predominantly inertia. |
HUD Engineer
July 15, 2025, 10:02:00 GMT permalink Post: 11922796 |
Preliminary Report - another possibility?
I agree that the Preliminary Report will have been a very carefully chosen synopsis, but I don't think it precludes a system behaviour that might or might not even be categorised as either an electrical or mechanical malfunction, as such. Consider that the Preliminary Report only references the CVR contents to indicate one crew querying why he (the other) cutoff, and the other denying it. Clearly very pertinent data to this investigation, so I'm not ruling out both of the crew being sincere, until I understand the aircraft system better. If the cutoffs weren't triggered by the crew moving the switches, then the most likely thing was that both FADECs encountered a situation that invoked cutoff, practically at the same moment. That might suggest that the data from independent Run/Cutoff switches via independent poles that possibly route via a PIP or PIPs to various RDCs and in turn the CCS (which means these paths have a common mode) resulted in data to both FADECs "failing" in a critical manner simultaneously. The only CCS vulnerability I'm aware of is what happens if the CCS is kept powered for 51 days, shortly after which, the Time Manager data integrity for ARINC 664 messages is compromised, possibly only considered critical n conjunction with a CDN Switch failure. AD 2020-06-14 therefore requires a maintenance action of cycling the CCS power at least once every 25 days. Two such omitted, or incorrectly executed maintenance actions in a row would have to happen for that known issue to be relevant. Perhaps there is some other input in the CCS system that influences both lanes of data to the FADECs for the Cutoff to be triggered, but I'm not aware of such. If the data to both FADECs became good again around the same time (perhaps no longer Stale Data, or an alternative source selected, or other mechanism), and given the unavailability of APU power at this point, might Engine 1 restart be initiated first, and might Engine 2 restart be triggered 4 seconds later by the FADECs? Is that possible without the crew cycling the switches? Regarding AD 2020-06-14, which was approved quickly, Boeing subsequently offered revised wording, which changed "may" to "will", see FAA Docket 2020-0205-0004, but as it did not alter the maintenance action, it was not adopted. A fuller description of possible consequences was in FAA Docket 2020-0205-0001_content, but it was quite wide ranging, and didn't attempt to characterise specific aircraft systems behaviour. For anyone wanting more information on the CCS, in the context of that Airworthiness Directive, I recommend reading A Reverse Engineer’s Perspective on the Boeing 787 ‘51 days’ Airworthiness Directive at IOActive. As a non-professional pilot, please accept that my knowledge of much beyond the CCS is patchy, but I welcome this scenario being critiqued. Thank you for your time, and I'm just donning my hard hat and flack jacket. Last edited by T28B; 15th July 2025 at 13:56 . Reason: formatting for easier reading |
jpsingh
July 16, 2025, 07:14:00 GMT permalink Post: 11923479 |
The deployment of RAT has announced Double Engine failure. Also the AutoStart of APU pretty much indicates the same thing . Release of CVR will definitely help.
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sabenaboy
July 16, 2025, 07:16:00 GMT permalink Post: 11923480 |
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Lead Balloon
July 16, 2025, 22:37:00 GMT permalink Post: 11924040 |
The recorded switch position data, the recorded engine performance data, the aircraft performance and trajectory after take off, the RAT deployment and open/ing APU inlet, the cockpit voice recording that has been selectively and carefully paraphrased in the preliminary report and the physical and electrical design and separation of the switches and the wiring for each system prove, to whatever standard of proof anyone wants to nominate, that BOTH fuel cut off switches were physically switched OFF 'shortly' after take off, then BOTH switches were physically switched back ON 'shortly' thereafter.
Give the FSC switch/wiring defect theory away, hamsters. |