Posts about: "CCTV" [Posts: 50 Pages: 3]

Thank you for your reply! There's a lot we agree on; unfortunately, I'll be cutting that from my response here.
Right. ADS-B is transmitted via radio, so reception can be patchy, or obstructed by someone else transmitting on the same frequency (e.g. other aircraft), so not every datagram that the aircraft sends gets received. When that happens, the live display of FR24 assumes the aircraft kept doing what it did, and when another datagram eventually comes in, it corrects the position. It also connects the locations of these datagrams, regardless of whether the aircraft actually went there. For example, in the AI171 there's a 4-minute gap between a datagram sent on the taxiway, and the next datagram sent when the aircraft was off the ground towards the departure end of the taxiway. FR24 then connected these points via the shortest route; but we know that the aircraft actually used the intervening 4 minutes to taxi to the approach end of the runway, where it then started its take-off run. So that was false. (Another source of errors is when different FR24 receivers don't have synchronised clocks, so a mixture of data from these can have weird artifacts as a result.)
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.

Have you ever seen a parabolic trajectory from "the short end"?
Yes.

It's uncertain because the 787 rounds all altitudes it sends to the nearest multiple of 25. The altitudes sent were from 575 ft to 625 ft., but that's MSL and not adjusted for the weather: low air pressure makes that number higher than the actual altitude. FR24 adjusted this to 21ft climbing to 71 ft, but it could've been 30 to 60 or maybe 10 to 80, as it's rounded. I think it's fairly close to 50 feet of climb, though.

1) people taking the MSL altitude literally (625 ft)
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.

The survivor likened the sound to a car engine revving up. If you've listened to a good version of the phone video, you'll have noticed the "vroom" sound at the start that some likened to a motorcycle. That sound is the RAT in action, and you can imagine what that would sound like when it rapidly spins up: like a driver stepping on the throttle with their car engine in neutral. The RAT deploying is a consequence of a dual engine shutdown. It says nothing about whether the TMCA was involved.

[Now I just hope your post is still there as I post this. ]

In the CCTV footage the aircraft's flight path appears to be a parabola. If there was any thrust remaining following rotation it would be very different.

How did you determine that the path was parabolic?
By eyeball, did you measure something, did you have something to compare it to?

PPL IR engineer in air trafic flow management, so not quite the right kind of engineer to talk about physics and total energy.

Nonetheless, I ran some calculation with the Eurocontrol BADA total energy model equation using a rough estimation of the height/time profile from the CCTV footage (wingspan yardstick method) and an approximation of the drag using a constant drag hypothesis of 200 kN. That amount of drag is about what a fully configured B788 on final approach generates, I felt it was an acceptable approximation for a less draggy configuration flown at higher than normal AoA but I could be very wrong. I used the total energy equation to calculate speed, inputs are the the height gains and the drag hypothesis.

I find a B788 with a mass of 207 t and an initial speed of 185 kts with no thrust from the rotation onward could theoretically have traded 50 kts for 190 ft before descending back down at speeds between 135 kts and 130 kts, coming back to take off elevation 1950 m after the last ADSB position. The flown distance is sort of the checksum in this, it is not an input but rather the output of the model.

I also did the same exercise with the EK521 incident, assuming a 285 t B77W rotating at 155 kts for a go around on idle thrust. I guestimated 350 kN of drag (normal approach in full configuration is supposedly 380 kN) and got a speed decay to about 130 kts at 80 ft which is how high that particular aircraft got before coming back down. I am not sure this is enough to give confidence in my attempt at doing total energy calculation, but at least it doesn't seem too far off.

FR24 has the GPS lat long position at each time - ground speed is then simply distance travelled over the time interval. The METAR quoted 25007KT and that should increase with height hence the nominal decrease in ground speed over the later ADS-B values - and probably the slight drift off the centre line once airborne.

An earlier poster defined the 787 ADS-B transmits with a height granularity of 25' - which explains the FR24 figures and I might posit that it was just about to transmit a 25' height increase when the electrical failure occurred.

The rooftop video records a nominal 14s flight time with RAT out throughout.
The CCTV video records a nominal 32s (from rotation) and subjectively the aircraft stops climbing 14s after rotation - meaning 18s of descent of which 14s are captured in the rooftop video.

If we accept the RAT is out then it must have been deployed about 12-16s after rotation, presumably immediately after the 4s of ADS-B data.

Another post referenced the RAT only supplying electrical power after 10s - I find that hard to believe, not instant obviously because there has to be some stabilisation time and startup/boot time but it would imply the LH flight instruments would only be active very late. Hopefully the RAT hydraulics would be effective quicker than that.

This has been very briefly mentioned - inside a "quote" from an "official" - uptread, but I don't think has been specifically discussed here.

It "seems" that the Gujerati Anti-Terrorism Squad (ATS) have recovered a DVR (digital video recorder) from the accident aircraft:

https://www.financialexpress.com/ind...dence/3879001/

As I understand it, the ATS were there to assist as necessary, not because of any posited terrorism link. The article posits that:

The DVR in aviation settings records footage from CCTV cameras installed throughout the aircraft, including the cockpit, passenger cabin and entry/exit points. This footage is crucial for investigators as it provides visual evidence of activities before and during the flight, such as passenger boarding and ground operations.

I'd not hear that Air India had such devices installed (the EAFR combo black box on the accident aircraft does have a "growth function" for adding video recordings at some future date). What's more the DVR seen carried by a "ATS Agent" in the picture and linked videos looks to me first undamaged and second like a domestic DVD player.....

Anyway, anyone know more of a DVR fitted to Air India aircraft for investigative purposes?

ADS-B stops 330' from the end of the runway, well before top of climb. A previous poster stated that ADS-B granularity on the 787 was 25'. As the aircraft obviously climbed higher than 71', it's reasonable to assume that ADS-B was about to transmit 96' when the electrics failed. Looking at the CCTV, subjectively, the aircraft reaches the height of a wing about 7s after rotate and top of climb about 7s later.

By wingspan, I mean one tip to the other tip.

Disclaimer: I'm not a pilot, only a non-aviation engineer, and so I'm a bit hesitant to criticise this. And yet:

The determination of the height AGL seems quite incorrect - it completely ignores the local pressure and temperature, which very much need to be corrected for. Applying those corrections even using rough rules of thumb (~30 ft per mbar, ~4 feet per \xb0C per 1000 ft) gives a figure that's around ~100 feet AGL. The follow-up sanity check also fails even without knowing the correct math because if you match the ADS-B data (timestamp + location) to when & where the aircraft lifts off and starts to climb in the CCTV video (versus just assuming that the peak height seen in the video matches the last ADS-B data point), the aircraft is very much not 300 feet above the ground when its transponder reports an altitude of 625 feet.

Also the estimation of the glide ratio with flaps 5, gear and the RAT deployed being 3.5 to 1 seemed incredibly low to me, but I'm the first to admit that I have nowhere near as much knowledge of gliding performance as I do of weather math. So I looked up the closest incident I could think of: Air Transat 236, an A330-200 (so of a pretty similar shape, wingspan and wing area as a 787). According to the final report (link: https://www.fss.aero/accident-report...1-08-24-PT.pdf ), the aircraft arrived at a fix approximately 8 nautical miles (48609 ft) from the runway at an altitude of approximately 13000 ft, at which point the crew decided to execute a 360 turn to lose altitude as well as to extend the slats and landing gear during the turn (the RAT of course had already long been deployed at this point), both to prepare for landing and to help further lose altitude. Sure there's some rounding here, and my understanding is that "flaps 1" on Airbus aircraft deploys only leading edge and not trailing edge devices, but this already suggests that their expected glide ratio was significantly higher than the raw 48609:13000 ratio (~3.74:1).

They re-established themselves on final in their landing configuration at an altitude of ~8000 feet and a distance of 9 nautical miles (54685 ft), so let's say that the true distance was somewhere between 8 NM and 9 NM to account for rounding. That gives a glide ratio of between ~6:1 and ~6.83:1. But on top of that, the crew still had to execute a series of S-turns to lose enough altitude to actually make the runway, so their "dirty" glide ratio must have been even higher than that. Unfortunately I don't think it's possible to determine conclusively what the ratio was since we don't know how many track miles were added on by the turns (the flight data recorder stopped when the engines flamed out and human testimony only goes so far), and again they did not have flaps extended, but I think it's fair to say that a glide ratio of 3.5:1 is a wildly low estimate for an airliner of the 787's calibre even with the gear down.

Sorry if this is off-topic or too much rambling, but considering how much speculation there tends to be both in this thread and elsewhere about real-world glide performance (especially in non-ideal configurations), hopefully these details are helpful.

Just before posting this reply, I noticed that nachtmusak posted something similar to what I wrote below. Since I spent quite a lot of time and effort fact checking that document and composing this reply, and I think it might clarify things a bit further, I decided to still post this. I'm not a pilot, my main field is software engineering but I also studied power engineering for a while. Watching the aviation world is just a hobby. I've read all posts in both threads since the beginning, but so far I refrained from making a post of my own. If this post is considered redundant, I don't have a problem with it is deleted.

Richard Godfrey first 7 points seem accurate as far as I can tell, but he makes a lot of mistakes at point 8, when he computes the altitude the aircraft reached. The first error is when he said "At liftoff from 180 feet AMSL, barometric altimeter would read: Pressure (ISA) = 1006.68 hPa." While that is correct, that only applies when the QNH is 1013.25. But the QNH was 1001.1, so the altimeter would actually read around 994.6 hPa.

He then said: "Aircraft later reported: Pressure = 990.573 hPa (corresponds to 625 feet in ISA conditions).". That is correct. Then he subtracts the two numbers, 1006.68 hPa and 990.573 hPa and gets a 16.107 hPa difference, which is wrong, since the field level pressure he uses is wrong, as mentioned earlier. The correct number would be 994.6 hPa - 990.573 hPa = 4.027 hPa difference. Using the approximation of 30 ft/hPA, gives us an actual around 120 ft altitude over the field (+/-12.5ft since the aircraft reported altitude has a 25ft granularity).

Instead, he gets a much higher altitude of 480ft over the field and, as a further mistake, he misinterprets it as altitude over sea level, so he subtracts 180ft (the field's altitude). This second error bring the value closer to reality, around 300ft, but still a long way from the correct value which would be around 120ft. I also did a quicker back of the envelope calculation with a different method, and got 80ft +/- 12.5ft, even further from his 300ft.

Then, he makes a third mistake, he assumes that the last altitude reported by the aircraft is the peak altitude it reached, when in fact it is almost certain the aircraft kept climbing for a while after the last ADS-B datapoint (the last datapoint was reported when the aircraft was still above the runway). So it might have indeed reached 200ft or more after all.

Finally, at point 9, he makes an estimation based on the ratio between the wing span and the altitude in the CCTV video, compared to the actual wing span. That's a good idea in theory, but you need the camera to be exactly behind the aircraft and very far away not to be affected by perspective significantly. For example, if you film the aircraft from a 45 degrees angle, the apparent wingspan would be reduced by about 30%.

I didn't fact check points 10 to 19, since most of them relied on incorrect numbers computed earlier.