EDML
2025-07-01T11:38:00
permalink Post: 11914210

Quote:

Originally Posted by Tailspin Turtle

This is my latest attempt to square the circle using all the data points and minimal assumptions. The main shortcoming of the analysis is not knowing the maximum L/D and the speed for maximum LD with the gear down, flaps 5, and the RAT extended. However, if I use a reasonable number in my opinion for the L/D in that configuration and assume that the airplane is being flown at the speed for it, it will not get to the crash site. The distance from the runway of the crash site is from a previous graphic (1.55 km); the rotation point from fdr, permalink 314; 200 feet max height above the runway being generally accepted; crash site 50 feet below the runway elevation cited previously. An average speed of 180 knots is consistent with the dimensions given and 30 seconds flight time. A flare at 50 feet will briefly increase the L/D to 20, maybe even 30 (500 feet more than shown) but still not enough to make up the shortfall, In fact, with a head wind the L/D will be lower than assumed as well as if the speed being flown is higher or lower than required for maximum L/D in that configuration. In other words, there must have been some thrust available.

You overlooked that they (the pilots) were trading speed for range/time. The aircraft slowed down by around 50kts while gliding. That is a lot of extra energy to use for range. It's visible in the video that the AoA slowly increases during the glide (I don't mean the flare at the end).

2 users liked this post.

nachtmusak
2025-07-01T12:06:00
permalink Post: 11914222

Quote:

Originally Posted by Tailspin Turtle

There is easily-correctable available data with the aircraft's altitude at pretty much the end of the runway and it is not at 200 feet (it's around 100\xb112.5 feet).

As the aircraft visibly continues to climb past that height (and for a longer period than ADS-B data covers, if the camera's perspective casts doubt on that), it seems rather clear to me that it reached its peak height past the end of the runway.

In light of this I find the fact that people keep calculating a glide from the runway to the crash site to be a bit strange. Wouldn't the first step of any math be to try to determine where it started descending?

adfad
2025-07-01T13:36:00
permalink Post: 11914278

Quote:

Originally Posted by Someone Somewhere

Thrust is non-linear and complex. Reaction engines (i.e. fans, props) are generally most efficient at minimum power - lowest excess velocity. Turbine engines are generally most efficient at high power. These cancel out somewhere in the middle. With two engines at low power, you also don't have the drag from the dead engine or the drag from the rudder countering yaw.

Cavitating destroys pumps rapidly - someone upthread said replacing the fuel pump immediately is SOP if it has suction fed. Expect end of life in tens of hours rather than tens of thousands.

Some aircraft have switched to using jet/venturi pumps powered by returned fuel, like the A220. The electric boost pumps there are mainly for redundancy and are shut down in cruise; only one in each wing tank. Some A320s replace the centre override pumps with venturi transfer pumps.

Thanks for the clarifications

My question is then: what is the minimum loss of thrust in both engines (perhaps more relevantly expressed as a % in fuel flow reduction from expected) that could produce the profile we saw. I appreciate this is a figure with many variables including timing and rate of loss.

The reason I think this question is relevant is because we pretty much have 2 prevailing theories at this point:

A failure, or reduction of thrust (below idle, indicated by loss of AC generators), that somehow impacted both engines, within 20s of rotation (explaining the RAT and gear orientation)
Somehow a loss of all AC power, leading somehow to a reduction of thrust or failure of engines (both engines impacted identically is assumed in this scenario since all AC is lost), and was of course below the minimum thrust needed to fly with gear down at this weight and temperature

I agree that if it is completely infeasible that loss of all AC power could do anything but cause thrust reduction of X where thrust minus X is not enough, even with gear down in high temperature and significant weight at the critical moment of takeoff to cause the profile we saw, then theory 2 is invalidated. I would love to invalidate any of the theories here but I do think some specific calculations, simulations or test data is needed

EDML 2025-07-01T11:38:00 permalink Post: 11914210	Quote: Originally Posted by Tailspin Turtle This is my latest attempt to square the circle using all the data points and minimal assumptions. The main shortcoming of the analysis is not knowing the maximum L/D and the speed for maximum LD with the gear down, flaps 5, and the RAT extended. However, if I use a reasonable number in my opinion for the L/D in that configuration and assume that the airplane is being flown at the speed for it, it will not get to the crash site. The distance from the runway of the crash site is from a previous graphic (1.55 km); the rotation point from fdr, permalink 314; 200 feet max height above the runway being generally accepted; crash site 50 feet below the runway elevation cited previously. An average speed of 180 knots is consistent with the dimensions given and 30 seconds flight time. A flare at 50 feet will briefly increase the L/D to 20, maybe even 30 (500 feet more than shown) but still not enough to make up the shortfall, In fact, with a head wind the L/D will be lower than assumed as well as if the speed being flown is higher or lower than required for maximum L/D in that configuration. In other words, there must have been some thrust available. You overlooked that they (the pilots) were trading speed for range/time. The aircraft slowed down by around 50kts while gliding. That is a lot of extra energy to use for range. It's visible in the video that the AoA slowly increases during the glide (I don't mean the flare at the end). 2 users liked this post.
nachtmusak 2025-07-01T12:06:00 permalink Post: 11914222	Quote: Originally Posted by Tailspin Turtle This is my latest attempt to square the circle using all the data points and minimal assumptions. The main shortcoming of the analysis is not knowing the maximum L/D and the speed for maximum LD with the gear down, flaps 5, and the RAT extended. However, if I use a reasonable number in my opinion for the L/D in that configuration and assume that the airplane is being flown at the speed for it, it will not get to the crash site. The distance from the runway of the crash site is from a previous graphic (1.55 km); the rotation point from fdr, permalink 314; 200 feet max height above the runway being generally accepted; crash site 50 feet below the runway elevation cited previously. An average speed of 180 knots is consistent with the dimensions given and 30 seconds flight time. A flare at 50 feet will briefly increase the L/D to 20, maybe even 30 (500 feet more than shown) but still not enough to make up the shortfall, In fact, with a head wind the L/D will be lower than assumed as well as if the speed being flown is higher or lower than required for maximum L/D in that configuration. In other words, there must have been some thrust available. There is easily-correctable available data with the aircraft's altitude at pretty much the end of the runway and it is not at 200 feet (it's around 100\xb112.5 feet). As the aircraft visibly continues to climb past that height (and for a longer period than ADS-B data covers, if the camera's perspective casts doubt on that), it seems rather clear to me that it reached its peak height past the end of the runway. In light of this I find the fact that people keep calculating a glide from the runway to the crash site to be a bit strange. Wouldn't the first step of any math be to try to determine where it started descending?
adfad 2025-07-01T13:36:00 permalink Post: 11914278	Quote: Originally Posted by Someone Somewhere Thrust is non-linear and complex. Reaction engines (i.e. fans, props) are generally most efficient at minimum power - lowest excess velocity. Turbine engines are generally most efficient at high power. These cancel out somewhere in the middle. With two engines at low power, you also don't have the drag from the dead engine or the drag from the rudder countering yaw. Cavitating destroys pumps rapidly - someone upthread said replacing the fuel pump immediately is SOP if it has suction fed. Expect end of life in tens of hours rather than tens of thousands. Some aircraft have switched to using jet/venturi pumps powered by returned fuel, like the A220. The electric boost pumps there are mainly for redundancy and are shut down in cruise; only one in each wing tank. Some A320s replace the centre override pumps with venturi transfer pumps. Thanks for the clarifications My question is then: what is the minimum loss of thrust in both engines (perhaps more relevantly expressed as a % in fuel flow reduction from expected) that could produce the profile we saw. I appreciate this is a figure with many variables including timing and rate of loss. The reason I think this question is relevant is because we pretty much have 2 prevailing theories at this point: A failure, or reduction of thrust (below idle, indicated by loss of AC generators), that somehow impacted both engines, within 20s of rotation (explaining the RAT and gear orientation) Somehow a loss of all AC power, leading somehow to a reduction of thrust or failure of engines (both engines impacted identically is assumed in this scenario since all AC is lost), and was of course below the minimum thrust needed to fly with gear down at this weight and temperature I agree that if it is completely infeasible that loss of all AC power could do anything but cause thrust reduction of X where thrust minus X is not enough, even with gear down in high temperature and significant weight at the critical moment of takeoff to cause the profile we saw, then theory 2 is invalidated. I would love to invalidate any of the theories here but I do think some specific calculations, simulations or test data is needed

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