Posts by user "Tailspin Turtle" [Posts: 11 Total up-votes: 7 Pages: 1]

My impression was very similar: they never had enough thrust for level flight with the gear down, used up the whole runway accelerating to something close to Vr, rotated and started to climb but very soon started slowing and had to reduce the angle of attack, starting the descent.

I wondered about that as well. The 787 reportedly has an L/D of 21. Unfortunately, Google didn't provide me with a gear-down L/D (one data set stated landing L/D was 7 but presumably that includes landing flaps as well) but it is clear from some technical reports that it is a non-trivial reduction, maybe 44%. That suggests a gear-down L/D of 12, not taking into account headwind, change of elevation, off-optimum glide speed, etc. Obviously a higher beginning altitude and airspeed and shorter distance to the crash site that stated as well as the benefit of a flare would affect the outcome. However, the difference between an L/D of 16 and 12 given the stated beginning altitude (200 ft) and distance to the crash site (1 km = 3,280 ft) raises the possibility that the engines were still producing some thrust or were coming back on line.

Aeronautical engineer, CFI, unused ATP rating, 3,000 hours in airplanes, rotorcraft, and gliders

Thanks for those numbers. Note, however, the max 787 L/D, flaps and gear up , is reportedly 21 (best in class), which casts shade on the credibility of those L/Ds, certainly 25 with the gear down. What is your source? IF the actual L/D with the gear down and flaps 5 is somewhat lower than 16 and given that starting height and distance traveled are correct\x97and considering that the aircraft might not have been at the speed for max L/D with the gear down and the possibility of a headwind\x97then there must have been some thrust during the descent (it did look to me to be somewhat flatter in that video taken from one side than I would have expected with the gear down, a relatively slow initial speed, and no thrust) or the engines were spooling back up at the end.

Based on approximate distances and heights, the time line, the aircraft configuration, surface wind, the published L/D (gear up reduced to a guess for gear down and RAT out), and probable off-optimum speed for maximum L/D in that configuration, it's my opinion (aeronautical engineer, unused ATP rating, and glider pilot, national contests) that we can't rule out both engines being at idle or very low thrust at or shortly after rotation, rather than shut down.

In prior posts, I used approximately the same numbers and came to a similar conclusion (we can't rule out the possibility that there was a least idle thrust available after the loss of thrust), using a guess of a 44% reduction for the configuration from a max L/D of 21. Starting with the height and distance being approximate however, we also don't know what the airspeed/angle of attack was at any point in the descent versus the speed for the maximum (and unknown) L/D for the gear and flaps extended and the RAT out, the amount of headwind, the increase in L/D in ground effect (at a height less than the wingspan) over other than a smooth surface, thermal activity in a urban area, the distance gained in the flare at the end, etc.

Therefore the crew could have manually deployed it... (there's a button for that). If I saw the engines winding down and couldn't be sure that they would stop at idle, I'd be inclined to deploy the RAT instead of waiting for airplane to do it.

An admittedly approximate assessment of the airplane's lift-over-drag capability, considering other conditions (head wind, ground effect, actual airspeed vs that required to maximize L/D, etc.) suggests that the engines were providing some thrust in order for it to get as far as it did. Note that simple calculations based on conversion of energy, e.g. velocity and height, to distance are even more approximate in this scenario.

Also note that either member of the crew can deploy the RAT in addition to the automatic modes, conceivably preemptively in response to the engines losing thrust (full disclosure: at least one poster has insisted than no competent crew would ever do that in those circumstances given the multiple actions with a higher priority to be taken). Others may know if their preferred scenario explaining the simultaneous loss of thrust and automatic RAT deployment could have then resulted in the engines recovering but not soon enough to stop the descent.

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.

Yes - Higher numbers are better. The 787 has one of the best for an airliner, almost 21 power off.

Thanks - I'm pretty sure that I read all the posts in both threads but missed that calculation as to the height at the end of the runway. I had originally guessed that the top of climb was 1,000 feet beyond the end of the runway (the current location is based on the referenced statement of the rotation point and an assumed ground speed, not air speed, of 180 kts). That still doesn't get the jet to the crash site, particularly if the post I relied on that it was 50 feet below the runway is incorrect. As far as the benefit of trading speed for distance, there wasn't that much extra speed to start with relative to the likely maximum L/D speed for that configuration and any slowing below it will reduce distance, not increase it, except of course for breaking the glide, i.e. flare at the end (there may have been a little benefit in rounding off the transition from climb to glide that I didn't take into account but I think it was small). My estimate for L/D based on known comparables that didn't include the RAT was actually 12, not 13, and I assumed that they were flying at the max L/D airspeed for that configuration even though it's likely that the crew didn't know what it was (and neither do I) but were following the prime directive, "don't stall". I also didn't take into account the headwind, which would reduce the maximum L/D available and require a slightly faster airspeed to make good than for no wind.