PPRuNe Forums Poster/Comments:
Careful examination of the OSCAR/NOAA surface current data provides 065°T x 22.5cm/sec at 3°N 31°W over the 5 day period centered on 2 June 2009.
I have therefore examined the positions in which bodies were recovered from on 6/7/8 June and constructed a likely current line based on what we know, i.e. that for the 3 days just mentioned and a calculated rate for the 5 - 6 June of 19cm/sec (9NM/day) back to the time of the accident at about 02:14:30Z on 1 June of 22.5cm/sec (10.5NM/day).
The reason for using the bodies as a check on the current is that they will have initially sunk to a point of equilibruim, and provided the depth was not too great, the water temperature would have commenced the decomposition process.
Then over a period of time each of these bodies would have gained enough buoyancy to become visible on or near the surface - which explains the number of days it took to find those that they did.
The point is that the bodies will have been subject to little or no leeway effects due to the surface wind. SHOM data shows that large easterly vectors on the surface become small westerly vectors the deeper you go, which helps to explain why some debris items floating with possibly little or no windage have been found to the east of the general drift line in which the bodies were found.
The graphic below shows 2 significant cumulonimbus cells, the one on the track and another left of the track shortly after passing ORARO. It seems that each of these mesoscale cells has played a part in this incident.
I surmise that for some unknown reason the WX radar has not revealed the presence of the cell the a/c penetrated at around 0209, but when everything turned pear shape at 0210 the PF made a decision to get out of the ITCZ and commenced a lefthand 180 and descent hand flying the a/c with somewhat degraded control systems provided in Alternate/Direct law.
The lefthand turn was unfortunately taking the a/c toward the Cb cell NNW of ORARO.
What happened during the SATCOM outage between 0213 and 0214 is of course speculative, but at some point in this rapid descent it can be assumed that IAS became available and an effort was made to stabilize the rate of descent. If a nose up attitude was adopted, the updraft associated with the next Cb cell may have resulted in a flameout of both engines.
Well the graphic shows the general idea, but if the current vector at 3°N 31°W was in fact 055°T x 20cm/sec, the impact point would have been about 10NM further east. This would give better GS but with a tighter turn - to be expected if the speedbrakes were deployed.
Thank you for this nice work.
I visualize the final trajectory in a very similar way than yours, two aspects expected maybe:
- this large route deviation would be unvoluntary, this would be the horizontal trace (large roll perturbation) of the high altitude cruise loss of control, this would occur as a consequence of a large exceedance of the MMO (Mach>0.89-0.90) and this severe overspeed would require ~1mn or over to occur [*]: the tight turn would start around ~02:11:00Z
- this turn would not be a constant load factor turn (a part of a circle) with such a high constant curvature radius but a trajectory where the shortest radius (highest load factor, highest roll) is at the beginning of the initial route departure and where it decreases (roll is being controlled) in less than one minute.
This tight turn would probably go with a rapid loss of lift/altitude ? Once the roll is back under control, the pilots were in position to try to regain control in the vertical plane ?
(AoA/incidence, pich). This would be the final part of the trajectory: rather linear in the horizontal plane and in the vertical plane, a rapid loss of altitude (~10 000 fpm), the AoA/incidence decreasing in a first time to regain the aerodynamical authority (also with the loss of altitude and the increasing air density - stall recovery) and increasing again to generate enought vertical Gs to try to break the catastrophic descent.
The plane being "en ligne de vol" (straight horizontal trajectory / wings leveled, small horizontal speed component / speed mostly vertical, possibly a slight nose up)
Indeed the drifting of the bodies or of the debris can be very different if you look where was recovered the left wing spoiler (this latter, recovered north of TASIL, seems like an outlier in the debris distribution, it has not been much deviated westward by the westernly surface winds derived from the satellite scatterometers, see windscat).
I will have another look to the surface current values over the first days of June since I have used values slightly lower than yours (and much lower than the SHOM values). I am studying the slopes distribution of the seabed in the area where the debris should have been colocated the 1st of June at 02:15Z.
I wish to validate my computations before I produce any graph results (missing values in the numerical terrain model) but it appear that between 5% and 10% of the seabed slopes are between 25° and 50°, using a 1.25km square bin resolution (narroy faults, slope details finer than this 1.25 km are lost/not observable). This bathymetry must be a real pain: towing up and down the multibeam sounder with the relief, varying scanning speed/resolution as a function of the slope, etc... Would it be a luxury to send another high resolution sounder to probe this area ?
The spoiler you referred to was I think the starboard outer which was picked up by a merchant ship north of TASIL on 13 July. If it had detached on account of an overspeed event, then that would help to explain its recovered position not falling within the range of expectation provided by the surface current/wind data.
This would also fit in with the high Gs roll/bank to port and the high speed descent toward the suspected crash site as postulated by you in a recent post.
I have extracted the Quikscat mean surface (+10m) winds from the NOAA site and determined that the mean surface wind at 3°N 30°W for the period 1 - 7 June was from 079.3°T x 11.06 knots (5.69m/sec). The vertical stabilizer has a very low profile to windage, in fact the only significant airfoil was the small piece of empennage skin rolled up and inward on the forward end.
The leeway for the v/s shown on the graphic below was 221°T x 11.1NM with reference to the general position of the 5 bodies recovered on 7 June. Without knowing the precise timings for either position (which could have been any time between sunrise and sunset) there is some room for positional error.
The effective windage factor calculated from the graphic is 0.74% which doesn't seem unreasonable. Applying the same factor to the Quikscat data provides a leeway of 259.3°T x 12.8NM over 6.5 days. Sun shadow on a photo taken at the time of the v/s recovery indicates that the sun was near or on the meridian, i.e. 1400z.
However, when comparing the Quikscat data with the MSL analysis during the period in question, the wind vector should be more northerly. Putting that aside, the calculated result puts the v/s where it should be +/- a couple of miles.