I thought it might be interesting to try to put together some diagrams of what people on this thread have said
ought to happen to an aircraft meeting a short duration down draught.
The diagrams below depict an aircraft flying from left to right, first through still air, then through a down draught, then into still air again.
Case 1
This diagram assumes that the aircraft weathercocks instantaneously to the new relative wind direction as soon as it enters the down draught by rotating about its center of gravity. If there is no change in engine thrust or trim, the lift vector, L, will be greater than in still air because the relative wind speed is greater, and it will act at the angle “a” to the vertical adding to the induced drag.
However I think that the increased lift and drag may be negligible and the aircraft will effectively emerge again into still air at the same altitude and speed that it started with.
For example, the lift, L, in the down draught will be increased over the still air lift by the square of the increase in the relative wind, or by a factor of (v*2+ d*2)/v*2. The vertical component of this lift is [(v*2+ d*2)/v*2][v/((v*2+d*2)*0.5)] = (1+(d/v)*2)*0.5.
We can get an idea of how big this factor (1+(d/v)*2)*0.5 is from some measurements made by NASA in 2000, when they flew an instrumented Boeing 757 through convective turbulence and in two encounters rated as “severe turbulence” measured maximum down draughts of 15.00 m/s and maximum up draughts of 18.41 m/s. However, these flights deliberately avoided direct entry into regions with the strongest radar returns.
If we assume a down draught twice as strong as the strongest measured by NASA, then d=30 m/s and the increased factor derived above (1+(d/v)*2)*0.5 = 1.02 if the aircraft is moving at 280 kts (as was NW 705), i.e. the vertical component of lift increases by only 2% with a down draught twice as strong as the strongest measured by NASA. So as I suggested above, these effects may be negligible and the aircraft may effectively emerge again into still air at the same altitude and speed that it started with.
Case 2
This case assumes that the aircraft takes a finite time to weathercock into the new relative wind.
In the first transition region the plane will experience a jolt downwards which will cause it to start to lose altitude, and during the second transition a very similar jolt upwards which will leave the aircraft flying about horizontally and at about the same speed but at a lower altitude. The wind and lift vectors are essentially the same as in Case 1. The difference between Case 1 and Case 2 is only that the jolt down initiates a loss in altitude which continues until the second, upward, jolt.
Case 3
This case assumes that in addition to everything that happens in Cases 1 and 2, each jolt will start a phugoid, the first initiating a pitch up rotation, while the second initiates a faster, potentially larger amplitude, pitch down rotation (assuming that there is no input from the pilot).
This sequence in Case 3 is reminiscent of the words in the CAB report on the NW 705 incident quoted in an earlier post (but talking about an up draught rather than a down draught):
Up draught causes
an initial weather cocking pitch down into the relative wind
followed “ultimately” by a pitch up and a gain in altitude.
Down draught causes
an initial weather cocking pitch up into the relative wind
followed “ultimately” by a pitch down and a loss in altitude.
I am sure that these diagrams are all too simplistic and that other effects come into play, but these sketches incorporate my impression of what people on this thread have been saying
ought to happen when an aircraft flying straight and level meets a short-duration down draught with no input from the pilot.
An extended abstract of the NASA turbulence measurements is available in pdf format at:
http://ams.confex.com/ams/13ac10av/1...acts/40038.htm
Cheers,
