Originally Posted by
Astra driver
In an earlier post I had hypothesized that this was not an overrun accident, but the presence of tire skid marks running through and off the end of the 1,000ft overrun area would seem to indicate it is just that. What is still baffling to me is that if this is an aborted take-off overrun, why did they not deploy their thrust reversers? I can only say that from having to perform a near V1 abort myself (due to multiple bird strikes) I was practically bending the T/R levers backwards along with pressing the brake pedals to the firewall when I saw the end of the runway coming up. The setting of Flaps 10 is equally puzzling to me from this airport, all Gulfstream jets use Flaps 20 as an almost default take-off setting except when taking off from high density altitude airports with long runways, where second segment OEI climb gradient becomes a greater performance limititation than BFL. That being said, I am aware of some crews preferring to use Flaps 10 as well as a minimum "Flex" (reduced power setting) in order to give their passengers a smoother take-off and less steep initial climb out angle. If this was the case it would likely increase the BFL from well under 4,000ft (Flaps 20, min EPR) by about an additional 2,000ft to just under 6,000ft, but still not enough to explain on overrun of 2,000ft beyond a 7,011 ft runway.
A few years ago, I spent a couple of months riding in the jumpseat of a GIV, being operated in Africa. Due to the lack of mx services at many of the airports, the aicraft owner wanted a mechanic onboard for all trips.
The only time I ever saw flaps 10 used for takeoff (as opposed to 20) was the very scenario you describe. Departing heavy from Abuja, Nigeria, on a day that was close to 40C. The crew had a long preflight discussion about second segment climb performance - most of which went over my head - but the decision to do a flaps 10 departure was determined to be the only safe option. I do recall that ground roll and v-speeds were quite a bit higher than what I was used to seeing.
Still, I'm no expert as to the operational/piloting aspects of the GIV, other than knowing that it would be difficult to accidentally takeoff with flaps 10 (when 20 was intended) due to the conflict between actual flap position and FMS takeoff init, which would prevent v-speeds from displaying if the two did not correspond.
Hard to say about the thrust reversers based on the photo. Both T/R and ground spoiler deployment require positive weight-on-wheels, but as I mentioned in another post, the T/R "buckets" are held open solely by hydraulic pressure via their actuators - there is no locking mechanism to hold them open. If hydraulics were instantaneously lost due to the ongoing disintegration of the fuselage as the overrun progressed, combined with rapid deceleration of the airframe, I can envision a scenario when the T/R doors POSSIBLY could have been thrown forward and (mostly or completely) closed from inertia.
One of the news reports on Sunday, quoted a woman who lived near the airport who said that the engines were "really screaming" just before the sound of the crash and breakup. Cound have been a last ditch attempt to firewall the engines to get airborne, but based on the skid marks, equally likely to have been the sound of an attempt to apply max reverse thrust.
Hopefully the FDR and CVR will shed some light.