For the accident, why did the aircraft veer off the runway near the end of the runway, instead of the moment it touched down (where the left engine went into reverse thrust and the right engine powered up to climb throttle)? The aircraft managed to maintain a (relatively) straight path for most of the runway length. I thought asymmetrical thrust that significant would immediately send it off course
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Right Engine thrust
The engine takes about 12 seconds to spool up from idle to full thrust.
Auto-throttle had held both engines near idle during the approach to touchdown. The throttle levers should both be retarded from "climb" to "idle" position prior to touchdown. But only in the last two seconds before touchdown, after the aircraft emitted a warning, was the left throttle retarded - but the right throttle lever was not moved from the "climb" position.
When the left throttle lever was retarded, this disengaged auto-throttle, so the right engine started to spool up to "climb" power setting as commanded on the right throttle lever.
So immediately after landing the right engine wasn't producing any thrust.
The thrust only developed gradually during the following 12 seconds as the aircraft traveled down the runway.
Rudder authority
At landing the aircraft might be travelling at 140 knots. At this speed, deflection of the rudder by the pilot can still have significant effect and can counteract asymmetric engine thrust.
As the aircraft traveled down the runway, airspeed would have reduced and rudder authority gradually diminish at the same time as the right engine was spooling up to CL (climb) power.
The aircraft ran off the runway at about 90 knots.
Nosewheel steering
When using the rudder pedals for nose wheel steering, the steering angle starts to reduce at 40 knots and progressively reduces to zero degrees at 130 knots.
According to the accident report,
When the accident happened, the volume of precipitation was 0.6 mm.
So a combination of speed and a wet runway may have reduced the effectiveness of nosewheel steering.
I don't know, but I suspect that nosewheel steering may not be able to fully compensate for the yawing effect of left engine at idle and right engine at climb power.
Differential braking
The FDR also recorded the use of the rudder and the differential braking by the pedals as the aircraft veered to the left (probably in an attempt to maintain the plane on the runway and stop it).
It seems that the crew really needed to shutdown or idle the right engine to complete a landing within the boundary of the runway.
answered Aug 27, 2015 at 16:18
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$\begingroup$ The effectiveness of nosewheel steering depends a lot on c.g. position. With a c.g. near the rear limit it would likely be overpowered by asymmetric thrust. $\endgroup$ Commented Oct 31, 2015 at 16:42
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$\begingroup$ @PeterKämpf Are you saying that the accident aircraft was near the aft limit at the time of landing, so this was a factor? $\endgroup$ Commented Nov 1, 2015 at 0:57
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2$\begingroup$ @Lnafziger: No, all I am saying that without knowledge of the cg position it is hard to know whether nose wheel steering was effective or not. $\endgroup$ Commented Nov 1, 2015 at 12:55
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$\begingroup$ I think 12 seconds spool up time is a good number for a A320 at ground-idle whilst standing still, but not when the engine is in flight-idle and moving at 140 knots. In the latter case, I would expect the engine to spool up within a few seconds. I believe the maximum allowable time in certification is 8 seconds. $\endgroup$– DeltaLima ♦Commented Oct 13, 2023 at 8:50