After touchdown, when the aircraft is rushing down the runway, can a pilot hold the nose up for a longer period and use the increased drag to help in braking? Or would the a/c come to a stop quicker if the nose wheel too is put down immediately after the main landing gear touches down?
And somewhat related to this, at what point are the wheel brakes applied? I know that auto brakes are set to 1,2,3 etc during approach. But at what point (during the landing process I mean, say for example at what speed) are the brakes physically applied? I believe they are not applied while touching down or the tyres would blow out right? So when?
After touchdown, when the aircraft is rushing down the runway, can a pilot hold the nose up for a longer period and use the increased drag to help in braking? Or would the a/c come to a stop quicker if the nose wheel too is put down immediately after the main landing gear touches down?
Putting the nose wheel down upon touchdown would result in quicker deceleration.
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And somewhat related to this, at what point are the wheel brakes applied? I know that auto brakes are set to 1,2,3 etc during approach. But at what point (during the landing process I mean, say for example at what speed) are the brakes physically applied? I believe they are not applied while touching down or the tyres would blow out right? So when?
Right, auto brake application occurs slightly after the main gear touches down. Brakes begin to apply when:
a) both thrust levers are retarded to idle, and
b) the wheels have spun up.
Deceleration also increases to the selected level once the pitch angle is less than a certain degree depending on the aircraft (this should also answer your first question).
It also brings up another thought. I feel that this could be because if the brakes were applied when the pitch is very high, because the CG is forward of the main landing gear, this might slam the nose into the ground? Could that be the reason?
It also brings up another thought. I feel that this could be because if the brakes were applied when the pitch is very high, because the CG is forward of the main landing gear, this might slam the nose into the ground? Could that be the reason?
At high speeds right after touchdown thrust reversers are more effective than wheel brakes. So upon TD the bulk of the deceleration is accomplished by the TR's. After deceleration down to 90-100kts (don't quote these figures) the autobrakes kick in to wash off the rest of the speed along with the TR's. You start to feel the brakes and the deceleration rate increases. At about 75-80 knots I just take over manually and apply brakes according to where I need to get off the runway.
There is an almost 50/50 split on keeping the nose up.
Some say the aerodynamic drag is negligible and practically ineffective. Keeping the nose up longer than necessary can expose you to unwanted crosswind effects. Some guys push the elevator down further after nose wheel ground contact to apply downward pressure to maximise wheel-surface grip.
Others swear by the aerodynamic effect for deceleration so keep it up as long as possible. The downside I see with this is that with decreasing speed and lift the nose comes down rather hard causing unnecessary shock to the strut and assembly. But it is supposed to slow you down faster.
There is an almost 50/50 split on keeping the nose up.
Some say the aerodynamic drag is negligible and practically ineffective. Keeping the nose up longer than necessary can expose you to unwanted crosswind effects. Some guys push the elevator down further after nose wheel ground contact to apply downward pressure to maximise wheel-surface grip.
Others swear by the aerodynamic effect for deceleration so keep it up as long as possible. The downside I see with this is that with decreasing speed and lift the nose comes down rather hard causing unnecessary shock to the strut and assembly. But it is supposed to slow you down faster.
Oh Okay! I was 50% correct with the answer - "putting the nose wheel down upon touchdown would result in quicker deceleration". I think it would certainly depend on other factors as well like weather, surface of runway etc.
But if similar aircrafts were to be tested under identical conditions with the two braking methods (nose wheel down/up on touch down) then will there be a significant difference in the deceleration? -- I doubt .
The question bugged me all through my last pairing and had to refer to the Bible.
According to the Boeing Flight Crew Training Manual.... and I quote:
Quote:
Fly the nose wheels down smoothly onto the runway without delay. Control column movement should not be required. Do not attempt to hold the nose wheels off the runway. Holding the nose up after touchdown for aerodynamic braking is not an effective braking technique and may result in high nose gear sink rates upon brake application.
There you go dude. From the horse's mouth. Testing has been conducted and the above answer should resolve any doubts.
Holding the nose up after touchdown for aerodynamic braking is not an effective braking technique and may result in high nose gear sink rates upon brake application.
That could be because as I suspected, on application of wheel brakes, if the nose is still up, since CG is forward of the main landing gear, it might slam the nose down!
Anyway thank you for wrapping that up effectively. All doubts resolved!
On the A320 the SOPs dictate delaying of nose wheel touch down when you have a nose gear that is bent 90 degs! (remember the JetBlue 320 incident?)
Otherwise normal derotation techniques are to be applied.
The only other time i can remember holding off nose wheel touchdown is in the Cessna 152 performing soft field landings...but then the reason clearly was not to aid braking but to save a prop strike!
Regarding AutoBrakes again as far as the 320 is concerned, there are 3 settings AUTO BRK LOW MED and MAX. For landings only LOW and MED are allowed.
LOW - autobrakes are active 4 seconds after spoiler deployment and decelerates the aircraft at 1.7 MPS (meters per second square)
MED - autobrakes are active 2 seconds after spoiler deployement and decelerates the aircraft at 3 MPS
MED setting is recommended only for contaminated runways.
Usually with autobrakes OFF one does not step on the brakes as soon as the MLGs touch down! That is not even an acceptable technique for a light aircraft!
Anyway for large jets the primary deceleration immediately after touchdown is provided by spoilers and reversers. Reversers ofcourse are effective most at high speeds only.
Further dangers of holding off NLG touchdown can be caused adverse weather conditions like strong crosswind or gusty winds. The B777 tailstrike on landing at Narita, Tokyo recently is a reminder as to what can go wrong!
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(remember the JetBlue 320 incident?)