Braking before nose wheel touchdown
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Braking before nose wheel touchdown
Do any of you use wheel braking in normal conditions inbetween main gear and nosewheel touchdown? My type has no autobrakes so I smoothly apply the brakes immediately the nose gear touches down. Just been reading about hard nose wheel touchdowns / excessive de-rotations and wondered if braking would increase the risk of a hard nose gear touchdown.
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I reckon the basic laws of physics apply here.
If you apply brakes before the nose is down, it will generate a nose down force commensurate with the achieved braking.
If you do it before either spoilers come out or the AOA is lowered, you'll most likely get some mighty flat spots on the tyres, not a lot of force and a smooth nose touchdown.
With the weight on the tyres, you'll more than likely get a good smack.
With this in mind, aircraft with spoilers and the like tend to have systems fitted to prevent early brake applications often linked to nose gear WOW switching and/or main gear wheel speed.
Thrust reverse or prop discing before nose gear touchdown is pretty common though, but you'd better be sure of symetrical application beforehand, mind, hence usual SOP not to do it.
Hope that helps.
If you apply brakes before the nose is down, it will generate a nose down force commensurate with the achieved braking.
If you do it before either spoilers come out or the AOA is lowered, you'll most likely get some mighty flat spots on the tyres, not a lot of force and a smooth nose touchdown.
With the weight on the tyres, you'll more than likely get a good smack.
With this in mind, aircraft with spoilers and the like tend to have systems fitted to prevent early brake applications often linked to nose gear WOW switching and/or main gear wheel speed.
Thrust reverse or prop discing before nose gear touchdown is pretty common though, but you'd better be sure of symetrical application beforehand, mind, hence usual SOP not to do it.
Hope that helps.
G'day Mungo,
On the B767 the autobrakes activate well before the nosewheel is on the ground, when 'truck untilt' occurs according to the manual, and from aircraft to aircraft the level and speed of application varies quite a bit, regardless of the setting, and the ensuing pitch control is not a problem even at max autobrakes.
The B707 I flew in my previous life didn't have autobrakes and the brakes were not applied until the nosewheel was on the runway.
Regards,
BH.
On the B767 the autobrakes activate well before the nosewheel is on the ground, when 'truck untilt' occurs according to the manual, and from aircraft to aircraft the level and speed of application varies quite a bit, regardless of the setting, and the ensuing pitch control is not a problem even at max autobrakes.
The B707 I flew in my previous life didn't have autobrakes and the brakes were not applied until the nosewheel was on the runway.
Regards,
BH.
Last edited by Bullethead; 7th Dec 2008 at 11:39.
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Originally Posted by Fredairstair
With the greatest possible respect..... could I suggest RTFM (if you have one)
My type has no autobrakes so I smoothly apply the brakes immediately the nose gear touches down.
Thanks Bullethead I thought as much because whenever I'm down at LHR I watch the big stuff land and sometimes they touchdown and hold the nosegear for a very long distance, certainly further than the distance I need to land and vacate in my type. They also appear to slow down considerably while the nosegear is still up and therefore must surely be braking.
On the type I fly there is indeed a sophisticated anti-skid protection, locked wheel protection and touch-down protection. Touchdown protection inhibits main gear braking for 3 seconds after touchdown or until wheel speed is above 50kts. This system is linked to the main gear weight-on-wheels proximity switches only (I have my mauals infront of me Fredairstair), but is not linked to the status of the nosegear so I could brake before nose gear touchdown, but since the nose tends to drop quite heavily anyway it would be foolish to try this hence the question about other types.
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because whenever I'm down at LHR I watch the big stuff land and sometimes they touchdown and hold the nosegear for a very long distance, certainly further than the distance I need to land and vacate in my type. They also appear to slow down considerably while the nosegear is still up and therefore must surely be braking.
Even on my minibus, and a decent runway, if I have a brake cooling issue, it is easy to delay braking to well below 100KIAS
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Nigel
I'm assuming you aren't referring to all Boeing types? My reading of Various FCTM (apart from the 747-400) tells me that holding the nosewheel off unduely does not help with braking performance, moreover on the type I am most familiar with the 757/767 the FCTM categorically states that the effect of holding the nosewheel off on braking is not significant and therefore not advised.
considerable Aerodynamic effect of a pitched up aircraft with Full Flap gives a LOT of Drag...
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777-200/300er
On the 777, autobrake application occurs just after main gear touchdown when
1. Both thrust levers are at idle, and
2. the wheels have spun up.
The only time this is different is with MAX AUTO, when Autobrake will limit the deceleration to Autobrake 4 until pitch angle is less than 1 degree, and then will increase braking to Autobrake Max.
1. Both thrust levers are at idle, and
2. the wheels have spun up.
The only time this is different is with MAX AUTO, when Autobrake will limit the deceleration to Autobrake 4 until pitch angle is less than 1 degree, and then will increase braking to Autobrake Max.
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B744
It is our SOP to always select autobrakes for landing (1,2,3,4 or max).
After landing, autobrake application begins when:
-All thrust levers are closed
-Ground mode is sensed
-The wheels have spun up
To maintain the selected deceleration rate, autobrake pressure is reduced as other factors, such as thrust reversers and spoilers contribute to the deceleration rate.
Braking occurs irrespective of the nosewheel position, which is normally smoothly lowered on during braking using the elevators.
After landing, autobrake application begins when:
-All thrust levers are closed
-Ground mode is sensed
-The wheels have spun up
To maintain the selected deceleration rate, autobrake pressure is reduced as other factors, such as thrust reversers and spoilers contribute to the deceleration rate.
Braking occurs irrespective of the nosewheel position, which is normally smoothly lowered on during braking using the elevators.
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The 747 Classic is similar to the 777. Min or Med Autobrakes (as selected) are applied at main gear touchdown (via tilt switches that indicate the entire truck is on the ground). If Max Autobrakes are selected (VERY rare), Med are applied until the nose wheel WOW switch is activated.
Autobrakes are used routinely, so brakes are applied before nosewheel touchdown about 95%of the time.
Autobrakes are used routinely, so brakes are applied before nosewheel touchdown about 95%of the time.
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Thanks guys, interesting responses. So it seems that braking doesn't make the nose slam down as one might expect. This ties in with watching 744s land at LHR, holding the nose off for ages whilst still slowing down considerably.
All correct, but on the 744 while at higher autobrake settings the nose does want to come down, its up to you to 'fly it' gently onto the runway. If you want to hold it off some effort is required, but I feel better braking and more importantly, directional control is with it on the ground. I am not a fan of 'aerodynamic braking' due to risk of tail strike if you over cook it. Also our company only selects reverse with the nose wheel on the ground. Again follow the manual and SOPs, you cannot go wrong. The 727 has nose wheel brakes, for example, not much point having the sharp end in the sky.
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The 727 has nose wheel brakes
A useful mod and VERY effective if needed. However, if you need, intentionally, to invoke the nose brakes .. then you are either in a spot of bother by design (RTO) or inadvertently have run out of ideas and options ?
A useful mod and VERY effective if needed. However, if you need, intentionally, to invoke the nose brakes .. then you are either in a spot of bother by design (RTO) or inadvertently have run out of ideas and options ?
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Boeing don't recommend aerodynamic braking, they say get the nose wheel down ASAP. Having done all the tests on the various types Boeing concluded that there was little to no advantage to holding the nose wheel off. Personally have done a complete 737 conversion at Boeing and 757/767 and 744 with Boeing instructors, they all say the same.
Sitting at the holding point you often see the earlier Airbuses, especially the A310 with a very high nose/pitch attitude on touch down and some hold it there, I don't know why. I agree with the above posts, get it on the ground.
I've used A/B 4 on a light weight 777-200 to get the first taxyway and the nose DID pitch down very quickly and drop onto the runway with a bump!!
Since then in the same situation I've used A/B 2 and as soon as the nose wheel is on the ground I disconnect the A/B and still make the first taxyway.
Ground roll about 2000'
Since then in the same situation I've used A/B 2 and as soon as the nose wheel is on the ground I disconnect the A/B and still make the first taxyway.
Ground roll about 2000'
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Yes watching the big stuff land with normal (low autobrake settings) you will see the nose coming down later rather than sooner as most of us try to fly it on smoothly...
However if you ever see an Airbus land with more than low (200) or autobrake 3 (600) the nose tends to come down very smartly as soon as the autobrakes apply! In my experience with the 200 medium autobrake means the nose is coming down and there is not enough elevator to fly it on. Autobrake applies and wack!!
Physics rule!!!
However if you ever see an Airbus land with more than low (200) or autobrake 3 (600) the nose tends to come down very smartly as soon as the autobrakes apply! In my experience with the 200 medium autobrake means the nose is coming down and there is not enough elevator to fly it on. Autobrake applies and wack!!
Physics rule!!!
