MCTSET
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Full brakes or rudder

Sun Aug 18, 2019 10:21 pm

For the pilots and engineers out the to give it a guess (pilots don’t try this). Imagine just after touchdown the pilot applied full left rudder on the left pedal and full brake on the right pedal, what direction will the aircraft turn too?

My guess is that at higher speeds such as after touchdown that the rudder would be more effective than the brakes however I could be wrong what do you guys think?
 
GalaxyFlyer
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Re: Full brakes or rudder

Sun Aug 18, 2019 11:10 pm

No idea and wouldn’t try it.

GF
 
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zeke
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Re: Full brakes or rudder

Mon Aug 19, 2019 12:25 am

Brakes would be more effective, they tell you in the qrh for jammed rudder to use differential braking. A test pilot would have had to try the procedure.
Human rights lawyers are "ambulance chasers of the very worst kind.'" - Sky News
 
Zeke2517
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Re: Full brakes or rudder

Mon Aug 19, 2019 12:48 am

That raises an interesting (at least to me) question:

When you touch down at, say, 140 knots, assuming you’re not using autobrake for some reason, does the pilot have full differential braking authority in both pedals? Like, say one were to push the top of one pedal juuust a little more than the other I’d imagine that this could be problematic at high speeds.

The obvious answer would be, “get good at pushing the brakes at the same rate.” But does the aircraft have a failsafe?
 
GalaxyFlyer
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Re: Full brakes or rudder

Mon Aug 19, 2019 1:52 am

Yes, we do have full authority and, yes, get good at using the brakes symmetrically.


GF
 
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zeke
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Re: Full brakes or rudder

Mon Aug 19, 2019 2:11 am

Zeke2517 wrote:
When you touch down at, say, 140 knots, assuming you’re not using autobrake for some reason, does the pilot have full differential braking authority in both pedals?


Auto brake does not start immediately on touchdown, there is a delay to ensure wheel spin up and weight on wheels.
Human rights lawyers are "ambulance chasers of the very worst kind.'" - Sky News
 
Zeke2517
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Re: Full brakes or rudder

Mon Aug 19, 2019 4:56 am

GalaxyFlyer wrote:
Yes, we do have full authority and, yes, get good at using the brakes symmetrically.


GF



I fully respect that point of view and your training. But my question is, What if...?

I guess it would depend on the generation of the aircraft, right? Like, what if during the course of the day a pilot didn’t take in enough salt (rushed schedule, lack of nutrition, etc.) and, at the moment of truth, got a leg cramp while trying to stop the plane. “Get good” may not be what the NTSB is looking for.
Last edited by Zeke2517 on Mon Aug 19, 2019 5:01 am, edited 1 time in total.
 
CATIIIevery5yrs
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Re: Full brakes or rudder

Mon Aug 19, 2019 4:57 am

A strong crosswind will somewhat lead to your example, although not intended by the pilot.
 
spacecadet
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Re: Full brakes or rudder

Mon Aug 19, 2019 6:49 am

Zeke2517 wrote:
I fully respect that point of view and your training. But my question is, What if...?

I guess it would depend on the generation of the aircraft, right? Like, what if during the course of the day a pilot didn’t take in enough salt (rushed schedule, lack of nutrition, etc.) and, at the moment of truth, got a leg cramp while trying to stop the plane. “Get good” may not be what the NTSB is looking for.


You can say this about literally anything a pilot is supposed to do. Being a pilot is a skill-based job. Sometimes people forget that. It's not just about sitting there pushing buttons. The most basic standard for any pilot is being able to control the plane.

Why not worry about rudder control on takeoff? Or not rotating at the correct speed or angle? Or having a tailstrike on either takeoff or landing? Or what we say just today - a hard landing that damages the airplane? Or a mid-air collision? Or any number of other things?

This is what pilots train for.
I'm tired of being a wanna-be league bowler. I wanna be a league bowler!
 
unimproved
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Re: Full brakes or rudder

Mon Aug 19, 2019 8:49 am

It's not like a slight difference in pedal pressure will run it off the runway. You might even need it to compensate for winds. If the pilot feels it moving to one side it's his skill to point it back, just like you would do in a car.
 
CosmicCruiser
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Re: Full brakes or rudder

Mon Aug 19, 2019 10:59 am

for the situation as the poster asked one should remember that with "full" rudder at touchdown the nose wheel steering will come into play as well. FWIW
 
chimborazo
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Re: Full brakes or rudder

Tue Aug 20, 2019 3:04 am

CFI at my flying club has the mantra “Do what you need to do” ie take positive action (which can also be positively doing nothing) in all situations to maintain control of the aircraft.

If you’re Landing with a crosswind and you are getting steered by the wind after touchdown by all means use some differential brake.

Some people take 5 mins to be able to learn how to control a plane manually, some people spend hours and hours and still can’t do it. First time I steered with my feet on the ground it felt weird but I got used to it pretty quickly and became better the more hours spent doing it. Now I don’t consciously think about it.

Airline pilots and of course most others are skilled enough to pre-empt and react to to how the aeroplane is behaving... it’s quite a natural feeling even as a PPL to feel/see the aircraft behaving in a certain way. Your nose is going right so you press a little more left foot without even thinking about it. Use the controls as necessary. It’s not a conscious decision most of the time as it’s what you train to do. The only time I can think of conscious decisions for control inputs are: control freedom check preflight on GA and before takeoff, soft field take off increased pitch up command and aileron deflection at start of a cross-wind takeoff. The rest of the time you just do what you have to do :-)

If a an airline pilot felt they didn’t have enough control on the ground with the rudder, chances are they’d be using the differential braking without thinking. Airliners generally have much higher cross wind limits than Light aircraft so the rudder/nose steering can handle it. Maybe if there was a nose steering failure on rollout in a crosswind the plane would drift as rudder authority decreased (and spoilers up so too late to go around)... then any pilot who was at the stops with rudder pedal would soon start pressing their toe on that side as well.
 
Max Q
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Re: Full brakes or rudder

Tue Aug 20, 2019 6:17 am

MCTSET wrote:
For the pilots and engineers out the to give it a guess (pilots don’t try this). Imagine just after touchdown the pilot applied full left rudder on the left pedal and full brake on the right pedal, what direction will the aircraft turn too?

My guess is that at higher speeds such as after touchdown that the rudder would be more effective than the brakes however I could be wrong what do you guys think?



I’m relieved you warned pilots not to try this..
The best contribution to safety is a competent Pilot.


Guns and the love of them by a loud minority are a malignant and deadly cancer inflicted on American society
 
SAAFNAV
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Re: Full brakes or rudder

Tue Aug 20, 2019 6:28 am

chimborazo wrote:
CFI at my flying club has the mantra “Do what you need to do” ie take positive action (which can also be positively doing nothing) in all situations to maintain control of the aircraft.

If you’re Landing with a crosswind and you are getting steered by the wind after touchdown by all means use some differential brake.

Some people take 5 mins to be able to learn how to control a plane manually, some people spend hours and hours and still can’t do it. First time I steered with my feet on the ground it felt weird but I got used to it pretty quickly and became better the more hours spent doing it. Now I don’t consciously think about it.

Airline pilots and of course most others are skilled enough to pre-empt and react to to how the aeroplane is behaving... it’s quite a natural feeling even as a PPL to feel/see the aircraft behaving in a certain way. Your nose is going right so you press a little more left foot without even thinking about it. Use the controls as necessary. It’s not a conscious decision most of the time as it’s what you train to do. The only time I can think of conscious decisions for control inputs are: control freedom check preflight on GA and before takeoff, soft field take off increased pitch up command and aileron deflection at start of a cross-wind takeoff. The rest of the time you just do what you have to do :-)

If a an airline pilot felt they didn’t have enough control on the ground with the rudder, chances are they’d be using the differential braking without thinking. Airliners generally have much higher cross wind limits than Light aircraft so the rudder/nose steering can handle it. Maybe if there was a nose steering failure on rollout in a crosswind the plane would drift as rudder authority decreased (and spoilers up so too late to go around)... then any pilot who was at the stops with rudder pedal would soon start pressing their toe on that side as well.


Great reply.
It seems that a lot of people think flying is a little recipe: add half a banana length of elevator, 2 eggs on the rudder, but in reality it is much like driving. You give an input, see if it works, then add to or subtract from it to get the desired response. And it all happens automatically.
L-382 Loadmaster; ex C-130B Navigator
 
MCTSET
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Re: Full brakes or rudder

Tue Aug 20, 2019 2:34 pm

Nice answers guys, I have another question can asymmetric reverse be used to correct direction?
 
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zeke
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Re: Full brakes or rudder

Tue Aug 20, 2019 3:27 pm

MCTSET wrote:
Nice answers guys, I have another question can asymmetric reverse be used to correct direction?


Yes
Human rights lawyers are "ambulance chasers of the very worst kind.'" - Sky News
 
Chemist
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Re: Full brakes or rudder

Wed Aug 21, 2019 4:50 am

But asymmetric thrust wouldn't act very quickly, unlike brakes or even rudder. Engines take time to react.
 
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zeke
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Re: Full brakes or rudder

Wed Aug 21, 2019 7:31 am

Chemist wrote:
But asymmetric thrust wouldn't act very quickly, unlike brakes or even rudder. Engines take time to react.


Depends on the aircraft, turboprops or those with clamshell style reverse can be rather responsive.
Human rights lawyers are "ambulance chasers of the very worst kind.'" - Sky News
 
mmo
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Re: Full brakes or rudder

Wed Aug 21, 2019 3:02 pm

In addition to what Zeke wrote, reverse thrust is more effective at high speeds. The time it takes for a hi bypass fan to go from idle to reverse is not very long and is easily controlled by using less reverse on the asymmetric engine on a 4 engine aircraft or rudder in the case of a twin. It's not a big deal at all.
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spacecadet
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Re: Full brakes or rudder

Wed Aug 21, 2019 10:23 pm

I could be overgeneralizing as I only have a good working knowledge of a few airliners' systems, but I don't know of a situation in which a pilot of a jet, at least, would *intentionally* use asymmetrical reverse thrust to help steer a plane. There are situations and planes in which you'd do the opposite - use steering inputs to help correct the effects of asymmetrical reverse thrust. But unless there's some abnormal/emergency situation I'm not thinking of, or there's some plane that has somewhat unusual steering and thrust reverser systems, I don't know when you would ever intentionally engage asymmetrical reverse thrust in order to help with steering.

Turboprops could be entirely different, I don't know. But I don't know of a jet airliner that uses asymmetric reverse thrust for steering in any situation. In fact the FCOM for the A320 specifically says "if directional control problems are encountered, reduce reverse thrust to idle until directional control is satisfactory" (emphasis mine).

In fact, usage of reverse thrust at all can cause directional control issues due to increased weathervaning, so it's not recommended to use reverse thrust at higher than idle during crosswinds. You certainly would not use asymmetric reverse to try to compensate for crosswinds. You would deploy both reversers and keep them at idle. If loss of directional control begins, you can go so far as stowing the reversers until you can re-establish control, then re-deploy them if needed. (They are only really effective at high speed, especially in idle reverse, so chances are you'd be slow enough by then not to need them anymore.)
I'm tired of being a wanna-be league bowler. I wanna be a league bowler!
 
chimborazo
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Re: Full brakes or rudder

Thu Aug 22, 2019 1:29 am

spacecadet wrote:
I could be overgeneralizing as I only have a good working knowledge of a few airliners' systems, but I don't know of a situation in which a pilot of a jet, at least, would *intentionally* use asymmetrical reverse thrust to help steer a plane. There are situations and planes in which you'd do the opposite - use steering inputs to help correct the effects of asymmetrical reverse thrust. But unless there's some abnormal/emergency situation I'm not thinking of, or there's some plane that has somewhat unusual steering and thrust reverser systems, I don't know when you would ever intentionally engage asymmetrical reverse thrust in order to help with steering.

Turboprops could be entirely different, I don't know. But I don't know of a jet airliner that uses asymmetric reverse thrust for steering in any situation. In fact the FCOM for the A320 specifically says "if directional control problems are encountered, reduce reverse thrust to idle until directional control is satisfactory" (emphasis mine).

In fact, usage of reverse thrust at all can cause directional control issues due to increased weathervaning, so it's not recommended to use reverse thrust at higher than idle during crosswinds. You certainly would not use asymmetric reverse to try to compensate for crosswinds. You would deploy both reversers and keep them at idle. If loss of directional control begins, you can go so far as stowing the reversers until you can re-establish control, then re-deploy them if needed. (They are only really effective at high speed, especially in idle reverse, so chances are you'd be slow enough by then not to need them anymore.)



Asymmetric thrust is often used when taxying, helps on tighter turns.
 
CosmicCruiser
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Re: Full brakes or rudder

Thu Aug 22, 2019 2:33 am

we didn't use it.
 
spacecadet
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Re: Full brakes or rudder

Thu Aug 22, 2019 3:03 am

chimborazo wrote:
Asymmetric thrust is often used when taxying, helps on tighter turns.


We're talking about asymmetric *reverse* thrust.

The question I was replying to (a couple posts above mine):

Nice answers guys, I have another question can asymmetric reverse be used to correct direction?
I'm tired of being a wanna-be league bowler. I wanna be a league bowler!
 
N766UA
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Re: Full brakes or rudder

Mon Aug 26, 2019 4:24 am

Zeke2517 wrote:
GalaxyFlyer wrote:
Yes, we do have full authority and, yes, get good at using the brakes symmetrically.


GF



I fully respect that point of view and your training. But my question is, What if...?

I guess it would depend on the generation of the aircraft, right? Like, what if during the course of the day a pilot didn’t take in enough salt (rushed schedule, lack of nutrition, etc.) and, at the moment of truth, got a leg cramp while trying to stop the plane. “Get good” may not be what the NTSB is looking for.


Lol what the hell are you talking about? That’s like saying “what if you tried to push the accelerator just a little but accidentally mashed it to the floor and ran over a pile of kids.” You push each brake as much as you need to, it’s freaking easy.

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