Braking action on a wett runway
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Braking action on a wett runway
I have come across a question on my ATP studies that I'm stuck on, wondering if anyone can give there opinion
During Landing, on a wett runway what is the most effective initail braking?
1.Rev/brakes
2.Rev/spoilers
3.Spoilers/brakes
4.brakes/Anti-skid
Obviously, reverse thrust is one but would you say spoilers or brakes?
Thanks in advance.
hya
During Landing, on a wett runway what is the most effective initail braking?
1.Rev/brakes
2.Rev/spoilers
3.Spoilers/brakes
4.brakes/Anti-skid
Obviously, reverse thrust is one but would you say spoilers or brakes?
Thanks in advance.
hya
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Originally Posted by issi noho
What's wrong with 4. brakes and a braking aid. Unless you're flying an airship you'll have plenty of mass attached to the tarmac if you applied the correct landing technique.
This means no matter how hard the brakes are applied you will not brake as effectively as with the spoilers raised. Why? Because if you have anti-skid it will keep backing off the brake pressure to prevent skidding, or, if you don't have anti-skid you will just, well, skid!
PP
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Correct me if I'm wrong since I have never flown a jet but I thought with a wett or contaminated rwy, your brake effectiveness will be less due to the water (wheels not contacting rwy initially) Therefore on TD use max amount of reverse and spoilers to put the weight on the wheels. Then apply the brakes...
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Spoilers put weight on wheels. Reverse gives better retardation at higher speeds (just after touchdown) than it does at slow speeds (late in the landing roll.
However, if using autobrakes you get a retardation RATE, so if you set autobrakes 3, you will pull up in a specified reference distance for the runway conditions. How much reverse you take is irrelevant as the more reverse you take the more the wheel brakes are backed off to achieve the same retardation rate. If manually braking then the above is not true, they compliment each other. The most effective is still standing on the brakes manually after the spoilers have been raised...
PP
However, if using autobrakes you get a retardation RATE, so if you set autobrakes 3, you will pull up in a specified reference distance for the runway conditions. How much reverse you take is irrelevant as the more reverse you take the more the wheel brakes are backed off to achieve the same retardation rate. If manually braking then the above is not true, they compliment each other. The most effective is still standing on the brakes manually after the spoilers have been raised...
PP
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I'm going to give my favourite answer: "it depends".
How effective is the anti-skid? How good are the brakes? How big are the spoilers? How much reverse thrust does reverse give?
Those all depend on the type. Without specifying a type this kind of "canned question and answer" tests nothing except memorisation of data, not understanding.
On some aircraft the reverses do next-to-nothing; on others they are pretty effective. Some aircraft have tiny spoilers, some huge barn doors.
FWIW, I checked one of our AFMs earlier today - loss of half brakes or half spoilers was about twice as powerful as loss of TRs, dry. Assuming a wet braking coefficient of about 25% of dry, that puts all three nicely balanced.
Just to add to the mix: how wet is "wet"?
How effective is the anti-skid? How good are the brakes? How big are the spoilers? How much reverse thrust does reverse give?
Those all depend on the type. Without specifying a type this kind of "canned question and answer" tests nothing except memorisation of data, not understanding.
On some aircraft the reverses do next-to-nothing; on others they are pretty effective. Some aircraft have tiny spoilers, some huge barn doors.
FWIW, I checked one of our AFMs earlier today - loss of half brakes or half spoilers was about twice as powerful as loss of TRs, dry. Assuming a wet braking coefficient of about 25% of dry, that puts all three nicely balanced.
Just to add to the mix: how wet is "wet"?
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To be honest I think the question is a little vague but generalized and I dont think it is meant to go into detail. All the info mad scientist has provided is all true and valid but I'm thinking in basic terms, to answer the question and assuming everything is operational; I think its rev thrust/spoilers..
At the same time I thought that most spoilers worked automatically when you applied rev/thrust?
At the same time I thought that most spoilers worked automatically when you applied rev/thrust?
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So, if you ask a different question or give different assumptions, you will get different answers...
Your original question was "spoilers or brakes" for INITIAL braking, given reverse thrust. My answer is still "Spoilers."
If you ask a different question, like which failure will have the most impact on TOTAL rollout distance, the answer for the 747 would be brakes, anti-skid, spoilers, and reverse thrust, in that order.
Your original question was "spoilers or brakes" for INITIAL braking, given reverse thrust. My answer is still "Spoilers."
If you ask a different question, like which failure will have the most impact on TOTAL rollout distance, the answer for the 747 would be brakes, anti-skid, spoilers, and reverse thrust, in that order.
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I agree that that is what actually happens in most cases. However, the reversers are initially more effective than the brakes on many airplanes in normal operations.
Depending on the brake system, there may be little or no brake pressure applied if full reverse thrust is used immediately after touchdown (e.g., 747 using Min [Classic] / 1 [400] Autobrakes). Use of brakes in lieu of reverse at high speeds in the 747 Classic will almost always lead to overheated brakes.
Depending on the brake system, there may be little or no brake pressure applied if full reverse thrust is used immediately after touchdown (e.g., 747 using Min [Classic] / 1 [400] Autobrakes). Use of brakes in lieu of reverse at high speeds in the 747 Classic will almost always lead to overheated brakes.
For those who may not be familiar with FAA questions, then you must realise that only the ‘stock’ answer applies. Logic, ‘what if’, ‘it depends’, and common sense do not apply. All you have to do is find the appropriate reference book – probably an FAA one.
This is not the FAA reference, but it is a good practical guide:- Managing Threats and Errors during Approach and Landing.
Spoilers and Brakes.
This is not the FAA reference, but it is a good practical guide:- Managing Threats and Errors during Approach and Landing.
Spoilers and Brakes.
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Braking
While it's true that thrust reverse is more effective at high speeds, and braking is more effective at slower speeds, the correct response to the question is "all of the above."
1. Spoilers,
2. Wheel Brakes (with anti-skid),
3. Thrust Reverse (at a level prescribed by SOP).
If you want to cloud the issue, throw in auto-braking and scheduled decel rates, etc.
However, both Boeing and Airbus say spoilers to get the weight on the wheels...good wheel spin-up...wheel brakes, then thrust reverse.
Think about a rejected takeoff where stopping capability is crucial. What do you ensure first? Spoilers. Then, wheel brakes. Then, whatever reverse you have.
(And, by the way, if you want to throw in autobrakes to muddle up the discussion...what do the autobrakes do in an RTO?)
PantLoad
1. Spoilers,
2. Wheel Brakes (with anti-skid),
3. Thrust Reverse (at a level prescribed by SOP).
If you want to cloud the issue, throw in auto-braking and scheduled decel rates, etc.
However, both Boeing and Airbus say spoilers to get the weight on the wheels...good wheel spin-up...wheel brakes, then thrust reverse.
Think about a rejected takeoff where stopping capability is crucial. What do you ensure first? Spoilers. Then, wheel brakes. Then, whatever reverse you have.
(And, by the way, if you want to throw in autobrakes to muddle up the discussion...what do the autobrakes do in an RTO?)
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Originally Posted by PantLoad
While it's true that thrust reverse is more effective at high speeds, and braking is more effective at slower speeds
Braking slows your aircraft down better at low speeds than high? OK, I presume that you've less lift at lower speeds, thus a greater normal force, and thus greater braking force. Are there other reasons?
Hawk
