Ask The Pilot

[MichaelD]

Reverse thrust does very little. The % of reverse power available is a small fraction of the possible forward thrust. In it's own right, it would take just about forever to stop the aircraft.

So why is it designed into engines at all if it does very little? Seems like a lot of added complexity for very little gain.

 

[meloz]

I would imagine to negate the forward thrust at idle???

Meloz

 

[jb747]

So why is it designed into engines at all if it does very little? Seems like a lot of added complexity for very little gain.

Two main things. It cancels out the quite large amount of forward thrust that the engines produce at idle, and the sideways blast of exhaust is quite effective at removing standing water, and sitting the tyres onto the tarmac.

The A380 was originally planned by Airbus without reverse at all, and it was forced to incorporate it by the FAA. It only has reverse on inboard engines.

 

[Flying Fox]

The A380 was originally planned by Airbus without reverse at all, and it was forced to incorporate it by the FAA. It only has reverse on inboard engines.

Does this mean that aircraft don't need reverse thrust to stop (ie. brakes only)?

Or were Airbus planning something else to aid in the braking effort?

 

[straitman]

It's effectively the same thing.
From the linked Wikipedia article:

Staying OT for just a moment.

Technically you are correct however in aviation circles it is referred to as the glide ratio for simplicity.

When flown at a constant speed in still air a glider moves forwards a certain distance for a certain distance downwards. The ratio of the distance forwards to downwards is called the glide ratio. The glide ratio is numerically equal to the Lift-to-drag ratio under these conditions; but is not necessarily equal during other manoeuvres, especially if speed is not constant. A glider's glide ratio varies with airspeed, but there is a maximum value which is frequently quoted. Glide ratio usually varies little with vehicle loading however, a heavier vehicle glides faster, but maintains its glide ratio.[SUP][2][/SUP]

 

[mjt57]

Does this mean that aircraft don't need reverse thrust to stop (ie. brakes only)?

Check out JB's videos on YT. Often he'll command "Idle only" after touchdown. Presumably these are runways (SYD/LHR) that are long enough. But on a recent Jetstar flight into MEL the guys driving the A320 used it. Dunno if it was or wasn't necessary, but JB would be able to answer that.

 

[jb747]

Does this mean that aircraft don't need reverse thrust to stop (ie. brakes only)?

Or were Airbus planning something else to aid in the braking effort?

Well, I'll vote for an arrestor hook....

As far as I know, Airbus did not want the weight, and so that rather drove their decision making.

Does it need it? Yes and no. The brakes become very hot as is, and without reverse would almost certainly need cooling fans at each transit. Whether installed on the aircraft, or on the ground, they become a weight/logistics/complexity problem in themselves.

The wet runway issue is a big one, which was sadly demonstrated by QF1. I think the FAA is right on this..

 

[jb747]

Check out JB's videos on YT. Often he'll command "Idle only" after touchdown. Presumably these are runways (SYD/LHR) that are long enough. But on a recent Jetstar flight into MEL the guys driving the A320 used it. Dunno if it was or wasn't necessary, but JB would be able to answer that.

The term 'idle only' is simply used as a reminder not to take full reverse. Arrivals into London often happen in the 'shoulder period' of the curfew, when use of idle reverse is mandated for noise abatement reasons. We need to allow an extra 300 metres (which is policy, not a performance impost), and the runway cannot be contaminated (i.e. snow, water). Even outside the shoulder time, it's still good manners to use idle if we can. In that instance too, we know the aircraft is not planned to operate again for some hours, so high brake temperatures are less of an issue. There are take off and taxi temperature limits that can be quite hard to adhere to after a 60-90 minute transit. Sydney and Frankfurt also have 'shoulder' reverse restrictions.

Other than that though, full reverse is generally used, although it might be deselected early in a 'roll through' situation. It is never NEEDED, and is not accounted for in the performance calculations.

 

[MEL_Traveller]

A330 Air Transat Flight 236 - Wikipedia, the free encyclopedia

A310 Hapag-Lloyd Flight 3378 - Wikipedia, the free encyclopedia


There is quite a varied list as you would expect:

Aviation Safety Network > ASN Aviation Safety Database > Index > (Contributary) Cause > ASN Aviation Safety Database results



Two , one just gets more press than the other since it happened 7 years earlier, and there was the SQ flight a few days after the BA event that almost lost 4.

KLM Flight 867 - Wikipedia, the free encyclopedia

There was also the air canada B767 (probably on the list but i haven't had a chance to read it). Some amazing flying by the pilot with some 'crab' approach IIRC to slow speed, lose height and change direction all at the same time.

 

[mjt57]

The term 'idle only' is simply used as a reminder not to take full reverse..

Oh, OK. Thanks.

 

[RobYg]

JB, how high do the brake temperatures rise, and what is the maximum brake temperature at which the gear can be retracted?

 

[mjt57]

JB, how high do the brake temperatures rise, and what is the maximum brake temperature at which the gear can be retracted?

How would the brakes get hot during takeoff?

 

[RobYg]

Sorry, badly phrased. What I meant was how far do the brakes have to cool before the next takeoff?

 

[harvyk]

How would the brakes get hot during takeoff?

An aborted high speed takeoff...
But then again it's unlikely (I would guess no chance at all) you'll be heading back to the start of the runway to try again without some sort of rest to sort out the reason why the take off was aborted in the first place.

 

[richie9x]

Sorry, badly phrased. What I meant was how far do the brakes have to cool before the next takeoff?

Boeing have got a video of a 747-8F at maximum takeoff weight doing a rejected takeoff with warn brakes at 320km/h and with no reverse thrust. It was done as part of the certification for the plane.
Boeing 747-8 performs ultimate rejected takeoff (Video)

Granted it is an very extreme example. But the brakes got to an estimated 2500 degrees C. The brakes and tires were damaged.

 

[harvyk]

Boeing have got a video of a 747-8F at maximum takeoff weight doing a rejected takeoff with warn brakes at 320km/h and with no reverse thrust. It was done as part of the certification for the plane.
Boeing 747-8 performs ultimate rejected takeoff (Video)

Granted it is an very extreme example. But the brakes got to an estimated 2500 degrees C. The brakes and tires were damaged.

That would be one loooong 5 minutes, just sitting there...
I have heard (although I don't know if it's a joke or not) that test pilots get paid a little extra on those days.

 

[jb747]

JB, how high do the brake temperatures rise, and what is the maximum brake temperature at which the gear can be retracted?

Highest I've seen is about 550c. Normally they are around 400c, but with 16 brakes sets you often have quite a range across them, with one or two both high and low.

Our take off temp limit is 300c, so that would also make a good retraction limit. Brakes will often still be at 200c or so when we push back, and it isn't hard to put another 100c into them during the taxi out to the runway.

 

[jb747]

That would be one loooong 5 minutes, just sitting there...
I have heard (although I don't know if it's a joke or not) that test pilots get paid a little extra on those days.

Sounds like a good myth. It's just a normal test.

 

[jb747]

An aborted high speed takeoff...
But then again it's unlikely (I would guess no chance at all) you'll be heading back to the start of the runway to try again without some sort of rest to sort out the reason why the take off was aborted in the first place.

Low speed abort perhaps. High speed....no way. You've quite probably deflated large numbers of the tyres ( there are thermal plugs in them).

 

[harvyk]

Sounds like a good myth. It's just a normal test.

It was an airbus engineer who said it, it was on a TV program talking about testing the A380. The way he said it though certainly had a hint of a joke about it...