Thrust reversers

[Brado78]

When the Airbus A380 lands, Why don't engines 1 and 4 reverse, only 3 and 4 do?

Thanks Guys

 

[Gerle]

Both inboard engines (2 and 3) have them. The reason the outboard engines don't is to minimize the amount of debris being blown around, since those two engines are closer to the edge of the runway. Presumably the runways are cleaner than the non traffic areas, I haven't seen sweepers out there.

 

[LTC8K6]

Yes, the outboard engines are often near the edges of runways where debris usually gathers. They would likely create a debris cloud, so the outboard engines do not have reversers.

 

[Genx87]

The A380 was originally designed with no thrust reverse mechanism and instead using braking to stop. But the FAA wanted them to include the ability to use the engines to slow the aircraft down. So Airbus added capability to the inboard engines. My guess as to why only the inboard engines is because of weight and maintenance savings on the outboard engines. And a bonus reduction in picking up debris in the outboard engines. But I dont think debris is the whole reason, The 747 for example still uses all 4 engines.

 

[LTC8K6]

There is no thrust reverser requirement for airliners. If the plane can meet braking standards without them, then it's fine.

In fact, the airliner must meet all braking requirements without using the reversers.

 

[momeNt]

The moment created by outside thrust reversers would be a lot higher, considering the A380 wasn't designed originally with thrust reverse they would have had to go back and redesign the wings and the connection at t he aircraft body to handle the increased moment.

 

[BarkingGhostar]

Think torque and momentum arm.

 

[GagHalfrunt]

Yes, the outboard engines are often near the edges of runways where debris usually gathers. They would likely create a debris cloud, so the outboard engines do not have reversers.

The debris is not located any differently than it is at takeoff. If the airport and runways can survive the debris cloud of an Airbus 380 taking off with all four engines at full throttle it can surely handle the same plane landing on the same runway.

 

[blankslate]

Think torque and momentum arm.

Yeah that was one of the first things that came to mind.

If the aircraft wasn't designed from the start with thrust reversers in mind then the wing structures might not have enough strength, reinforcement, structural integrity(?) or whatever the engineering term is for being structurally sound enough to safely handle reversers on all four engines.


________________

 

[Crusty]

The debris is not located any differently than it is at takeoff. If the airport and runways can survive the debris cloud of an Airbus 380 taking off with all four engines at full throttle it can surely handle the same plane landing on the same runway.

Engines reversing will blow debris forward and then possibly get ingested into the engines.

During takeoff that debris would be moving away from the engines, not towards it.

 

[randay]

yes but can you land it on a treadmill

 

[cbrunny]

Could be that its just not necessary. slam on four high powered brakes and peoples stomachs could be flying out their mouths or just extremely uncomfortable for passengers? I assume that for the most part there is little control over the degree to which these brakes are applied. Either on or off. Maybe halfway, but even that might just be too much.

 

[freeskier93]

Yeah that was one of the first things that came to mind.

If the aircraft wasn't designed from the start with thrust reversers in mind then the wing structures might not have enough strength, reinforcement, structural integrity(?) or whatever the engineering term is for being structurally sound enough to safely handle reversers on all four engines.


________________

It seems very unlikely that a wing designed to handle 10s of thousands of pounds of thrust in one direction can't handle a small amount of thrust in the other direction.

 

[garndawg]

Gents,

Vmcg is why. Vmcg = Velocity, Minimum Control, Ground

A failure of an outboard thrust reverse mechanism would create a yawing moment greatly in excess of the aircrafts' control surfaces and/or nosewheel steering to maintain control.

In short, a failure would cause the aircraft to veer off the runway and potentially ground loop.

Other aircraft that have outboard reverse typically have a derating mechanism to restrict total thrust available below a certain speed.

Hope that helps...

Signed,
An aerospace engineer, and a pilot...

 

[rudder]

Could be that its just not necessary. slam on four high powered brakes and peoples stomachs could be flying out their mouths or just extremely uncomfortable for passengers? I assume that for the most part there is little control over the degree to which these brakes are applied. Either on or off. Maybe halfway, but even that might just be too much.

Thrust reverser power is controlled by the throttles. The pilots are in 100% control of how must reverse thrust is applied.

 

[IronWing]

yes but can you land it on a treadmill

It always possible to land a plane.

 

[garndawg]

Thrust reverser power is controlled by the throttles. The pilots are in 100% control of how must reverse thrust is applied.

I'll just leave this here...

https://www.faa.gov/regulations_pol....cfm/go/document.information/documentID/22468

 

[Jeff7]

It seems very unlikely that a wing designed to handle 10s of thousands of pounds of thrust in one direction can't handle a small amount of thrust in the other direction.

Perhaps. It depends how the design was optimized.

A beam that's really strong in tension could buckle if put in compression.
(Take a piece of solid 14GA wire and try to make it fail structurally by pulling it. You'll have to tear it apart with a lot of force. Now try to make it fail structurally in compression. It'll fail under much less force, not by damaging the material, but by buckling as the straight beam member bows.)

I have no idea if that's how the A380 was built; that's more just a generalized description of how improper loading can cause problems.

It always possible to land a plane.

:hmm:
I guess even if you ditch in water, it'll eventually sink, and thus land.

 

[tynopik]

:hmm:
I guess even if you ditch in water, it'll eventually sink, and thus land.

You can't land a plane that's already on the ground

 

[Red Squirrel]

My guess is that it's just not necessary, there are plenty of 2 engine planes, the extra 2 on 4 engine planes is mostly for extra power and redundancy mostly, so as far as braking goes it was probably not worth adding the reverse on the far ones.

Might also be an aerodynamics thing, if it a far end reverse thruster was to fail to activate on one side and work on the other it would send the plane into a dangerous sharp turn flipping the plane before the pilot has a chance to react. Not really sure how likely that is to happen though.

 

[Red Squirrel]

yes but can you land it on a treadmill

Maybe this has already been done before. Did the WTC have a gym?

 

[Linux23]

well i guess if the brakes fail, then the reverse thrusters would help to postpone briefly your impending death as you're careening towards the end of the runway.

 

[skyking]

Gents,

Vmcg is why. Vmcg = Velocity, Minimum Control, Ground

A failure of an outboard thrust reverse mechanism would create a yawing moment greatly in excess of the aircrafts' control surfaces and/or nosewheel steering to maintain control.

In short, a failure would cause the aircraft to veer off the runway and potentially ground loop.

Other aircraft that have outboard reverse typically have a derating mechanism to restrict total thrust available below a certain speed.

Hope that helps...

Signed,
An aerospace engineer, and a pilot...

Thanks for a concise explanation. Adverse yaw works both ways

 

[Crusty]

Thanks for a concise explanation. Adverse yaw works both ways

Is 'ground loop' a fancy term for cartwheel?

 

[LTC8K6]

The debris is not located any differently than it is at takeoff. If the airport and runways can survive the debris cloud of an Airbus 380 taking off with all four engines at full throttle it can surely handle the same plane landing on the same runway.

Thrust reversers blowing the rain on the runway forward, in front of the engines during landing.

http://cdn-www.airliners.net/aviation-photos/photos/8/4/0/2753048.jpg

http://cdn-www.airliners.net/aviation-photos/photos/1/0/3/2748301.jpg

http://cdn-www.airliners.net/aviation-photos/photos/9/7/6/2619679.jpg