Thrust Reverse

[Winkiepinkie]

I’ve got a quick and simple question (or so I thought) that I hope a few pilots could answer.

After chatting to a number of pilots (757’s, 767’s etc) I’ve almost always ended up asking what percentage (roughly) of braking action during landing is accomplished by reverse thrust.

The answer I nearly always get is "very little", and even frequently "just to cancel the forward thrust at idle", most of the breaking is achieved from the tyres/wheels that become effective as lift from the wing is destroyed by the spoilers that deploy upon contact with the runway.

So my question is one of two:

1) How much is reverse thrust actually responsible (esp. on high bypass engines eg, RB211, GE90 etc) for the braking action during landing

and if it is so little as only to cancel the fwd thrust;

2) How would it explain the in-flight break-up of the Lauda Air 767. Surely if the thrust reverser deployed on no. 1, with the effect of cancelling fwd thrust, shouldn’t it act like a normal engine out with the associated drag?

Thanks in advance for any replies

W.

[lomapaseo]

I'll let some big-iron pilots comment on the decell performamce aspects of using reverse thrust vs just brakes on landing in your first question.

However, your second question implies an assumption that the reverse thrust in the air produces large mecahnical loads on the aircraft and its systems to cause inflight breakup. This is not true. There have been a number of inflight deployments both wanted and unwanted where no breakup occured. The breakup in the Lauda incident was due to the aerodynamic loads suffered late in the event (after the reverse effect had ceased) entirely due to increased speed in an uncontroled dive.

Similar breakups have been documented in the CI B747 event several years ago (recovered) as well as the TWA800 event.

[niallcooney]

Winkie,

1) Thrust reverse is accomplished using doors redirecting bypass flow, which is approx imately 65 - 75% of total thrust. So the max reverve thrust available is only 65 - 75% of total (forward) thrust. Generally only idle reserve is used. It DOES make a difference, but it's not taken into account for official stopping distances.

2) In the case of the Lauda accident, the engines would have been at a higher thrust setting. When the reverser unlocked, bypass air would immediately begin to flow backwards, giving the drag of an engine out plus a h*ll of a lot of unwanted thrust (minus core thrust, plus maybe 70% assuming engine was at MAX CONT or TOGA). So the aircraft would have yawed HARD toward the malfunctioning engine, rolling the aircraft and subjecting everything to huge stresses, including the engine pylon. Below crossover point (speed at which aileron overpowers rudder) the aircraft may have been very difficult, if not impossible to control.

Nial

[dolly737]

1.
First let me point out that reversers lose their effectiveness progressively with decreasing speed for obvious reasons and are almost useless at speeds below 80 knots. The time between ground contact and full rev thrust is typically 4 seconds.

Then we have to differentiate two basic scenarios here:

a) the case where we want the shortest possible landing ground roll (usually a theoretical consideration): As we hit the ground, wheel brakes are applied to its maximum, which gives us an overwehelming deceleration. When the engines finally have spooled up in reverse, we would be already in a speed range way below 100 knots, which leaves the reversers quite ineffective.

b) the practical case - lots of runway available: Here we would use minimum wheel brakes ($$$) and the contribution of reversers to the total decelerating force will therefore increase.

Generally speaking, at high speeds (150-120kts) reversers contribute up to 50%, at lower speeds about 15% to the total decelerating force capability.

2.
May I add to the above posts that on the 767, if reversers are deployed inflight, a huge area of airflow over the wing is “shaded” by the upwards (and forward) flowing engine thrust IN FRONT OF THE WING, destroying lift. The wing almost instantly created a rolling moment together with yaw - and left the aircraft uncontrollable within seconds. The breakup was a result of the developing spiral/spin/dive.

[Intruder]

To put it in perspective, full reverse thrust alone gives more deceleration on a 747-400 (GE engines) than the brakes alone when set at Autobrakes 1 (the lowest setting).

Also, the net reverse thrust available is about 40% of forward thrust. The core thrust, which accounts for about 25% of total thrust, is not reversed. Therefore, that 25% has to be subtracted from the fan thrust that is reversed. 75%-25%=50% remaining. N1 is limited in reverse, and there will be mechanical efficiency losses when reversing the flow of air, so 40% is the most that can be expected.

While less reverse thrust may be used on smaller airplanes, it is used significantly in the 747.

[basil fawlty]

yes, it is correct that the core airflow is not reversed, only the bypass, but also remember that the reversed airflow is not directly opposing the movement of the aircraft. It is ejected at 45 degs from the cascades or target, therefore only a component (50%) of the reversed fan thrust actually opposes the horizontal movement of the aircraft. So, if a typical fan produces 80% of the thrust, then the maximum effective reverse thrust available is half of this...i.e 40% of max forward thrust. in reality it is a bit less than this, as max reverse is "derated" to prevent surging problems and such like. It is very useful, especially when landing on a wet runway.

[m&v]

The general concept is that reverse converts fuel to noise,but it does reduce forward speed to some extent if used properly upon initial touchdown.A specific 'value' of the use of reverse thrust is evident in the 320 FCOM 2.04.10p2/3 which refers to the penalty of payload ,whilst takeing off on a wet runway,with the reverser inop.(1-2 tons???):rolleyes:

[Checkboard]

The following is a quote from a Boeing article, Landing on Slippery Runways Guide, Airliner/Oct-Dec 1992

The total decelerating force available on a dry runway is quite large, approximately .5g deceleration capability. This means the total stopping force available on a 500,000 pound Model 747 is 250,000 pounds, or 45,000 pound for a 90,000 pound Model 737. At high ground speeds approcxiamately 35% - 55% of the total force available is provided by drag and thrust reversers and 45% - 65% is provided by the brakes. At lower speeds, the brakes provide 80% - 95% of the total decelerating capability.

On wet runways the total stopping force available is less than on dry runways due to the reduced braking effectiveness. The reversers and speedbrakes become more important since they now represent a larger protion of the total force capability. Wet runway braking capability is smallest at high speeds and increases as speed decreases. With the speedbrakes deployed, the drag and reversers furnish 50% - 80% of the high speed stopping force, whereas the brakes furnish 70% - 95% of the low speed stopping force. Overall, the wet runway stopping capability is 50% - 80% of the dry runway capability. Failing to extend the spoilers on a wet runway reduces the stopping capability by an additional 20% - 30%.

This paragraph is accompanied by a graph (which I can't be bothered scanning for you) that indicates that braking without reversers adds about 10% to the stopping distance on a dry runway, 20 - 30% on a wet runway, and up to 100% on an icy runway.

The reason some pilots believe that the reversers only "produce noise", is due to two reasons. First, modern autobraking systems determine a set deceleration rate, and the brakes are modulated to achieve that. i.e. Haul on the reversers, and the brakes release pressure a little, producing no noticeable effect (unless you are on a slippery runway!) and second, pilots in trouble braking generally only notice this at the end of the runway and (as can be seen form the article above) thrust reversers are most effective at high speed, so hauling on them just before you exit for the bush doesn't do much (unless you are still travelling at 100 knots!).

[Shore Guy]

A couple of comments relating to reverse thrust…..We operate 757’s with both the Rolls Royce and Pratt and Whitney motors. Reverse thrust with the Pratt’s will lean you forward in the seat……with the Rolls, mostly noise, very little deceleration moment. My point being that the same airframe can and will have very different deceleration characteristics because of engine/nacelle/reverser design.

Because they are not required for certification or performance computations, I have never seen any charts showing deceleration characteristics using reverse thrust (if anyone has a link for some, I would appreciate you posting it).

Regarding in-flight deployment….the grand old DC-8 still allows for in-flight deployment of the inboard reversers. Many operators have restricted their use because of pylon fatigue/cracking, but the aircraft certification still allows their use. Shakes the aircraft pretty good, and if one side deploys before the other, you will definitely feel the yaw. I believe the DC-8 is the only civil transport aircraft that allows for in-flight deployment of reversers. (Note: the DC-8 has no in-flight spoilers (except for roll), and with the high residual thrust on the 70 series aircraft (CFM’s), particularly with A/I on, the aircraft does not really want to come down).

[Jhieminga]

Now that's a shame....

During some quiet hours of sitting behind a desk I once worked out the precise (well... based on just a few little assumptions) forward thrust for all three reversers on a DC10. This was based on the angles that the cascade vanes deflect the bypass airflow by. I seem to remember that the end result of a pleasant afternoon was one monster Excel sheet and some interesting figures. Unfortunately I cannot find the file on my PC, and therefore must conclude that this calculation must be reported missing.

If I find it in the not too distant future I will let you know...

[mutt]

Shoreguy,

Boeing supplies a Landing Performance comparison chart as part of their performance engineers manual. It shows various configurations of brakes, speedbrakes and reversers. Unfortunately this chart isnt in the B757 book.

For Example: B777 / Flaps 35 / 440,000 lbs l/ anding distance.

Brakes, Auto speedbrakes and reversers = 3000 feet.
Brakes and Auto speedbrakes = 3200 feet
Brakes and Manual speedbrakes = 3400 feet
Brakes Only = 4300 feet
Auto speed brakes and reversers = 9600 feet
Manual speed brakes and reversers = 11500 feet.

Mutt.

[EGLG777]

Without wanting to digress from the original question too much, I seem to remember that Concorde can also use reverse thrust in flight.

Is my memory playing tricks with me?

If anyone could confirm or deny, I'd be most grateful

Thanks

[Capt. Crosswind]

If you aquaplane or land on an ice contaminated runway, reverse thrust is the only braking you have,so don't fall for the SOP of only using idle reverse. If you have a very wet runway pull in full reverse until you're happy all is under control.

[Winkiepinkie]

Thank you all very much for the replies. Just the sort of info I was after. Does anyone know where I can look up information on the net about the Lauda Air crash, like one can look up the Pan Am 103 incident.

Thank you all once again.

w

PS: Shore Guy, the old Trident could deploy the thrust reversers in-flight.

[dolly737]

The Lauda-air B767 accident report is available here. (http://www.rvs.uni-bielefeld.de/publications/Incidents/DOCS/ComAndRep/LaudaAir/LaudaRPT.html)

[BmPilot21]

One of the main purposes of reverse thrust is to reduce brake wear (this is especially true on short turnarounds - think low cost and 30'). On the Boeing, autobrake 1-3 is selected before landing. This gives a certain deceleration (in m/sec squared). When reverse thrust is selected a few seconds after touchdown, the autobrakes actually ease off slightly to give a constant deceleration - thus reducing brake wear.

However, once you resort to manual braking, the effect of reverse is added to the wheel brakes, so producing a maximum effort stop if required.

You can definitely notice a difference when you stow the reversers and return to forward thrust. I believe the net affect of reverse is about 50% eg) Full reverse on a 20000lb engine gives 10000lb of reverse thrust.

Also, as someone said earlier - reverse can be invaluable on a wet / slippery runway when brakes are liable to skid. A word of caution though - reverse can lead to a loss of directional control on a slippery runway in a crosswind, so must be used with care.

A reverser unlocked in flight is very serious indeed - the thrust lever should retard to idle automatically to reduce the effect, but the engine must be shut down without delay. At low speed and high power it can lead to loss of control and possible structural failure - I believe the fin failed on the Lauda due to the excessive roll and yaw cause by the reverse.

[Bellerophon]

EGLG777

Concorde can use reverse thrust in the air, during subsonic flight.

Only the two inboard engines can be used, at reverse idle power, plus one or two other limitations.

The descent profile is planned without the use of reverse thrust, but it is available, and used, if required.

Regards

Bellerophon

[niallcooney]

Guys,

Was just reading the Lauda accident investigation report, which brought up some very interesting points.

10 seconds after the reverser vanes unlocked the left engine FCS was positioned to CUTOFF. At the high operating Mach number (with reverser unlocked) it would not make a difference if the engine was shut down as N1 would still be windmilling at a high speed, redirecting bypass flow anyway. After 6 seconds the aircraft would be uncontrollable. An *almost* unavoidable accident. The reason I say *almost* is that had the PF applied FULL control input (both aileron and rudder), the aircraft MIGHT (in theory, and that's what it says in the report) have been saved. Although who would think of doing that? We're not trained to apply full input! Another interesting thing was that the aircraft broke Mach 1 in the dive and then broke apart. Most Boeings (well the 747 and 737) can get up to Mach 1.2 with only minor damage... and that's been documented. What could have been done in this situation? Two things: 1) TRUST THE EICAS\ECAM MESSAGES! 2) Be paranoid. It might just save your life.

Nial

PS: I know we can all be wise after the event, but personally I would rather learn from every accident.

[lomapaseo]

>Was just reading the Lauda accident investigation report, which brought up some very interesting points.

10 seconds after the reverser vanes unlocked the left engine FCS was positioned to CUTOFF. At the high operating Mach number (with reverser unlocked) it would not make a difference if the engine was shut down as N1 would still be windmilling at a high speed, redirecting bypass flow anyway. After 6 seconds the aircraft would be uncontrollable. An *almost* unavoidable accident.<

This certainly gives the impression that shutting down the engine will not affect the fan flow through the reverser. I don't agree with that!

It is true that the auto-shutdown feature activated by the inflight deployment is too slow to get rid of the unwanted thrust before the pilot can reasonably be expected to get the corect control inputs. However the torque drag on the fan rotor would bring it down to windmill speed in about 4 secs and at windmill speed at climb conditions the fan would not be putting out much eflux through the reverser at all.

The core of the engine does get to keep up a reasonable rotation speed at windmill, and that helps to relight an engine if needed.

In summary the best protection against an inflight deployment upset is to shut down the engine *before* the actual deployment. as this will remove most of the eflux out of the fan reverser cascades