Question on Reverse thrust ?
 

1. when you deploy the reverse thrust , is there a minimum speed beyond which you cant use it ? if so , why is that ? (because i've heard people say u cant use the thrust reversers for a 737-8 below 60 knots )

2. And when do you start braking during the landing roll ? simultaneously when you use reverse thrust ??

Josh,

During normal operations, reverse thrust is usually discontinued below 60 to 80 knots. My employer directs us to begin stowing the reversers at 80 knots, and have them stowed by 60. In an emergency or when absolutely necessary, the reversers can be used all the way to a full stop. A high speed rejected takeoff might be such an occasion.

The primary reason for stowing reversers below eighty knots or so is that as the aircraft slows, the potential for exhaust gas re-ingestion increases, as well as the potential for reverse flow gasses to cause or permit foreign object ingestion. Direction to stow reversers is there to protect the engine. Ingestion of exhuast by products can cause a flameout, and some engines aren't very stable in deep reverse at slow speeds; they may compressor stall and flame-out on their own. Reverse thrust isn't very effective at low speeds, and offers little advantage.

Wheel brakes are generally employed soon after landing, but the circumstances dictate when and how much braking is used. If the airplane is light and it's a long runway with light traffic, very little braking may be used, instead preferring to allow the airplane to roll to the end while using reverse thrust.

In other cases, operators use "autobrakes," which apply braking at a preset value upon touchdown. Autobrakes have settings that correspond to acceleration values. For example, an autobrake system set to "medium" may slow the aircraft at a rate of 6 feet per second per second; if reverse is applied at the same time, the airplane still slows at the same rate, but the brakes get used less (and therefore aren't as hot; an important consideration for short turn-arounds).

We calculate our landing performance based on autobrake settings, because these give known rates of acceleration (deceleration, if you will), and therefore predictable stopping distances.

You could'nt have explained it any better .. very helpfull ! thanks for the awesome reply SNS3guppy :ok:

That may be your company procedure because of specific airfield conditions. The Boeing 737 recommended procedure is different to your company procedure. The Flight Crew Training Manual states:
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"After touchdown, with the reverse levers at idle, rapidly raise the reverse thrust levers up and aft to the interlock position, then to number 2 reverse thrust detent. Conditions permitting, limit reverse to the number 2 detent. The PM should monitor engine operating limits and call out any operational limits being approached or exceeded, any thrust reverser failure, or any other abnormalities.

Maintain reverse as required, up to the maximum, until the airspeed approaches 60 knots. At this point start reducing the reverse thrust so that the reverse levers are moving down at a rate commensurate with the deceleration rate of the airplane. The thrust levers should be positioned to reverse idle by taxi speed, then full down after the engines have decelerated to idle. The PM should call out any indvertent selection of forward thrust as reverse thrust is cancelled. If an engine surges during reverse thrust operation, quickly select reverse idle on both engines".

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Idle forward thrust on the 737-300 is approx 23 percent N1. Idle reverse is the same. If the reverse levers are rapidly selected full down fron the full reverse position the engine will come out of reverse around 62 percent N1 giving significant forward thrust just when you are trying to slow up. Not a good look but a very common habit among some pilots.

Note the Boeing advice is to maintain reverse as desired up to the maximum until the airspeed approaches 60 knots. It is often seen where the PF is not heeding the airspeed during the roll-out and instead relies completely on the PM calling 60 knots. Only then, will the PF start reducing reverse thrust. The 60 knot call is purely a reminder - it is not an action call. You would be surprised the number of pilots under training in the simulator will forget to reduce reverse until the aircraft has stopped simply because the PM may have forgotten to call out 60 knots.

However 60 knots is not an action call because under FCOM Normal Amplified Procedures, Boeing also advise "By 60 knots, start movement of the reverse levers to be at reverese idle by taxi speed". In other words before reaching 60 knots airspeed reverse should be reduced as above. The operative word is "By" not, "at" 60 knots

Done correctly, the N! should be at idle reverse (23 percent N1, by taxi speed. If the procedure is followed the chances of ingestion are minimal. And Boeing should know.:ok:

Reversers should only be stowed when stopping is assured, as a re-deployment plus spool-up to get useful reverse thrust takes ages. Obviously, at 60 kts halfway down a 4k dry runway on a sunny CAVOK day, that might not be an issue. But it's a bad habit to get into, as one day you will be landing on a worse-than-expected contaminated runway, and your habit will bite you.

Thinking in a different way, if you land with full reverse, would you stow them straight away at 60kts? If not, why change it? Plenty of good reasons in the Boeing FCTM:

No, when safe stop is assured and at taxi speed. You can leave the runway at 50kts+ at some RETs.

Why does it feel wrong? Reverse at idle does not hurt anybody and the risk of FOD at reverse idle is minimal.