Reverse during taxi
 

Hi all,

Good someone please explain to me the use of idle reverse thrust during taxi as an alternative procedure to braking. Apart from possible fod ingestion are there any other disadvantages? is this practice prohibited? I referring to the B737 CL and NG series.

Thank you in advance.

Generally idle reverse isn't used for braking during taxi. Reverse is most effective at high speed. FOR ingestion is also a problem. Different airplanes,airlines and manufacturers have different restrictions, but generally using reverse during taxi is either not recommended or prohibited except for emergencies when brakes are inoperable or the surface is so slippery brakes are ineffective.

Not a good idea on a/c with wing mounted engines but quite common to see biz jets like Gulfstreams taxing about with one in reverse to limit taxi speed.

Some operators of types like the MD80 back themselves off the gate using reverse thrust or at least they used to in the US.

On aircraft like the 744 and 777, you will lose your V-speeds for takeoff if reverse is used during the taxi out. Best only used in that situation if necessary for control reasons such as on slippery surfaces. I wonder if the newer 737's are the same.

Used to use idle reverse on engine #2 on the 727 although I'm sure all three engines could be used if desired.

Loss of V-Speeds ???
 

JS, on what basis do you loose your V-Speeds if reverse were to be used during taxi out? I am intrigued by that statement.

Pax here but I remembered quite well back in December 1989 when Romania was under communist control that on longish taxi on 732 in Brussels (European Airline) the reverses were used during taxi - they were so obvious on those aircraft. During my visit to the FD, the captain stated it helped the breaks during the taxi out.

Thanks for the quick replies,

I'm sure this has been debated for some time. To cut a long story short we getting hot brakes and the lifespan of these brakes are not good. Some guys have reverted to using idle reverse thrust on long down hill taxiways. Their brakes are cooler. But of course one fod incident will cancel out this attempt to save the brakes. Are there any airlines that do not prohibit this practice?

How about single engine taxiing as a way of keeping speed under control? No FOD problems, less thrust, reduced fuel consumption, less engine hours etc.

Until JM comes back- I can't find a FCOM reference but certainly in the event of an RTO in the sim on both the aforementioned types the V speeds are "wiped" from the FMC, quite what the trigger is ( RTO autobrake operation, REV Thrust being selected, or sim re-position) I'm not sure...

PM

In the context of taxiing to the runway for take off FWIW on some types (certainly at least one heavy I know of) there is a limitation that you can't single engine taxi if you are above Maximum Landing weight

A little bit surprised at this statement as because the reversers operate in the vertical plane its going to be stirring up a lot of FOD that is likely to end up in the engine intake.


On aircraft like the 727 or Falcon 50/900/7x using the centre engine where the reversers operate in the horizontal plane you wouldn't have that problem

Used to be quite common. Lots of videos on-line.

https://www.youtube.com/watch?v=-Zkxh903s_w

Procedures need to be approved by the Chief Pilot if they're not stated in the manuals People doing their own thing points to poor discipline and leaves your​ head on the block if anything goes wrong.

From Ground Ops 2.5 in an old Qatar 777 FCOM:

Loss of V-Speeds ???
 

KRviator. Thank you. As one from an earlier time I did not pickup on the real meaning of the comment, i.e. that the V-Speed display would be lost. I thought the inference was that the originally determined V speeds, which would be set on the bugs in my era, would become invalid because of having applied reverse.

I've noticed larger business jets, the G-V and such, often use reverser on one side. I suppose it's to reduce wear on the brakes.

KRviator

Many thanks for the reference, makes sense. I may have missed it but I can't find similar in our current FCOM or FCTM so I suspect if it was ever there it has been edited out.

Only once off the gate on an FAA checkride on a 727. DFW at night. Many many moons ago. Never used it again there or anywhere else. Not a good procedure

There's a requirement by law for the reversers to be visually checked stowed before the take-off. This is usually done during the pre-flight walk-around inspection. Once you used your reversers you must check them visually again, if you are unable to do it from the cockpit/cabin (as the case be with many airliners) you need to stop, shutdown, get out, do a walk around and then continue. Not very practical.

Last time I used it on a jet was on the #2 on an L-1011 TriStar at Gander taxying out for departure a long long time ago; it was approved practice.

After a lengthly debate with the Captain about use of reverser on taxi, the reverser then refused to re-stow correctly when commanded. The ensuing headaches afterwards convinced me that it's best to leave them until you get to the landing runway!

They use a chalk test to determine relative reverse thrust levels vs aircraft speed to visualize the reverser efflux flow and potential for ingestion.

These visuals show that the small pieces of chalk are blown forward off the tarmac too far to be ingested especially when the aircraft is backing up.

At high landing forward speeds they are overrun before they bounce off the runway unless of course you eject too much efflux at lower speeds. If this is the case then you definitely want to be over a runway blown clean by other aircraft operations

Aside from landing, I have, on the A319/320, used only idle reverse during only taxi out at only one airfield, Stuttgart.

It is somewhat downhill to the threshold of 25 and idle reverse helped the brake temperatures to remain below the max brake temperature for take off.

The ops. manual, it has been many years since I retired, at that time, and may still, allowed idle reverse while taxying.

Can you give a reference? This is the first I hear of such a "law".

The 737 Classic FCTM discusses use of reverse after landing.


Quote: The thrust levers should be positioned to reverse idle by taxi speed then to full down after the engines have decelerated to idle. Unquote.
This suggests that ingestion is not a problem at reverse idle.
Depending on your point of view, it could be argued that providing you don't go above reverse idle (approx. 22% N1) you could slip into reverse idle without harming the engines in order to keep taxi speeds from building up in a long taxi and thus minimising brake heat increase.

I only ever used reverse (idle) on taxi out on the B737-200 with the clamshell reversers (best reversers I have ever used!), usually only ever in the summer on hot days. As an aside, the 737-family was also the only aircraft type with no brake temp display that I have flown.

At one point on the B737-200 the manuals did permit reversing back from stand, and it was practised in the sim, although never used in reality at BA.

As Finncapt says, taxying out at Stuttgart for Runway 25 was a challenge in summer in the early A320's at BA without the brake fans, although I never had to resort to using reverse, as far as I can recall.

Backing off gate using reverse thrust was the normal operation on B727 DC9 MD80 at my US airline in the 80's/ 90's supposedly to save money. They changed back to push back crew with tug in late 90's again to save money .

Reversing from gate (powerback) is a bit different, because these days there are noise & safety regulations which may limit or prohibit such practice.

Typically at most major airports powerbacks (if allowed) may only be conducted from specific gates during specific times, with prior approval from the airport authority. The airline/operator must also have an approved OpSpec and associated procedures to conduct powerbacks.

OK, time to inject a bit of logic to this. The aircraft mentioned above that have used reverse in taxi tend to have clamshell reversers and all of the flow is directed by them.

However, aircraft with high bypass engines and sleeved reversers only use the fan air to create a wall of air that causes the drag to slow you. It's less the forward momentum of the air than the large area of disturbed air that acts like a parachute effect.

At idle, the majority of thrust you are getting out the back is the core. SO reverse idle is providing f**t in a spacesuit type usefulness in this case. If you need reverse you need to give it a HANDFUL to do much.

Better to brake to a near standstill and taxi slowly. The friction of braking or riding the brakes is far greater at higher speeds, so controlling the aircraft down a slope is going to generate a lot less heat at walking pace than 25kts.

Also this gives you more time to get to the single engine taxi point and makes for a smoother and quieter taxi in. Just my 20c worth...

Many jets with high bypass engines can powerback, perhaps most notably the 757.

However, most new jets today are not approved for powerbacks -- even if they are physically capable to do so. And as already mentioned, most airports have restrictions against powerbacks.

What's the reason for that?

@Amadis of Gaul: checking correct reversers stowage during the exterior pre-flight inspection is a required check for almost any modern jet. You should be able to find it in your FCOM/FCTM.

Probably just your company procedures? I know of no such law.
It would be very unpractical as well, any (low speed) RTO would lead to a taxi back to gate for an inspection.

It is irrelevant that we check reversers stowed during the walkaround. IThat still does not make it 'law' to visually inspect reversers after you used them during taxi.

That's not what I asked. I was inquiring about the "law" that requires shutting down, getting out and checking them again after using them during taxi.

Oh all-right, no, that's an extrapolation. How else can you *visually* ensure the reversers are properly stowed before take-off?

You don't. It is not required to visually ensure anything. Do you visually ensure your flaps are out on every takeoff? No.

Yes you check the reversers during your walk around. But a walk around is classified as pre-flight. It has nothing to do with the actual operation of the aircraft once moving under power. So no, unless you can quote the article, I'd say there is no general rule or law prohibiting the use of reversers during taxi other than the usual type-dependent operational guidelines which may or may not prohibit their use.

It's OK, Penko, he was just extrapolating.