Aircraft Braking Question
 

Dear all,

Question: upon touchdown which method would you recommend, thrust reverse or brakes. Obviously each has its own disadvantages; thrust reverse has an effect upon cyclic counts and therefore the life of the engine, whilst using brakes would mean less braking life and shorter inspection/replacement/overhaul intervals. What do you think the relative advantages of each are? You’re thoughts would be appreciated. Thanks

stu

If you're looking for a total commercial point of view, the brakes win any time.

Usually it's a combination of both. The restricting factor for reverse use for me is mostly noise consideration.

It depends on the condition of the runway and airport regulations. Some airports dont allow thrust reversers during normal opperations due to increased amounts of noise.

If the runway is contaminated with snow/water, using reverse thrust is usually the safest method because the brakes will not be as effective due to the loss of traction.

When thinking from an economical stand point, it is better to use the wheel brakes as they are cheaper to maintain than the thrust reversers.

Remember that during aircraft certification, the landing distance is measured without the use of thrust reversers.

/regards

stuharri - brake cooling also has to be taken into consideration, and appropriate reverse will mean cooler brakes if the turnround is short.

In the case of manual brake application...
 

Reverse, or idle reverse, upon touchdown is the immediate action item. No need to heat up the brakes at 145+ knots. Brake application below 120 knots is more effective...assuming adequate runway remaining.

stuharri2002 - What type of aircraft are we talking about, e.g. transport category jets?

Let's not forget the tire wear caused by liberal brake application. How many $9000 tires do you think it takes to equal one engine change when you're changing them every 120 cycles like some airlines.
TWA took the autobrakes off and got to 300 cycles. They also got to over 10000+ hours between MD80 engine changes.
Doesn't seem to support using brakes over reversers does it?

Reversers
 

Using Autobrakes in either MIN or MED position on a normal landing:

Weight 255 tons
Brakes on at 130 kts
Sea level
ISA (15'C)
No reverse thrust

Brake energy released 23 million ft lbs
Cooling time 1h 10min

However if one would use full reverse until 100 to 80kts and then use autobrakes in either MIN or MED with same conditions as above:

Brake eneregy releases 8 million ft lbs
No cooling time required

On a B747 after Anti-Skid touchdown protection releases with Autobrakes selected, brakes will provide full deceleration with no modulation until reverse thrust comes available (Approx 3 to 4 sec) and modulation of brake input starts. It is this high weight, high speed combination that causes most heating as well as wear and tear of brake units and tires.

One needs to understand that reversers are most effective at high speeds where brakes are at their best al lower speeds. This goes for any airplane.

A brake unit, wheel and tire combination can cost between 100 000 and 300000 USD x 16. Smart use of reverse and brakes can save company millions per year.

AD

Yes, Atlanta-Driver,

Your final comment is quite true, amongst many other things that are pilot choice.

It's a wonder airline companies don't seem to understand this when they treat the flight crew in a most unfriendly and unhelpful manner, together with marginal salaries, as has been the case over the last several years!

Just food for thought!

Cheers,

FD

Are brakes and reversers also meant to provide redundancy for each other?
Is it not also a good idea procedurally in case one is um, "forgotten?"

I was on a flight once that had quite a bounce after touchdown, I'd guess 10 feet, after which we rolled on the runway for a good 5-10 seconds before the reversers kicked in. I mention this because if something distracting like that happens, isn't it best if some automatic system (brakes/spoilers) takes effect?

FakePilot
 

Sitting at the pointy end of an airplane with the end of the rwy looming ahead one would have to be very thick or blind to forget braking.

On another note. Use of reverse thrust is not a consideration when calculating landing performance. Just anti-skid and auto-spoilers operative (Type specific variations in that).

If you read above posts, use of reverse thrust is a good practice.

As far as I know airports can not prohibit the use of reverse thrust, PIC is is the one that will make that decision. However there are serious financial penalties involved with noise violations to the company.
Reverser use does not contribute to cycles on the engines I have operated only starts and shutdowns so that should not be an issue.

JJF

JJflyer, i disagree that the use of thrust reverse does not contriubute to the cycle count and therefore the degredation of life of the engine. It does add to the cycle count - not a complete cycle but part of one. The use of thrust reverse is basically an engine transient, which depending upon how much reverse thrust is selected and for how long, can use up life. It is well proven that transients are a big contributer to engine life counts due to the thermal expansion, soak, mechanical forces etc that they produce compared to say an hour of normal cruise operation. This becomes even more important considering the alt above sea level that this transient occurs at.
The reason i had posted this message was to get the opinion of pilots as to the use of thrust reverse or brakes as recently i had read that some engine manufacturers were looking into recommending that the use of thrust reverse be limited to improve life usage on certain engines. Any comments?

cheers

stu

Cycles
 

Never seen or heard that use of reverse thrust would contribute to cycles... I have never seen a tech log for any company reflect reverse use. The use of reduced or normal T/O thrust use is logged (Company procedural variatons). Starting and shutting down engine is a cycle.

You are right partly though. Use of reversers does contribute to wear and tear of the engine. But as far as I know it is not detrimental to the engine service life, unless one picks up FOD, over temps or causes compressor stalls on a regular basis.

It would be stupid to say that using reverse would not reduce engine life than normal cruise at FL350, of course it does. However operating the engine within it's approved envelope, where start-up, taxi, T/O, climb, cruise, descent, landing using reverse, taxi and finally shut-down, are all normal and have been reflected in the engine design life.

My point is that using reverse at high speeds and brakes at low speed one will get best out of both systems where these operate at their best efficiency. One will minimize FOD , stalls and re-ingestion of exhaust gasses as these occur at low speeds and minimize wear and tear of brakes.

AD

does anyone know of any studies that have been carried out with regards to the advantages and disadvantages of thrust reverse versus braking?
Also i believe that the Airbus manual states use max thrust reverse, whereas we dont. Am i right? Does Boeing also state the same?

thanks for responses so far

stu

JJflyer

Re: A colleague tells a tale of passengering in a 737 landing, in Indonesia I think, that ran off the end of the runway, with the crew sitting there for much of the landing roll, wondering why the auto brakes hadn't activated! :uhoh:

Ditto to the above. Carbon brakes wear less at high temperatures, thus, for those aircraft so fitted, it saves money to use more brake and less reverse. Also, cold carbon brakes wear unevenly, reducing braking effectiveness for the day you'll really need it, the RTO from close to V1.

The original question asked the relative effectiveness between Brakes OR Reverse.

Reverse thrust is not considered for Rejected Takeoff or Landing performance on DRY runways. This is not because of any anti-Reverse Thrust stipulation in the certification rules, but rather that the same certification rules DO stipulate that one means of retardation must be held in reserve. The choice is up to the manufacturer, and 'by-laws' within the regulations do allow the use of all means of retardation, if a 15% penalty is taken upon the ASDA.

To date, I do not know of any manufacturer who has favoured Reverse Thrust over Brakes as the primary means of retardation, simply because the manufacturer wants the best possible results, and that is invariably achieved with brakes.

Most modern aircraft have 3 systems of retardation - Brakes, Spoilers, and Reverse Thrust. The manufacturers still put reverse thrust last, after spoilers, not because they apply significant retardation force on the ground, but because they spoil lift, put more weight on the wheels, and make braking more effective. For one of Seattle's products, with which I was very familiar in the numbers area, the contribution to the reduction in Accelerate-Stop Distance Required from Reverse Thrust amounted to 3 aeroplane lengths. The Brakes and Spoilers did the rest.

Wet / Icy / Slick runways are another story.

In practice, what is required is a sensible application of all 3 means of stopping, as alluded to in the many posts here.

There's a DL crew who now knows it's better to get on the binders and reversers yourself rather than wait. Happened in 1997/98 in CLE. They didn't arm the autospoilers so the brakes didn't brake and by the time they got around to realizing it they were the subject of much attention from the gray concrete building across the airfield off their right wing. Of course when AA did the same thing the very next night the focus changed rather quickly.

As to studies done:
TWA did a couple in the 80's and that's perhaps the strongest reason why there were no autobrakes on the MD80's.
They used to rebuild their own engines in MCI so they were in a unique position to make the observations and evaluations.
Use the reversers, it's cheaper and safer, was the result.

does anyone know where one can get hold of these reports?
what other considerations should one take into account when deciding whether to use brakes or thrust reversers?

cheers

stu

I hate to say it but when the big fish ate the little fish most of the people that did the reports and evaluations went the way of the dinosaurs and retired. If you have any gray-haired contacts you may be able to find out who actually gathered the data but I'm afraid that someone would take great umbrage if I put their names in this forum.
Suffice it to say that at least two of the TWA guys were still there as of five months ago.

stuharri,

You might like to try this NASA Technical Memorandum Report :-

NASA TM-109158

Title: "Why do Airlines Want and Use Thrust Reversers? - A compilation of Airline Industry Responses to a Survey Regarding the Use of Thrust Reversers on Commercial Transport Airplanes"

Author: J.A. Yetter, NASA Langley Research Center

Date: January 1995


You can get this directly from NASA or via a university library if you have access to one.

With regard to the issue of the costs of brakes vs. thrust reversers this report states that on average thrust reverser maintenance costs are more than four times greater than the cost saving associated with brake wear assuming that thrust reverser use gives a 25% saving in brake costs.