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stuharri2002
26th Jul 2004, 08:47
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

RadarContact
26th Jul 2004, 09:20
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.

fritzi
26th Jul 2004, 09:26
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

BOAC
26th Jul 2004, 09:57
stuharri - brake cooling also has to be taken into consideration, and appropriate reverse will mean cooler brakes if the turnround is short.

GlueBall
26th Jul 2004, 21:44
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.

80/20
26th Jul 2004, 21:54
stuharri2002 - What type of aircraft are we talking about, e.g. transport category jets?

avioniker
26th Jul 2004, 22:02
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?

Atlanta-Driver
27th Jul 2004, 00:46
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

Flight Detent
27th Jul 2004, 02:55
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

FakePilot
27th Jul 2004, 04:30
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?

JJflyer
27th Jul 2004, 05:04
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

stuharri2002
27th Jul 2004, 08:41
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

Atlanta-Driver
27th Jul 2004, 10:17
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

stuharri2002
27th Jul 2004, 11:49
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

Capt Claret
28th Jul 2004, 09:06
JJflyer

Re: 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.

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:

Old Smokey
28th Jul 2004, 16:18
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.

avioniker
28th Jul 2004, 23:38
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.

stuharri2002
2nd Aug 2004, 15:48
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

avioniker
2nd Aug 2004, 23:00
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.

merlin505
18th Aug 2004, 13:28
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.

df1
21st Aug 2004, 10:10
Wasn't there also a similar incident/accident with a Lufthansa A320 in Warsaw some time ago to what avioniker refers to?

I'll stand corrected but wasn't there some combination of tail-wind and water logged runway which caused hydroplaning? And was it this that prevented wheel spin-up which in turn prevented spoiler deployment? This (if I'm not mistaken) had the effect of not getting the weight on the squat switches so the reversers were locked out!

Sorry if this is/was deemed to be inaccurate.

Dan

Easy226
23rd Aug 2004, 12:35
While travelling back from Crete on a MON A321, I noticed that the reversers made a terrific noise from inside the cabin unlike a 737. I have not been on an airbus for a long time, but was wondering weather all airbus's have to use max reverse on landing or weather it was just pilot preference at this time?

Reverse thrust cannot contribute to much of the stopping potential of a jet aircraft on landing if the landing distances are calculated without the use of reverse thrust?
Many Thanks
Dan

hawk37
23rd Aug 2004, 14:57
Easy,
No aircraft "has" to use max reverse thrust on landing. Sounds like it was pilot technique. Certainly, pilots will use more reverse on a short and/or wet runway.
Reverse thrust will contribute to the stopping distance based on the retarding force. Assuming a constant reverse thrust force (not necessarily the case) then the reversers will dissipate more of the aircraft's energy if used at higher speeds, versus used at lower speeds. Hence, a 10 second use of reverse at higher speeds will mean the brakes have to absorb less enery than a 10 second use of reverse at lower speeds.
I suspect this is why it is often said "reversers are more effective at higher speeds"
Hawk

Easy226
23rd Aug 2004, 17:27
Oh right i see thanks for the reply. I read in an earlier post that in the airbus manual it says to use max reverse, hence why I thought this was the case with airbus aircraft.
So reverse thrust is more efficient at higher speeds - is this why the reversers are cancelled when at 60knots?
Many Thanks
Dan

hawk37
23rd Aug 2004, 18:05
Dan, I haven't seen any airbus manuals, but I doubt they say to use "max" thrust for all landings.

On small jets, use of reverse thrust can poses some controllability problems, and hence those pilots are hesitant to immediately go to full reverse on landings. Typically, in these cases, the pilot concentrates on applications of brake, and steering of course. Reverse takes a much lower priority, and the usual sequence of events are to first get reverse thrust deployed at idle, and then slowly increase power in a controllable way. And often the aircraft is well decelerated by the time full, or close to full, rev thrust is attained.

And here's one to think about. With any crab on, eg for a slippery runway with a cross wind, then the added use of reverse thrust will actually pull the aircraft towards the downwind side of the runway. When this starts to happen, immediate reduction of power, among other things, can be required to keep the aircraft from drifting off the downwind edge of the runway!!

Yes, you could say rev thrust is more "efficient" at higher speeds, I prefer to think of it as the reverser dissipates more energy when used at higher speeds than at lower speeds, hence less brake wear.

The aircraft type/engines can have different speeds to be out of reverse. For example, for some, only idle thrust with reversers deployed below 60 kts, and some even allow an appreciable amout of thrust up til the time the aircraft has stopped.

Hawk

Easy226
24th Aug 2004, 19:30
Oh right i understand fully now. So when you are slowing down a large jet aircraft, does it require a large amount of concentration to keep the aircraft travelling in a straight line.
Do the large forces assosciated with applying braking and reverse thrust make the aircrafts movements more liable to become unstable?
Many Thanks
Dan