747-100/200 Brake Overheating

[cf6-80c2b5f]

I have a friend who used to fly a 747-100/200F into Kabul on a fairly regular basis. I was amazed when he told me about the problems they used to have with brakes overheating and tire fuse plugs thermaling.

He said they almost always landed at their maximum landing weight (tankering fuel), which usually required a medium setting on the autobrakes at Kabul. But, he said if they didn't reach up and go to minimum on the autobrakes once they touched down and got into reverse, they would likely thermal the tires due to the brake temperatures exceeding 800 to 1,000 degrees F once they taxied in and parked on the ramp.

In other words, their runway analysis stated that they could land on a particular runway with medium autobrakes at a certain weight and atmospheric condition, but using medium autobrakes all the way down to a stop would result in thermaling most of the tires.

I thought the landing performance was predicated on getting it completely stopped within 60% of the runway with maximum braking. Wouldn't this imply that there is something wrong here if doing so would cause the tires to thermal? Shouldn't maximum braking to a full stop mean that they should even have been able to do it with the autobrakes on max. Thoughts? Thanks.

[EW73]

I used to fly 747-200 around the Saudi, UAE and Somalia areas for some years, and had only one single tire on one occasion 'thermal' out on me!
The technique we found worked best for us was to not select any autobrakes at all, use max reverse at the earliest opportunity, then use manual braking, as required, below 100 knots, and always use the entire available runway. With that technique, as I said, we had few brake overheat problems. Oh they certainly got hot, but stayed up!
The one time we did deflate a tire, this technique wasn't used and we were very lucky not to have many more than one deflate, and we only had one spare on board!
Great to have an FE on board, isn't it...got it changed using assorted local lifting/inflation equipment, and we were away back to Jeddah in good time! :ok:

[Intruder]

"Runway analysis" just tells you if you can stop, not how hot the brakes are going to get. "Maximum brake energy" is a LOT more than the energy required to get them hot enough to melt a fuse plug or trigger the overheat warning.

Indeed, braking too hard, too early will exacerbate the situation when the brakes or stopping distance is marginal. 747 Classic crews should realize that great care must be taken when more than Min Autobrakes are required.

[cf6-80c2b5f]

Thanks for the replies. It sounds like the crews do all kinds of unorthodox things to keep from overheating the brakes. Runway analysis tells you if you can stop, but if the analysis says you must use medium autobrakes -- not low -- why would you have to switch to low autobrakes once you touched down to prevent thermaling the tires? It seems to me that something is wrong if he can't leave medium autobrakes all the way down to a full stop.

[18-Wheeler]

I used to fly 747-200 around the Saudi, UAE and Somalia areas for some years, and had only one single tire on one occasion 'thermal' out on me!
The technique we found worked best for us was to not select any autobrakes at all, use max reverse at the earliest opportunity, then use manual braking, as required, below 100 knots, and always use the entire available runway. With that technique, as I said, we had few brake overheat problems. Oh they certainly got hot, but stayed up!

That's what I used to do flying the freighters into Dubai, hot and at MLW most of the time. By not using autobrake for touchdown it'd give you about one unit better on the brake temps, usually enough to have them hovering around near the top of the yellow band. Autobrake would almost always have the brakes just into the red and so you'd have to call for brake cooling carts.

[Cough]

CF6,

Take the 737-400. Take a hot day into an airfield at altitude with no reverse at MLW with max brakes.

This also puts you into fuse plug melt zone.

Use the procedure above - Reversers and no autobrakes till 100kt means you won't have any limits on a subsequent quick departure, because the brakes will be cool enough...

Not just the jumbo!

[mutt]

cf6-80c2b5f, are you sure that the runway performance was based on Autobrakes? In our case with the classics, the AFM data was based on Maximum Manual Braking.

[bcgallacher]

I handled 747-100 and -200 staging through Dubai - some of them flown by 18 Wheeler - and found that it depended on who was landing the thing as to what the brake temps would be.The same individuals would arrive on the gate with the brakes smoking every time while others would have brakes in the green or low amber.For some reason the Canadian pilots would have consistently lower temps than most others.From what I recall those that did not use auto-brake arrived with lower temps. In Dubai there really was no excuse as the runway length did not require heavy braking. Hiring 2 airconditioning units plus operators from Dnata was not cheap and we had delays when they were not available. Do not ever allow anyone to pour water on hot brakes - Boeing allows a fine mist only.
Good to see my old friend 18 Wheeler is still around.

[cf6-80c2b5f]

They use an OPS computer program, just like a lot of carriers do on the 747-400's. You enter your landing weight, landing runway and ATIS information and it spits out the stopping distance with various autobrake settings. In the case of the classic, that would be minimum, medium or maximum. If the stopping distance with that autobrake setting is greater than the runway length, then that autobrake setting is grayed out so you can't use it.

In the case of going into Kabul at max landing weight, the minimum setting is usually grayed out, so the crews would have to use medium. What was happening was that crews were often late to go to minimum once they felt they could get it stopped by the end of the runway with the lower deceleration rate that min autobrakes would give (also using max reversing). As a result, they would end up thermaling tires and getting stuck there.

It just seems odd to me that under normal conditions, any amount of braking would result in thermaled tires if the analysis said you can dispatch to that runway. For instance, if you truly were right at the limit on a dry 10,000' runway, F.A.R. 121.195 requires that you get it stopped by the 6,000' mark (60%). I would presume that this is with maximum manual braking as Mutt pointed out. As far as I know, you can outbrake maximum autobrakes (but not RTO), so even a max autobrake setting would actually be something less than max manual braking.

Given the example above with max braking on the 10,000' runway, you should be able to leave the autobrakes on max all the way down to a stop and still not thermal the tires, right? These guys were thermaling tires with autobrakes on medium.

Also, FAR 25.125 requires that "[t]he brakes may not be used so as to cause excessive wear of brakes or tires." I can't imagine that thermaling tires would be acceptable to the regulators but excessive brake or tire wear would not.

[Old Fella]

In my time on the Classic (200-300 & 200F) with a well known HK operator Auto Brake was rarely used, predominately Full Flap, Max Reverse and Manual Braking. Never had a fusible plug melt or an overheated brake in the ME or anywhere else. Something to be said for the maintenance of manual skills in the Pilot world.

[main_dog]

I had the good fortune to fly the same airframes Old Fella did, albeit in the last three years of their operational lifetimes... they may have been twenty-year-old airframes and probably not performing as well as the tables and graphs suggested, but no autobrake/full reverse after touchdown and a smooth, gentle application of brakes below 100 kts was definitely the best way to go on the -200, especially in BOM or DXB in the summer.

I miss the old girl, but with the -400's carbon brakes life is so much easier! :ok:

MD

[grounded27]

I used to handle a 742F out of KMIA that used to thermal tires on a regular basis on a 13,00 ft rwy and had understood that it was the operator's manual that encouraged autobrakes was the fault. The damn pisser was once one blew I was guaranteed 4-6 more.

[cf6-80c2b5f]

I appreciate all the replies. The consensus seems to be to refrain from using autobrakes and use superior airman skills to get it stopped without overheating the brakes. But I'm still trying to figure out whether there was something wrong with their aircraft when a medium autobrake setting would often cause thermaling, given that the analysis (approved by Boeing) is predicated on max manual braking to a full stop. I understand that the 60% rule of 14 C.F.R. 121.195 is for dispatch purposes only and not actual landing where you can take credit for reverse, but it's difficult to believe that the numbers produced by the analysis would result in thermaled tires if actually carried out.

[Romasik]

If they make brakes that hot on landing, touching down in the beginning of the RW, then what do they have on their takeoff performance with presumably higher weight rejecting in the end??????
Something is wrong here...

[747dieseldude]

We had on our airport analysis a "maximum quick turn weight". It was usually well below max landing weight for a given runway and it guaranteed that you can get the brakes cold enough for another takeoff in an hour.
Our technique for dry runways was to land with autobrakes MED, and once touchdown was made at the proper point and reversers out you would disconnect the aurobrakes and let the t/rs do their job.
On wet or worse there was no overheating problem.
Also the taxi-in technique was important, not riding the brakes and shutting down #3.

[cf6-80c2b5f]

Romasik: That is a good point. If the brakes couldn't absorb the energy to get it stopped on a landing without thermaling tires, did they really have the ability to stop it on a rejected takeoff?

Dieseldude: It sounds like everyone had their own technique for not thermaling tires, but I am trying to find an answer to this question: If the runway analysis says you can dispatch to a particular runway, shouldn't you be able to apply max brakes to a full stop on landing and not thermal tires if the brakes/tires/aircraft are properly maintained? I know it's not the preferred technique, but the question still stands.

[18-Wheeler]

Romasik: That is a good point. If the brakes couldn't absorb the energy to get it stopped on a landing without thermaling tires, did they really have the ability to stop it on a rejected takeoff?

Not a good point sorry - It's not unreasonable to expect the thermal plugs to stay intact with a normal landing, and it's not unreasonable to expect the thermal plugs to blow with a maximum-effort stop.
That's what they're for.

Hey Mr B, ya wee fella!

[cf6-80c2b5f]

I would expect the thermal plugs to melt on a rejected takeoff because the brake energy charts state that they will above a certain speed and weight. I think the point Romasik was making wasn't that the thermal plugs would not melt on a reject -- he was saying that if the brakes were so poor that they couldn't withstand a normal landing with autobrakes in medium, do they really have the capacity to stop on a reject, as advertised.

When you say that it's not unreasonable to expect the thermal plugs to stay intact during a "normal landing," would you consider a medium autobrake setting to a full stop to be a normal landing?

[Intruder]

Applying more brakes early will force the brakes to absorb more energy, therefore increasing the chance of overheating. Remember that Boeing Autobrakes (other than at Max or RTO) attempt to maintain a constant deceleration rate, not a constant energy-absorption rate. Because energy absorbed is proportional to the square of the ground speed, the brakes can withstand a higher deceleration rate at low airspeed without overheating. Clearly, using max Reverse immediately after touchdown will help prevent brake overheating by dissipating much of the energy that would otherwise be absorbed by the brakes.

AFAIK, OPS only 'worries' about the Maximum Brake Energy published by Boeing, and NOT whether or not the thermal plugs might melt. Also, OPS (as configured for our Classics) did not use Thrust Reverser credit for the Operational Landing Performance.

Note that the limit certification criteria for a rejected takeoff only require that a severe fire will not break out for 5 minutes after a max-TOGW rejected takeoff. Thermal plugs merely help prevent tires from exploding when they are overheated, so they may melt well before the max brake energy is absorbed if a tire is heated quickly by hard braking at high speed.

[cf6-80c2b5f]

Intruder:

I won't argue with what you just said, but the question still remains:

If the runway analysis says you can dispatch to a particular runway, shouldn't you be able to apply max brakes to a full stop on landing and not thermal tires if the brakes/tires/aircraft are properly maintained?

[Intruder]

No.

OPS does not analyze brake heating or heating rate; it only analyzes projected total brake energy absorption for the brake limit part of the equation. If you use up all the projected early by stomping on them at high speed, thereby not allowing air and other drag to do their part, you will use MUCH more brake energy than projected by the analysis. Also, using the energy early means the brakes are absorbing energy at a higher rate, and may not be able to dissipate it quickly enough to the air before the thermal plugs overheat.

[cf6-80c2b5f]

Assuming, arguendo, that some classic operators did not use OPS -- they used the old runway analysis charts which, if I'm not mistaken, are predicated on 121.195's stopping within 60% of the runway with maximum braking. That is really all that's required anyway -- the dispatch performance. OPS is nice but really not required by the regulations if you can legally dispatch to the runway under 121.195.

Are you saying that if the pilot actually applies maximum braking when he lands there in order to get the performance guaranteed in the runway analysis, there is no assurance that he will not thermal tires and this is all within the confines of the aircraft certification? Boeing is okay with this?

[Intruder]

OPS is built around all those charts.

Just because 121.195 sez you have to be able to stop in 60% of the runway, doesn't mean you try to do it on every runway every time. Margins are built in for contingencies and emergencies, NOT for everyday operation. Also, 121.195 does not prohibit Reverse Thrust credit.

Yes, that is effectively what I am saying. Yes, both Boeing and the FAA are "okay with this."

Remember again that the max TOGW rejected takeoff certification is a one-time requirement. The brakes and tires don't have to be usable afterward.

[cf6-80c2b5f]

Okay. Thanks for your input.

[Willit Run]

When you apply medium brakes at 163 knots, you are getting the maximum amount of energy into the brakes as soon as possible.
I only have about 5000 hours on the classic, but can assure you, that landing with flaps 30 and no auto brakes and reverse as quickly as you can, you can delay brake application until about 80 knots, and barely have them register in the green.

The problem that I think your "friend" is running into, is the companies SOP of mandating using auto brakes; dumbing down the procedures for the lowest common denominator. I used to regularly NOT use auto brakes, and would often arrive at the chocks with condensation on the brakes.

[vilas]

Last I flew the classic 200/300 series 10 years ago. I basically agree with Wiiit Run. I used this exact technique when ever I wanted to exit on closer taxi track without heating breaks. Kabul elevation is 5877 ft. so that will add to the problem.