Brake energy limits

[deskjobssuck]

Can someone please explain to me what Brake Energy Limits are? We are currently researching the Hawker, but I can't give a good explanation as to what Brake Energy Limits are because I've never dealt with them before. Thanks for the help.

 

[Kenny]

I'm pretty sure it has something to do with the amount of energy the brakes can disperse during the take off run if you have an RTO at or below V1. The limit can affect the V1 speed and therefore your MTOW.

Someone who's done the JAR ATPL will probably have a better explaination for you though.

 

[Flythis]

Will it stop at a given weight ? That's about it. Over the limit on the chart and their is a good chance of going agricultural. Very simple really.

 

[gutshotdraw]

Deskjob,

Whenever an aircraft is stopped using the wheel brakes, the kinetic energy stored in the moving aircraft is transmitted to the brakes. The by-product is heat. The faster the aircraft is moving or the heavier the aircraft when the brakes are applied, the more heat is generated in the brake assembly. Once the brake reaches a certain temperature, it can no longer absorb any more energy and therefore brake effectiveness deteriorates to the point of no longer stopping the aircraft. Depending on the aircraft and brake manufacturer and the material used (steel, carbon), the brake energy limits can vary greatly.

Brake energy limits are usually only an issue either on a quick turn or after a rejected takeoff. As I recall, the Hawker was somewhat sensitive to brake energy limits after a landing or RTO. The performance manual will always inlcude a brake energy chart that compares factors including aircraft weight, ambient temperature, cool down time between takeoff attempts, etc. Plug in all the appropriate numbers and you come up with a weight limitation for the next takeoff attempt or a minimum cool down time before you can launch. If you exceed the limit and have to abort, you just became a test pilot and may very well depart the airport boundary, blow a tire, both, or worse.

 

[con-pilot]

BEL (Brake Energy Limit) is a major factor in takeoff performance planning for 'high and hot' operations. On some jet aircraft the most limiting factor on a high altitude airport coupled with high ambient temperature will be the BEL.

All aircraft have BEL limits, as anyone who has flown a DC-3 with the old style brakes can attest to.

Now, I'm going to say something that may get me into trouble.

The fact that anyone who is operating jet aircraft, or who holds an ATP to ask such a question has no business doing either. I would highly recommend that they run, do not walk, to the nearest Flight Safety School (or CAE) and get some basic jet aircraft operational training.

I am glad you asked, now get some proper training.

(And no, I do not work for Flight Safety.)


(Now putting helmet on and going to the bunker.)

 

[ultrarunner]

Hey Con, the original poster is NOT operating a jet aircraft, according to their profile. Valid question and good responses.

 

[Blackjet]

The Hawker has its own aircraft specific limitations. Typically, it's never an issue in the 800 series. Where you get into trouble with the Hawker is on the RTOs (Rejected Take Offs). The key to the Hawker is 90 Knots. It's be awhile, but I believe you can abort once and give it another go before the limitations begin to apply, PROVIDED you never reached 90 Knots on the attempted take off. If you abort above 90, the charts are going to be useless to you, because you ar enot going to be able to get the braking system inspected faster than the brakes can cool off. Yes, you read that correctly, any reject above 90 requires an inspection, even if you let it roll to the end and just used the T/Rs, never touching the brakes.
Most of the time, it's either Hot/high ops or you are doing short hops (ramp dumps) and just tankering fuel. That's where you start to have to let the aircraft brakes cool down before you can put it back into the wind.

 

[con-pilot]

Hello, does anyone here know the difference between;

"Brake Energy Limits"

and

"Brake Cooling"?

They are two different things!

One can have cold brakes and still be limited by "Brake Energy Limits".

After a maximum performance landing (brakes to anti-skid) or an RTO does "Brake cooling" come into effect. Well that and a very long taxi after landing coupled with a short ground stop and a long taxi back for takeoff. In some aircraft, such as the 727, that is very important, the 737 it is not that important.

Brake temperature has nothing to do with figuring takeoff performance at an high/hot airport in regards to "Brake Energy Limits".

Yes, the warmer the brake is the less effective the brake is, no argument. However, that is not the subject under discussion.

Now I have noticed that some folks have mentioned Hawker 800s, I don't anything about Hawkers, 800s or any other Hawker Jet series. However, I would like to see the brake charts you are referring to and see how they are labeled.


(Back to the bunker.)

 

[ultrarunner]

Con, it's not always a chart. It some types it can also be a table. If I remember, I'll post the table from the aircraft I fly.

I was actually BE limited a week or so ago at a SL airport at high weight and high temp when I ran the perf to see if I could take off with a 5k twc. I was BEL with the twc, but not so with a hwc. Naturally we took off with the hwc and adjusted out departure with respect to the weather. It always needs to be considered.

 

[HA25]

Just one bit of advise...

Get a Hawker with the TR's and..

USE THE THRUST REVERSERS TO THEIR MAX POTENTIAL ON LANDING or RTO! And you should not have to worry about real world Brake Energy Limits on a Hawker.

I've flown everything from 15,000lbs to many hundreds of thousands, and if you use those TR's the way they're intended, you hardly need touch the brakes on most landing until you're about to take the high speed..


Short runways, then all bets are off..

 

[Blackjet]

Be careful with that school of thought. Though I agree with you, there are LOTS of people on here who believe you are a hack puilot if you are not dogging out the brakes out as soon as the mains touch because you should be concerned about the aircraft on final having to go around. I know, that's laughable, but plenty of folks out here who think that way and have no problem telling you all about it.

Also, about T/Rs on the Hawker; if you get an 800 series, T/Rs for sure. But if you get a 700 or older, I would find one with out the T/Rs for performance at altitude reasons. It's a 500 pound system and a killer in the climb. They sux anyways on the 700s, so I do without.

Just one bit of advise...

Get a Hawker with the TR's and..

USE THE THRUST REVERSERS TO THEIR MAX POTENTIAL ON LANDING or RTO! And you should not have to worry about real world Brake Energy Limits on a Hawker.

I've flown everything from 15,000lbs to many hundreds of thousands, and if you use those TR's the way they're intended, you hardly need touch the brakes on most landing until you're about to take the high speed..


Short runways, then all bets are off..

 

[HA25]

Be careful with that school of thought. Though I agree with you, there are LOTS of people on here who believe you are a hack puilot if you are not dogging out the brakes out as soon as the mains touch because you should be concerned about the aircraft on final having to go around. I know, that's laughable, but plenty of folks out here who think that way and have no problem telling you all about it.

Well, while I by no mans subscribe to the "Screw the guy on final, I own the runway" mentality... I'll be darned if I'm going to lay on the brakes of a 368,000lbs landing weight L1011 so I can clear the runway a bit faster for the guy that was sequenced too close to me. It's all context and good habits formed on a Hawker will pay off in the future.

If you're in a Citation V or Lear 35 that's one thing, if you're in a large or heavy that is the ONLY way to safely land/roll out and as a guy starting out in (whether in Jets, or otherwise) flying, it's a bad habit to get into where you get on the brakes at 140+ kts touchdown speeds of some jets.

 

[HA25]

double post

 

[JimNtexas]

Deskjob,

Whenever an aircraft is stopped using the wheel brakes, the kinetic energy stored in the moving aircraft is transmitted to the brakes. The by-product is heat. The faster the aircraft is moving or the heavier the aircraft when the brakes are applied, the more heat is generated in the brake assembly. Once the brake reaches a certain temperature, it can no longer absorb any more energy and therefore brake effectiveness deteriorates to the point of no longer stopping the aircraft. Depending on the aircraft and brake manufacturer and the material used (steel, carbon), the brake energy limits can vary greatly...

That was pretty much exactly the issue on the good old F-111. Can a business jet generate enough heat to explode the tires?

The Vark did have the possiblity of a tire exploding a few minutes after stopping if you were at the high end of the BEL.

The Space Shuttle got a drag chute a few years ago after they blew some tires a few minutes after landing.

 

[gutshotdraw]

Well Jim, the short answer is yes, but. A former co-worker was performing high-speed taxi tests in the Hawker at the behest of the maintenance manager (why? I haven't a clue). After the third one, the brake temp rose so high that the fuse plugs in two tires melted out allowing the tires to deflate more safely than an explosive event. All business aircraft that I have flown are equipped with such fuse plugs. I would have thought the swing-wing was similarly equipped.

As to con-pilot's point: Yes, there can be a brake energy limit on an aircraft with stone cold brakes depending on the aircraft weight, field elevation, ambient temperature, etc. and whatever the chart says. My feeble memories of the Hawker are that the 700 that I flew could easily have a BEL for a proposed takeoff under the right conditions in the event of an abort. My current aircraft (G-IVSP) won't even come close to the BEL unless you just landed or rejected a takeoff. Then, you enter the chart to include cooling time to make sure you won't exceed the BEL on the next takeoff.

 

[HA25]

It does merit noting that the max temps of the brakes aren't reached for a good 3-5 minutes AFTER the landing roll ends. I've witnessed this on a TriStar landing in ATL 5 years ago.. no TR's were used for some operational reason that I forget, and they were heavy.. I recall the brakes set fire about the time they were about to pull up to their gate.

 

[stknrdr]

The physics of it all are at rocket-scientist level for me, but yes to keep it simple: the brakes can only absorb/disperse so much energy. Few airplanes with standard brakes can withstand a full-on abort and return to the runway right away for departure; most will blow fuse plugs within 20min after the abort when the brakes reach their highest temp. The gotcha here is when planning arrivals into high altitude airports in these aircraft. That thin air doesnt aid in brake cooling, and your kinetic energy will be higher (think ground speed at a higher elevation) and the brakes will work harder getting you stopped. The charts basically tell you at what weight your brakes will start losing their effectiveness which is more than an attention-getter landing at Telluride, Sedona, Quito and a several others.
Get a Lear 45 with their sweet carbon brakes and no "turn around times" and go on your merry way.....

 

[Hawker800]

Deskjob:
The Hawker does have brake energy limits but they rarely come into play except for high,hot&heavy ops. The 800a that I fly requires a 5 minute cool down after taxi in to taxi back out. Also an RTO below 90kts. requires a 25 minute cool down before trying again. Found that one out the hard way.
Good luck on your search.

 

[MauleSkinner]

Deskjob:
The 800a that I fly requires a 5 minute cool down after taxi in to taxi back out.

Unless, of course, your takeoff weight is above the limit specified in the AFM table for the field elevation and conditions present. Then it's 20 or 25 minutes.

Fly safe!

David