View Full Version : Brake Application versus Heat.

19th Aug 2002, 20:55
In bringing an airplane without anti lock breaks,i.e. KingAir 100, to a stop on a long runway; Which technique would generate the most heat.
1) Heavy brake application for a short period of time.
2) Light break application for a longer period of time.

oxford blue
19th Aug 2002, 21:46
If it's a long runway, why not hold the nose high for as long as possible and use aerodynamic braking?

That way, you might not have to use the wheelbrakes at all, or only for a very little at the end.

19th Aug 2002, 21:51
The amount of heat generated is proportional to the mass of the aircraft and its ground speed (Kinetic Energy = 1/2m V squared). The brakes extract kinetic energy from the aircraft and convert it into thermal energy. Whether you use heavy braking for a short period, or light braking for a longer period, makes no difference to the amount of heat produced, provided the mass and overall deceleration is the same in both cases.

This does not mean that both methods will produce the same brake temperatures. The brake temperature also depends upon the amount of heat that the brakes can dissipate to the atmosphere during braking. while friction is putting heat into the brakes, the airflow over them and conduction into the rest of the landing gear, draws heat away from them. The temperature rise depends upon the relative rates of heating and cooling.

Harsh braking for a short period will reduce the time available to dissipate the heat, so the brakes are likely to reach a higher temperature. But this higher temperature will also increase the rate of heat disipation, so the overall relationship between braking time and brake temperature is not a simple linear one.

The key to judging the overall effect is to note that in order to benefit from the increased heat dissipation rate, the brake temperature must be greater. So harsh braking for a short period will tend to give higher brake temperatures than light braking over a longer period. It will also wear the brakes and tyres out more quickly, and increase general wear and tear on the aircraft.

20th Aug 2002, 00:49
If there were no other sources of resistance then what Keith Williams has said would be totally true. However, on a normal landing, there are a number of other forces that will come into play..Rolling resistance, Reverse Thrust/Pitch, Aerodynamic resistance and of course good old brakes.

By delaying application of the brakes, or by using them in a gentle fashion you will increase the effective time that the other stopping methods have to reduce the kinetic energy of the aircraft. i.e. Reverse will take out a bit of energy, etc and this will lower the energy requirements of the brake units, i.e. let them produce less heat. I suppose taking this to the limit by using no brakes at all, and just reverse/aerodynamic braking the roll out will be longer, but your brake temp rise will be nil.

Poor english -> time for bed!

20th Aug 2002, 12:40
It probably varies with aircraft type and the brake designs and materials, but I can tell you what works for the DC-8s with steel brakes.

A long application with light pedal pressures will cause VERY hot brakes. We try to delay brake applications until below 100 kts (preferably 80 kts), and then apply them firmly. If possible, get off them completely and then re-apply firmly once again. We have experienced significant cost savings by doing this. (Of course, short, wet or slippery conditions dictate a different technique.) ;)

20th Aug 2002, 15:26
Um, if it's a Kingair then don't you just use reverse thrust??

In the Metro I flew for a few years, the only time you'd use the brakes is for the last little stop when parking. The pads and discs would last for years.

20th Aug 2002, 18:55

You are of course correct.

I must confess that I had limited my comments to the use of brakes only. If the choices being considered are limited to touching down then either putting the brakes on very hard or very lightly, and doing nothing else to stop the aircraft, my comments are essentially true.

The coolest brakes and longest brake life will of course be achieved if you don't use them at all, until you park the aircraft.

Captain Stable
21st Aug 2002, 00:28
There are, of course, different characteristics to carbon brakes, which don't actually do much braking until they heat up.

21st Aug 2002, 01:44
Here's what can happen with draggy brakes on take-off. www.tsb.gc.ca/ENG/, click on "reports", then click the aircraft icon in the upper left and click on New Released Reports. The report number is A98Q0087.

21st Aug 2002, 03:49
I have no doubt that there is a good reason (?) why we don't ... but wouldn't it be nice if civil runways had distance to go boards so that, with the appropriate data .... one could have an idea if the actual performance were somewhere near the book guess ?

21st Aug 2002, 16:26
18-Wheeler: I use reverse and brakes.
The question is more hypothetical.
I have never been able to find reference material on brake applications and have always wondered which would generate the most heat hard/short or soft/long brake applications.

Like turbine engines some require fast /hot starts others long/cool starts.

Ignition Override
22nd Aug 2002, 05:53
Our malfunction manual has a brake cooling chart for high speed aborts. How about a max effort landing (max braking) during a divert to a short runway? Would every pilot use the same cooling chart and then call Maintenance Control?