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Nick 1
11th May 2008, 17:55
So you are taxing to the rwy (brake fan instatalled) , temperature lets say 250 (fan on) . Max temperature for t/o is 150 , so you turn off the fan and start the take-off , because now the limit is 300 .
Right ? Wrong?
Cheers.

Nick 1

Hand Solo
11th May 2008, 18:49
Wrong. The Fans On limit is lower because the fans cool the temperature sensor faster than the brakes. A sensor reading of 150C shortly after turning the fans on may correspond to an actual brake temperature of double that. If you have a Fans On temperature reading of 250C it will almost certainly correspond to an actual brake temperature in excess of 300C and a subsequent RTO could lead to an exceedance of the brake energy limits.

Swedish Steve
11th May 2008, 18:59
Is there not an ECAM message that tells you to 'Delay T/off' (or similar) in this scenario?

Yes. With the fans off this is set at 300degC.

Conan The Barber
11th May 2008, 19:04
The 300C limit is not due to brake energy. It is to prevent hydraulic fluid ignition in case of a leak in the main gear bay.

Dani
11th May 2008, 19:28
The brake fan operation is a well disputed thing. Generally there is no general guideline, except the expanded checklist in FCOM.

It depends mainly on your airline's managers and/or flight instructors. You have to take into account the brake temperature, the temperatures in case of a RTO (rejected take off) and the brake wear.

There are also different brake supplier for Airbus, which have different wear characteristics. There are some guidelines on the web. Look in smartcockpit.com.

There are basically 2 different schools of thought: The first says that the brakes have to be as cool as possible, those guys switch on the fan all the time. Then there are those who want to have the brakes always within a reasonable range of temperature. They hardly ever use them, only after a RTO or an operational necessary hard brakes application. Then there are those who operate Airbusses without brake fans. They have to think a bit more, but it's also possible to make quick turnarounds.

Make up your mind which is best for you, consulting your airlines opinion leaders before ;)

hth,
Dani

PS: In your case there is no y or n, it also depends how long is your turn around, how is ambient temp, how long is taxi time, how fast where temps rising aso.

Nick 1
11th May 2008, 19:41
But the limit depend of the position of the switch ?
Fan installed or never set to on 300
If fan on 150 .

Cheers.

Dani
11th May 2008, 19:45
If you don't have them installed: 300 is your limit.

Switch position (it's not a switch but a button) doesn't matter, because ECAM and the button tells you if the fans are working or not. If there is a problem you have an ECAM error message.

The difference comes from the fact that the temperature sensor is an air temperature sensor, and if there is more airflow through the brakes discs through the fan, there is more colder ambient air flowing around the sensor, which lowers the indicated temperature. 300 with fans off and 150 with fans on gives you about the same disc temps.

Dani

SIDSTAR
14th May 2008, 00:06
Dani is correct: 150 fans on = 300 fans off approx. Carbon brakes work best when they're warmed up to a reasonable temp, not when they're cold. Different brake manufacturers give slightly different advice on the use of brakes and fans. Just do what your company SOPs say as they will have been set up with input from the brake manufacturer. Also, different airlines may have different cost structures so may decide to use/not use the brakes and reverse differently. For example, some use normal reverse with manual braking below 100 kts while others use autobrake and idle reverse (runway length and condition permitting) to get the same landing performance. Turnaround times also come into the equation. It's always best not to put the fans on for at least 5 mins to allow the brakes to thermally equalize. Personally, if I were writing the SOPs I'd leave them OFF until pushback on the next sector unless the temp was very high. The limiting factor appears to be the flash point of the hydraulic fuel in case of a leak.

Nick 1
14th May 2008, 07:33
Discussion is born because we where cooling the brakes with fan on , cleared for t/o temperature was 250.
So we turn off the fans , i'm my opinion if we never used the fan, the limit is 300 .
But in the moment you start to use them the limit is now 150.

Cheers.

Wingswinger
14th May 2008, 07:49
Wrong. Switch them off and if you are below 300, take off. In 9 year's A320 series flying on aircraft both with and without fans I have never seen temperatures go up more than 20 -30 degress after fans have been turned off.

If you have carbon brakes, they are most efficient if temperatures are in the range 150 -200 when heavy braking commences.

FlyUK
14th May 2008, 10:29
It also depends how long the fans were on for?! If they were 350 degrees and you taxi from the stand and then turn them on for 5 mins and get airbourne, then you will most likely find that when the wheels go into the bay that you get an ecam saying they are hot (>300) as they will hardly ever cool in the wheel well. The issue you then have is very hot brakes in the wheel well. Not really a great position. If you have 250 degrees with fans on and don't want to delay takeoff then turn the fans off and get airbourne, leaving the gear down for 2 mins. Works well. Obviously an RTO should be considered.

IMHO the takeoff decision was incorrect.

goeasy
14th May 2008, 16:30
I think its common sense, and prudent, once you have fans on to wait for 150deg before takeoff. To do otherwise is outside the limitations. Once the fans are on, you are stuck with lower limit as I understood the temp sensor is not at the friction surface. Therfore the indicated temp may not be the actual temp in the critical zone.

I dont turn them on on the turnround for exactly that reason. Even 350deg will be 200-250 after 20-30min turn and taxi.

PantLoad
14th May 2008, 19:53
Gentlemen,

We are talking about several factors. I will list them:

1. The 300 degree limit is a certification issue. Due to the flash point of Skydrol (It varies a bit, based on individual manufacturer of the Skydrol.), AND due to the fact that the Bus hasn't wheel well fire warning, you are limited to 300 degrees of brake temperature for takeoff. (Typically, the flashpoint of Skydrol is around 425 +/-.) So, ostensibly, if you take off with brake temps at or below 300 degrees (fans-on or fans-off temperature), you'll have plenty of margin. (Yes, I know, you can't take off with fans on...you turn them off before takeoff...but, say you turn off the fans as you're taking the runway for takeoff...and the brake temp at brake release is 300 degrees...well, the actual temp of the brakes is around 425 or so.)

2. Which brings up the next point: Yes, due to the cooling of the brakes by the fans, you'll have an actual brake temperature of around 125 to 150 degrees more...than what is indicated...when the fans are on.
300+125=425.....What a coincidence!!!

3. It's true that carbon brakes wear better when they're a bit warm...not hot! Depending on the brake manufacturer (They all have slightly different engineering data.). However, the stopping capability of the brakes decreases with increases in temperature. Your best stopping capability is with brakes cold. You must not confuse stopping capability with wear...they're two different things.

4. The single-aisle Bus has a little-known caveat. The accelerate stop distances you see published are predicated on the brake temperatures (at brake release for takeoff) being no more than 150 degrees. So, you can take off with brakes at 300 degrees (and be OK with regard to fire in the wheel well), but if you're accelerate-stop limited on your takeoff, you might want to reconsider things. You may or may not get it stopped in the runway available.


If you follow the FCOM and FCTM, you can't go wrong. I hope this clarifies things.


Fly safe,

PantLoad

Dream Land
15th May 2008, 03:02
Great explanation. :ok:

flyingins
15th May 2008, 05:38
Keep it simple.

If you never turned the fans on before departure the maximum brake temperature for take-off is 300 degrees.

If you did turn the fans on? The maximum brake temperature for take-off is 150 degrees.

Nick 1, in your scenario where the temperature is 250 degrees and the fans are on, you must leave the fans on until the temperature reaches 150 degrees, switch them off and then depart. If you had never switched the fans on the limit remains at 300 degrees, so you could depart immediately.

Why? The brake temperature sensors are located in a position immediately next to the flow of air from the fans. When the fans are running the air artificially "cools" the sensors so that they indicate a significantly lower temperature than the actual brake temperature. In reality, with the fans on, an indicated temperature of 150 degrees equates to a "real" brake temperature of 300 degrees.

Returning to the simple part of this;
1) Fans on? Keep them on until 150 degrees is indicated.
2) Fans off? 300 degrees is acceptable for take-off.

PantLoad
15th May 2008, 06:26
Gentlemen,

Flyingins is correct. I was in error. Sorry,


PantLoad

Nick 1
15th May 2008, 07:01
Thank you gentleman....

Dream Land
15th May 2008, 07:06
Clear as mud. :}

lfbb
15th May 2008, 10:52
Great explanations on this post!:ok: