Clarification sought re Airbus Brake temp limitations

[nnc0]

From the A320 FCTM

"The FCOM limits brake temperature to 300 Deg C before takeoff is started. This limit ensures that any hydraulic fluid that may come into contact with the brake units will not be ignited in the wheelwell after gear retraction. However the brake temperature should not rise above 150 Deg C (fans off) in order to respect brake energy limitation in the case of rejected take-off."

I find this confusing. Guaranteed any RTO will see brk temps exceed 150 degs so what is the point of that statement, and if they are stating that the limit for T/O = 150 deg C, why confuse us with the 300 deg limit.

[Hand Solo]

IIRC the brake fans cool the temperature sensors much faster than the actual brakes so the sensors underread. I think the idea is that if you've had the fans on for the duration of the turnaround and the brake temps are still indicating 150C then the chances are the brakes are actually at 300C so you should delay takeoff. This may be a typo as our manuals stated the limits as 300C fans off or 150C fans on, but it's still a bit of a muddle.

[nnc0]

Thanks for the response HS.

I went back and rethought this. We don't have the fans and the FCOM clearly states it's 300 deg (or 150 if we had fans) so I believe you are correct.

I've put the question to AI themselves just to be sure and I'll post here if I get a different explanation.

[nnc0]

I spoke with Airbus about this and it basically comes down to:

The maximum brake temperature for takeoff is 300°C, as indicated in the FCOM. Crew are advised of the limit exceedance by the associated ECAM warning (triggered when the temperature exceeds 300°C).

The 150°C limit given in the FCTM is simply to ensure that any hydraulic fluid that may come into contact with the brakes will not be ignited in the wheelwell after gear retraction.

It does not ensure that brake energy limitation will be respected in case of rejected takeoff. That limit is addressed by the 300° warning.

So if brakes are at 250°C, you are authorized to takeoff. If you then have an an RTO, the brake energy limitation may be exceeded but certification takes into account the low probability of a rejected takeoff being energy max limited, with hot brakes and that there is some further margin available since energy max was demonstrated with worn brakes.

[gyro]

Seem to recall reading that although the max temp for take off is 300 deg, Airbus has never demonstrated an RTO initiated with the temps at this level.

[TomConard]

Gentlemen,

Please reference the OEM Digest 2004 "Summary of Braking Recommendations in the SOP." Also, please reference a Flight Safety notice dated 29 OCT 2003 with reference to FCOM Vol 3 03. "Airbus Performance and Operations Conference Reports 1998/2001."

Extract from the OEM (reference to the 300c limit), "Prevents hydraulic fluid ignition in the main landing gear bay in the event of a fluid leak."

In my opinion, this figure is related to the ignition point of Skydrol which is around 450c (plus or minus, depending on the manufacturer). It has to do with brake fans, the location of the brake temperature sensors, the reading (low) of the temps by about 125 degrees or so, etc.

This is how (again, in my opinion) Airbus designed the 320 series without any wheel well fire warning. By following this limitation, no wheel well fire warning system is necessary.

A totally separate issue is the 150c temp figure and RTO certification. For this discussion, I direct you to the "Airbus Performance and Operations Conference Reports 1998/2001."

Let me start by emphasizing that "Airbus recommends to keep the brakes temperature as cool as practicable for take off."

I'm going to quote:

"The A320 has been certified according to FAR 25 amendment 42 (extracts): RTO at max brake energy has to be demonstrated with new brakes (initial temperatures at around 150 degrees)."


"The A321 has been certified according to FAR 25 post amendment 42 / JAR 25 NP 244 (extracts): RTO at max brake energy has to be demonstrated with 90% worn brakes (initial temperature around 150 degrees) and a bench test must demonstrate that max energy can be absorbed by 100% worn brakes."

I quote the paper further:

"Consequence: FAR / JAR intentionally disregards any higher initial brake temperature influence. Therefore, on an ASDA limited RWY, even with new brakes (A320) or worn brakes (A321), without reverse, nobody knows if it is possible to stop before the end of the RWY in case of RTO if the initial brake temperature is between 150 and 300 degrees (and actually, it is not for any airliner in the world)."

To further explain (and to quote):

"A320 (amendment 42) Maximum brake energy should* be sustained in the case of an RTO, with 100% worn brakes up to 150 degrees only on dry RWY with reversers."

"*should because all of this has not been demonstrated but is the result of a mathematical model."


I hope this clarifies the issue. In fact the 150 degrees issue and the 300 degrees issue are totally separate. One is for an RTO, the other is for a wheel well fire.


Tom

[bluesideup]

My Summary:
If you have the 'Brakes Hot' ECAM then Airbus are saying it is possible that when you retract the gear the brakes may be hot enough to ignite hydraulic fluid. Consequently if this ECAM occurs when wheels are up, the ECAM instructs you to put the wheels down.

If you have brake fans on, they cool the temperature sensors down more effectively than they do the actual brake discs. Airbus have calculated that 150 degrees indicated with fans on means that brake temps are equivalent to 300 degrees indicated without fans on.

Hence if you have been using fans, and the indicated brake temp is greater than 150 degrees, when you switch the fans off the indicated brake temp will gradually return to greater than 300 degrees. So if you have been using fans and the temp is indicating greater than 150 degrees and you then switch off the fans and takeoff, you may well get a 'Brakes Hot' ECAM when airborne, asking you to put the gear down again. This is the Airbus' only defence against wheel well fires as there is no fire detection system in the gear bays.

'Brakes Hot' ECAM has nothing to do with RTO's and brake energy, it is only there to indicate the potential for a wheel well fire. Indeed as the previous contributor states RTO trials are not carried out with brake temps indicating 300 degrees, but typically much less, in the region of 150 degrees.