Auto Speed Brake MEL Performance question
 

Hi - I am posting this under a new thread title that might attract a performance or certification whizz to respond.

Can you help me understand the MEL weight restrictions imposed by an inoperative automatic speed brake extension?

Each airplane imposes a substantial penalty to landing runway limit weight but no penalty to takeoff runway limit weight. The amount of landing penalty and lack of takeoff penalties are counterintuitive. Examples include:

757: 25,000lb landing penalty, but no takeoff penalty
767: 63,000lb landing penalty, but no takeoff penalty

How were these penalties calculated?

Thanks for any thoughts

OH

Hi Oilhead,

I think it may be because:
1) Take off flap setting is normally less than landing flaps
2) RTO is conducted with attitude zero.

1 & 2 means there is lots of weight on wheels so brakes will work reasonably well during the time it takes to select ground spoilers manually.

On landing without AGS, there is a greater chance of a skip or bounce. Due nose up attitude and Landing Flap, there is less weight on wheels (even when nose wheel down) and delayed ground spoilers will delay satisfactory braking.

Edit, the landing weight penalties quoted will affect the minimum landing distance required. Normally the landing runway is so long that it won't be a problem.

I would suggest that the aircraft was certified using Manual Speed Brakes for takeoff rather than automatic, hence no need for a penalty.

During landing, Automatic Speed Brakes were most probably used for certification, hence the penalty.

Mutt

You dont want to break it up on landing, too much stress on the airframe. Figure all that weight on the center tube translating to the wingbox connections.

The only 'penalty for takeoff is real time performance related.

It could be that the weight penalty is an easy calculation. Stopping distance at fixed brake energy is a function of weight. On B738 you increase the ELW and so increase the required LDR with manual spd bks. On takeoff I suspect that stopping distance was calculated/certified using manual spd bks. B737's the spd bks deploy with Thrust Revs, but you are supposed to do them manually. can't remember B5757/767; too long ago.

Stopping distance at fixed brake energy is a function of weight

.. plus speed squared to a first approximation (0.5mv^2 comes to mind ?). Final approach speed - and hence touchdown speed - will be linked to weight (stall speed).

Unless you have evidence to the contrary, it is not unreasonable to presume that the performance landing distance trials were done with brake application immediately after touchdown - hence the need for early/prompt spoiler operation.

For example, B738 landing distance is developed by flight test using MAX MANUAL BRAKING, SPEED BRAKES UP, DRY RUNWAY and AGGRESSIVE TOUCH DOWN TECH. No thrust reverse is used or certified in the flight test demonstrated landing and takeoff distance. During Landing, auto speed brakes operate when A/C is on ground, speedbrake lever is in ARM and thrust levers are idle. However, during RTO, auto speed brakes operate when A/C is on ground, wheel speed is more than 60kt and thrust reverser levers are moved.

Concur...
but given that max landing weight is given, irrespective of the runway length, it would infer issues with airframe/assembly issues, rather than stopping distance parameters.

The term oilhead uses in the OP is "landing runway limit weight" which to me is the weight as limited by available landing runway and therefore considerations of performance are the likely root cause of the Limitation.