B767 MEL Weight Penalties on RTOW numbers

[Gards]

This is a question for you more experienced 67 drivers. I am fairly new on the aircraft and to airline flying in general.

I have had to training captains teach that when you have an MEL weight penalty not only do you adjust your max allowable take off weight but you add that weight penalty to your actual take off weight to adjust your assumed temp (not take off speeds). No where in my manuals can I find anything that says this, so at our airline it is not SOP. My question is, at other airlines do you have this procedure? If so why?

To me if you adjust the Max T/O allowable weight then you are compensating for the weight penalty you are facing. I do not understand, other then to be more conservative and add in an extra safety margin, why you would add this penalty to your actual weight and adjust only your assumed temp.

Any insight or explanation would be much appreciated.

Cheers!

[Checkboard]

Most of the weight penalties are compensations for increased drag due to missing fairings and such. On any aircraft I have flown these penalties also need to be taken into account for reduced thrust take-off calculations.

The assumed temperature method is simply reducing the thrust until the limiting performance requirement is just met. If the aircraft is operating with a higher than normal drag - then it makes sense that this needs to be taken into account, and the thrust not reduced quite so much.

[None]

Some of the info is in the MEL (or MDM) itself. Other info is in your Flight Ops Manual.

We cannot know what your company's perspective is, but you can (and should) request where you can read that MEL policy in print.

[haughtney1]

Gards, it sounds very much like a company specific interpretation.

From memory a fault brake unit being one of the most restrictive etc...
FWIW, when I was on the 767, our view was that it ia a trade off between the loss of revenue payload and the performance restriction/safety factor, which was left entirely in the hands of the flightcrew.

[Gards]

None - I do not make random postings without doing any research first. Nothing is stated in our FOM or FCOM and nothing other then the standard instructions relating to weight penalties in the DDPM.

Haughtney1 - As far as a company interpretation, you may be correct but it is not a technique that is standard amongst our training staff. Nor do we have any documentation or information outlining the theoretical practicality of this method; other then is beingmore conservative and more safe if you will!

Thanks for the response!

Gards

[Old Smokey]

This is one that really needn't be written into the SOPs, it belongs to the basic application of performance principals as applied by operating crews.

If your MEL specifies a Performance Penalty, then there's 2 actions which must be taken -

(1) Ascertain the normal RTOW, and SUBTRACT the performance penalty to find the "new" performance limited weight, and

(2) If your actual weight is below the "new" limit, ADD the performance penalties to the actual Takeoff Weight to find the "artificial" weight for which you will calculate Reduced Thrust / V-Speeds etc.

Sometimes you'll find 2 limits to apply, one for the Climb limit, and a second for the Field/Obstacle limit. Apply both in both cases stated above, and fly at the most limiting.

You asked the question for the B767, but the philosophy applies similarly from the DC3 right through to the A380:ok:

Regards,

Old Smokey

[None]

The MEL below (767) states what must be complied with.

Let's say that Runway Allowable Takeoff Weight and T-O Climb Limit weights are calculated for a jet with no write-ups. Later, due to a maintenance discrepancy you have to apply this MEL. In this case, the RATOW and Climb Limit weights must be reduced to 7000 pounds lower than the RATOW and Climb Limit weights that were previously calculated prior to applying this MEL.

Then, your actual takeoff weight must not exceed the new RATOW and Climb Limit weights. The other limits must be complied with as well.

There is no directive in this MEL saying that I must add the 7000 pounds to my actual takeoff weight. Therefore, if someone told me that was the procedure, I would ask them to show me where that procedure is written.

Many operators will have their weight and balance computer programs apply these MEL performance corrections automatically. If so stated on the final paperwork, all that needs to be done is to make sure the correct MEL number was applied, and that your actual weights are in compliance with the MEL and the performance numbers.

Personally, I never heard a policy where you apply the penalty to actual takeoff weight, but I will not pass judgement on it.



26-18-01B
FLIGHT PLANNING
REQUIREMENTS
1. Reduce ENROUTE LIMIT WT by 7,000 lbs.
2. Increase BLOCK FUEL by 250 lbs. per hour for APU usage inflight and on the
ground.
3. Increase BLOCK FUEL by 1.2%.
NOTES Make a notation on the Flight Plan when Enroute Climb is limiting.

TAKEOFF
PERFORMANCE
WORKSHEET
TAKEOFF
1. Reduce RATOW by 7,000 lbs.
2. Reduce T/O CLIMB LIMIT by 7,000 lbs.
LANDING
1. Reduce RWY ALLOWABLE LANDING by 7,000 lbs.
2. Reduce LANDING CLIMB LIMIT WT by 7,000 lbs.

[Gards]

None, I agree entirely as to what you just posted. However others are saying it is basic performance to add this penalty to your actual weight to come up with an adjusted assumed temperature. Look up at Old Smokeys post. I am just not sure where this is coming from. I have yet to read or be shown anything in writing that describes this procedure in any of our manuals or other companies manuals.

Thanks for your post!

Gards

[A Comfy Chair]

How about this scenario. (Elaborating on what Checkboard and Old Smokey have said)

You have an MEL that requires a 10,000kg reduction to takeoff limit weight.

In this situation, they are effectively saying (depending on which MEL it is, and why the performance penalty exists) that the aircraft will perform, on takeoff, as if it is 10t heavier.

You have calculated that for the given runway, its 30 degrees outside and you can take off at 150 tonne using FULL RATING. The MEL means that you can now only take off at 140tonne. If you are at 140t you really going to derate your thrust to the 140tonne figure? After all, the MEL has said you are performance limited to 140t! Full Rating Please.

If you are 135t - You are reducing thrust as if you are 15tonne below the performance limit weight, when you are actually only 5 tonne below. If you added the 10t to your actual weight (135 + 10 = 145), then you would get the appropriate conservative derate. Note, that you are working your performance out on 145t, but only taking off at 135, which is below your adjusted limit weight.

Clear as mud?

When I try to understand it, I just think that the performance pentaly MAY mean that the aircraft performs as if it is that amount heavier, and think about taking the conservative option.

[Old Smokey]

Think of this one as you would a Anti-Ice ON penalty, or as A Comfy Chair has put it.

In similar style to A Comfy Chair's input, imagine an aircraft with an RTOW limot of 100,000 Kg. Along comes an MEL (or Anti-Ice ON) reducing your RTOW by 10,000 Kg. Now, the new maximum weight for operation is 90,000 Kg, and the aircraft will perform at 90,000Kg as it would in normal circumstances at 100,000 Kg. So far so good, this is for a Full thrust operation.

What now if the ACTUAL weight of the aircraft is 75,000 Kg? After applying the MEL (or Anti-Ice) correction of 10,000 Kg, the weight for performance calculation is 85,000 Kg, some 5,000 Kg less than the prevailing adjusted limit. We are less than the limit by 5000 Kg, and may use Assumed (Flex) Temperature thrust reduction.

From the performance perspective, that's perfectly legal, safe, and within accepted conventional procedures.

There is a kicker to this. Many operators sensibly dictate that for non-normal operations thrust reduction is not to be used (even though it could be). This is a matter of policy erring on the conservative side. If the operator does do things this way, there will probably not be any instruction for other than full thrust takeoff. Gards, does your operator in these circumstances specify that Full Thrust MUST be used, or say nothing at all? Interested to know.

Regards,

Old Smokey

[Gards]

No we do not have any restriction for having to use Full Rated Thrust on MEL penalties.

Discussion has been going on and I think I am making headway as there has been a bit of confusion. I appreciate your response!

Cheers,

Gards