I'm thinking in terms of the Concorde accident in 2000 having just read part of the report which states that they were in effect several tonnes heavier than they should of been factoring in the tailwind and tyre speed restriction imposed for that reason. The crew took 3 tonnes of taxi fuel of which they only burned 800kg and that put them at least a tonne above MTOW not taking the tailwind into consideration which the report states would have been negligible. So I was wondering, is there any margin for takeoffs with weights marginally higher than MTOW? (obviously in this case they were ridiculously heavy amongst a whole host of other poor decisions taken by the crew)
The crew took 3 tonnes of taxi fuel of which they only burned 800kg and that put them at least a tonne above MTOW not taking the tailwind into consideration which the report states would have been negligible.
I can't speak to operations in today's environment, but at the two 747 carriers I worked for in the late 1980s and the 1990s, it was quite common to load more taxi fuel that you knew you would actually burn prior to takeoff. This was especially true for the 747-100, which had only a four bay center tank as opposed to the -200 which had a five bay center tank. Justification was easy. For example, if you were taxiing out at JFK to a trip to Tel Aviv, you well might burn 4000 lbs if you got caught in a line up. However, with any luck you'd get to the runway without delay, and you'd only burn 2000 (maybe even a little less), and you'd have the extra fuel for later. You wouldn't have the whole 2000 extra, because it would take about 20% of that to carry it.
Why was the extra fuel desirable? Well, in general as they say, the only time you have too much fuel is when you're on fire. Seriously, though, an extra 1500 to 2000 pounds could make the different between having to divert to Athens for fuel or going on to Tel Aviv.
So I was wondering, is there any margin for takeoffs with weights marginally higher than MTOW?
There is a practical margin but it's a judgement call as to how much it really is. If on the 747 all four engines stay running, the airplane will march right out, albeit with reduced performance, even if seriously overweight, but you've compromised the airplane's ability to stay in the air if an engine fails, especially if the failure is catastrophic.
Sometimes you're overloaded but don't know it. Either accidentally or on purpose the cargo weight you're carrying is misreported. The worst case that I experienced was taking a full cruise line crew and equipment from Miami to Trieste, where they would board a brand new cruise liner and put it in service.
We were at max gross and had enough fuel for that weight to reach Trieste. Once airborne at at cruise, we realized we were burning more fuel that we should. Using our mach number, altitude, and fuel flow we worked backward to find our weight and discovered we were somewhere between 30 and 35 thousand lbs heavier than we should have been. Our dispatch confirmed our calculations when we contacted them, and we made a fuel stop in Paris.
If you're flying freight, and pallets of uncovered dry goods are left out in the rain, they weight a lot more than when they were dry. That happened at Madras, India (now called Chennai) and that put us well over gross.
In a perfect world, neither experience should have happened. Welcome to the real world, at least as it existed in the 1990s.,
Is it ever permissible to takeoff on an aircraft heavier than its MTOW?
Exceeding the maximum certificated weight for aircraft is extremely limited by FAR Part 91. The regulation shown below refers to operations within the state of Alaska and applies to airplanes certificated under a very old rule and limits this authorization to aircraft operating under FARs 121 and 135 as well as certain U.S. Department of Interior airplanes:
Here is the regulation:
§91.323 Increased maximum certificated weights for certain airplanes operated in Alaska.
(a) Notwithstanding any other provision of the Federal Aviation Regulations, the Administrator will approve, as provided in this section, an increase in the maximum certificated weight of an airplane type certificated under Aeronautics Bulletin No. 7-A of the U.S. Department of Commerce dated January 1, 1931, as amended, or under the normal category of part 4a of the former Civil Air Regulations (14 CFR part 4a, 1964 ed.) if that airplane is operated in the State of Alaska by—
(1) A certificate holder conducting operations under part 121 or part 135 of this chapter; or
(2) The U.S. Department of Interior in conducting its game and fish law enforcement activities or its management, fire detection, and fire suppression activities concerning public lands.
(b) The maximum certificated weight approved under this section may not exceed—
(1) 12,500 pounds;
(2) 115 percent of the maximum weight listed in the FAA aircraft specifications;
(3) The weight at which the airplane meets the positive maneuvering load factor requirement for the normal category specified in §23.337 of this chapter; or
(4) The weight at which the airplane meets the climb performance requirements under which it was type certificated.
(c) In determining the maximum certificated weight, the Administrator considers the structural soundness of the airplane and the terrain to be traversed.
(d) The maximum certificated weight determined under this section is added to the airplane's operation limitations and is identified as the maximum weight authorized for operations within the State of Alaska.
[Doc. No. 18334, 54 FR 34308, Aug. 18, 1989; Amdt. 91-211, 54 FR 41211, Oct. 5, 1989, as amended by Amdt. 91-253, 62 FR 13253, Mar. 19, 1997]