If a 777 ER has to immediately land after takeoff on a planned 14,594 km trip (in the case of a fire, e.g.), the landing will be executed with a very heavy aircraft.

  • What are the major concerns of doing such a landing?
  • What will, on average, be the consequences of this overweight landing?
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    $\begingroup$ Because it's an emergency, you're likely to already have a "problem" on top of the fact you'll have to conduct an overweight landing. Depending on the emergency, you can add much more concerns and consequences to the main ones explained in the answers below. (Fire: engine fire ? hydraulic failure due to a fire ? etc.) By the way, fire fighters can't properly work on a landed aircraft if there's a risk to make passengers suffocate under foam (hence evacuation duration) Brakes will overheat. Better not have leaking fuel nearby; heartbeating with a 777 full of fuel... 14594km.. quite precise :P $\endgroup$ Oct 2 '15 at 13:22
  • $\begingroup$ Max landing vs. takeoff weight $\endgroup$
    – fooot
    Oct 2 '15 at 15:59
  • $\begingroup$ Aren't there systems for dumping fuel? I believe that you could significantly decrease the weight by this measure alone, even in a short time. Reference $\endgroup$ Oct 2 '15 at 16:30
  • $\begingroup$ Now of course in the case of catastrophic failure of main components on take-off for a long range flight this might not be possible, as there can be tens of tons of fuel that needs to be jettisoned. $\endgroup$ Oct 2 '15 at 16:37
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    $\begingroup$ @JPhi1618 a special inspection is required after any event outside of the limits for which the plane was certified. See faa.gov/regulations_policies/handbooks_manuals/aircraft/… page 8-16. $\endgroup$
    – alephzero
    Oct 2 '15 at 19:06

Two things:

  • Loads on the landing gear. This can be alleviated by carefully touching down, but you never know what exactly will happen in the next landing, so better don't count on it to be smooth
  • Deceleration distance: The higher mass requires a higher approach and touch down speed (higher by the square root of the mass ratio between your actual mass and the maximum certified landing mass). Now you have more mass and more speed, but the landing strip and the airliner's brakes are still the same. The risk is that you overshoot the runway with an airplane full of fuel or that you overheat the brakes (which will result in their failure, and again an overshoot). Since the kinetic energy is proportional to mass times speed squared, the energy which needs to be absorbed by the brake goes up with the square of the mass increase.

If the selected runway for your overweight landing is long enough and the weather is calm (so a smooth landing is easier), an overweight landing is not such a problem.

Now let's look at the 777-300ER in detail. I use the figures from Wikipedia, because the manufacturer's site is trying much harder to hide the relevant data. The MTOW (maximum take-off weight, which is actually a mass) is 351,534 kg, and the maximum landing weight is 251,290 kg, about 100 tons less. Now let's assume that taking off and returning to the airfield uses 5 tons of fuel, so the mass ratio is 1.379. Therefore, the approach speed has to be 17.4% higher and the total kinetic energy at touchdown is higher by a factor of 1.9 compared to the case at maximum allowable landing weight.

If we now assume that speed is reduced linearly during the landing run and that the wheel brakes will just absorb as much energy over time as in the case at maximum landing weight, the deceleration needs to be lower by the mass ratio, so the aircraft decelerates with only 72.5% of the deceleration at maximum landing weight. To shed its kinetic energy, the landing run will take longer by the increase in kinetic energy, i.e. 1.9. Since the average speed is also higher by 17.4%, the landing distance will be higher by a factor of 2.233 or 223% of the landing distance at maximum landing weight.

This neglects that the heating of the brakes is an instationary process, so you cannot simply brake with the same intensity for more than twice the time they were designed for. But this little estimation should illuminate what impact a higher landing mass has on the energy and distance involved.

  • 1
    $\begingroup$ Re: loads on the landing gear, are you talking about landing gear collapse, tire puncture or what? $\endgroup$ Oct 2 '15 at 11:40
  • 2
    $\begingroup$ @JonasG.Drange, when the vertical momentum is more than what the shock absorbers can handle, the shock transfers to the surrounding structure, which usually gets damaged before the gear would collapse. Not so much that the airplane would fall apart, but such damage is usually very hard to repair. $\endgroup$
    – Jan Hudec
    Oct 2 '15 at 12:12
  • 1
    $\begingroup$ @JanHudec What happens to the plane next day and after is probably a little a concern if you have an emergency now, right? :D $\endgroup$
    – yo'
    Oct 2 '15 at 14:57
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    $\begingroup$ @alephzero: Not true. The same airplane just took off on the same tires with the same load. And take-off speed is higher than landing speed. Unless the pilot slams the plane into the runway, the tires and gear will be fine. Note that static load means typically 2/3 of oleo deflection. The problem is the brakes and the long distance needed for deceleration. $\endgroup$ Oct 2 '15 at 20:29
  • 2
    $\begingroup$ @PeterKämpf But the dynamic loads on touchdown will be higher than static while you are compressing the oleos (even for a normal touchdown), the landing speed for a heavy plane will be faster than for normal weight, and the braking forces have to go through the tire walls and treads. You may be right but it's not "obvious" (at least to this mech engineer!) that everything would be OK. $\endgroup$
    – alephzero
    Oct 2 '15 at 21:01

The major issue with landing a plane that is overweight is over-running the runway.

If you are trying to land a plane that is over it's rated landing weight, you will need to be going faster to keep enough airflow over the wings to generate the lift required to keep the plane in the air. This, of course, has the consequence that it will take longer to stop when on the runway.

Another factor that contributes to it taking longer to stop when on the runway is that the extra weight of the plane adds to the momentum. The calculation for momentum is p=mv where m is mass and v is velocity (for this example we can think of it as speed.)

So not only are we now landing with a higher mass (m) we are also landing with a higher speed (v) so our momentum increases very quickly.

For example, if we had a plane that was 660,000 pounds, and needed to travel at (lets say) 290km/h (156 knots) to keep giving enough lift, it's momentum would be: 191400000 pounds km/h

If we look at a normal landing weight and speed of a 777 524,000 pounds landing at a normal speed of 250km/h then the momentum would be: 135500000 pounds km/h

you can see that even a small increase in speed causes the momentum to increase significantly.

The other issue is the load put on the landing gear which have the potential damage them, although I think that the deceleration speed is the bigger factor here.

(I am not a mathematician, my calculations may be wrong so feel free to edit the answer if I did get them wrong)

EDIT: One other thing I forgot to mention was that most larger airports have a special kind of surface at the end of the runway that serve to slow planes. They are constructed (iirc) out of some sort of very strong foam like material that is very effective at slowing a plane. I don't remember exactly what it was made out of though.

As mentioned by NoviceInDisguise and ChrisW, this is actually called the Engineered materials arrestor system and is made out of lightweight, crushable concrete blocks.

You can see an image of the effect here where the plane's wheels have dug through the soft layer which helps slow the plane.:

Aircraft overrun

  • 5
    $\begingroup$ Momentum is not very important here. More important is kinetic energy which has to be converted to heat in the brakes (and brakes heated to 900°C are not funny). Kinetic energy grows with mass and square of speed, but since landing speed only grows with square root of weight, energy of landing plane grows with square of its mass. It is still an awful lot. $\endgroup$
    – Jan Hudec
    Oct 2 '15 at 12:02
  • 1
    $\begingroup$ I believe they use a form of aerated concrete. $\endgroup$ Oct 2 '15 at 16:31
  • 1
    $\begingroup$ Wikipedia: Engineered materials arrestor system $\endgroup$
    – ChrisW
    Oct 2 '15 at 18:38
  • $\begingroup$ Yep, that's what I was thinking of. I'll add it to the answer. $\endgroup$
    – marcus
    Oct 3 '15 at 19:05

An overweight landing is defined as a landing made at a gross weight in excess of the maximum design (i.e., structural) landing weight for a particular model.

In general, landing overweight (though not desirable) is safe. FAA has stated:

There has been no adverse service experience with airplanes certificated under Part 25 involved in overweight landings.

The major concerns for overweight landing are:

  • Loads on the Landing Gear The touchdown rates are higher in case of overweight landings. Touchdown at higher-than-normal sink rates may result in structural limits being exceeded.

    However, unless the pilot does a very hard landing, this should not be a problem. This is because regulatory requirements for landing gear design are based on a sink rate of 6 feet per second at the maximum design takeoff weight, while the usual sink rate is nearly half of that.

    Also, the wheel, brake and tire design is based on the most severe landing stop conditions at maximum wear out or a rejected landing at maximum take off weight, whichever is worse.

  • Landing Field Length This becomes an issue only when the selected runway is smaller than the requirements. As the landing weight increases, the approach and touchdown speeds increases and the aircraft require more distance to decelerate.

    The landing length required is more as the weight increases and the aircraft field lengths are calculated for landing at MTOW. For example, for 777, according to Boeing, excess field length is available even for landing at MTOW.

Landing at maximum TOW

Image from Overweight Landing? Fuel Jettison? What to Consider

  • Performance Higher weight involves performance penalties as the control forces required are more.

  • Control In most aircraft, for example in Boeings, the autopilot is not certified for overweight landings.

In case an overweight landing has been carried out, there are some inspections required to be carried out:

  • Phase I The aircraft is inspected for obvious signs of structural distress, such as wrinkled skin, popped fasteners, or bent components in areas which are readily accessible.

  • Phase II This is a much more detailed inspection required in case the Phase I inspection shows signs of excessive stress and requires opening access panels to examine critical structural components.

In general, overweight landings should pose no problems as long as the landing is performed correctly.

  • $\begingroup$ Sorry to catch it so late. rejected landing at maximum take off weight I have only heard of rejected takeoff at MTOW. Am I missing something? $\endgroup$
    – vasin1987
    Jun 10 '16 at 9:13

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