Why can you take-off but not necessarily land?

[Nicholas49]

So that's my question, from a discussion raised in the SLF thread.

Please could someone explain in a nutshell why if an aircraft is not too heavy to take-off, it can still be too heavy to land? In other words, why are maximum take-off and landing weights not necessarily the same?

Thanks
Nick

[Joao da Silva]

Nick

This will give you something to think about, until a professional pilot comes along.

Fuel dumping - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Fuel_dumping)

It is about fuel dumping, but it explains why. If you read carefully, you will also see why dumping fuel enroute was not an option for the A319.

[redsnail]

Nic,

Think about it. A take off is comparatively gentle on the gear and wing mounts but a landing is much more brutal.

[Bealzebub]

Most airliners have maximum structural weights for various conditions. The usual ones that concern us for day to day operations are:

The maximum take off weight (mass)
The maximum Landing weight (mass)
The maximum zero fuel weight (mass)

The MTOW is the structural maximum weight the aircraft can be at take off, after an allowance has been made for taxying fuel burn. That figure may (and usually is) be reduced still further by the performance limitations imposed as a result of such things as runway length, and ambient weather conditions etc.

The maximum landing weight is the maximum structural weight that the aircraft is certified to land at. This figure is often significantly below the MTOW.

The maximum zero fuel weight, is the maximum structural allowable weight of the aircraft whereby any additional weight must simply be fuel up to the MTOW. This allows for such things as stress relief generated by wing bending as a result of carrying fuel in the wing tanks etc.

The certification limits do normally allow for an aircraft to land in an overweight condition (up to the MTOW) in an emergency or extraordinary situation, without causing structural damage, provided certain criteria are adhered to. In many cases this obviates the need for a fuel dumping system on those aircraft. Similarly, on many other aircraft the same criteria do not allow for an overweight landing right up the structural MTOW. Those aircraft do have a fuel dumping system.

Taking two examples of aircraft that were developed almost in tandem, the Boeing 757 and 767.

757-200 MTOW = 113398kg MLW = 90,000kg diff = 23,398kg
767-300 MTOW = 184612kg MLW = 145,149kg diff = 39,463kg

The 757 has no fuel dumping sytem and the 767 does.

The difference in allowable weights reflects the stresses that must be allowed for in the landing case that are not present during take off.

[Intruder]

It is also a certification issue. The brakes have to be good enough to stop an airplane at max takeoff weight from V1 ONE TIME. However, those same brakes have to stop an airplane at max landing weight MANY times. So, the brakes (and many other components) are built just large and heavy enough to do the required job (i.e., absorb all that energy without damage or undue wear), so more weight can go to passengers, cargo, and/or fuel.

[Romulus]

Think of g forces.

[Northbeach]

not too heavy to take-off, it can still be too heavy to land?


Hitting the runway at 400’, 600’ foot per minute descent (or pick your number) generates additional forces throughout the airplane that running down the runway for takeoff does not.

If the jet weighs 200K and experiences a 1.5G touchdown force the effective weight is no longer 200K it is now 300K a 50% increase.

[Doors to Automatic]

The 757 has no fuel dumping system and the 767 does.

What happens if a 757 at MTOW has a major problem on departure and needs to get on the ground quickly?

[Bealzebub]

It can land up to its MTOW in an emergency.

The certification limits do normally allow for an aircraft to land in an overweight condition (up to the MTOW) in an emergency or extraordinary situation, without causing structural damage, provided certain criteria are adhered to. In many cases this obviates the need for a fuel dumping system on those aircraft. Similarly, on many other aircraft the same criteria do not allow for an overweight landing right up the structural MTOW. Those aircraft do have a fuel dumping system.

[MarkerInbound]

Some light reading for you:

AERO - Overweight Landing? Fuel Jettison? What To Consider (http://www.boeing.com/commercial/aeromagazine/articles/qtr_3_07/article_03_1.html)

They pretty much say if you had the performance to takeoff from a runway you should be able to land on that runway and meet performance limits.

[Nicholas49]

Well, that makes perfect sense. Thank you all for the replies.

If I can add a follow-up question. Imagine the scenario where you take-off at MTOW but you need to return to land immediately (i.e. there is no time to circle to burn off / dump fuel) and you are over the Maximum Landing Weight.

From your answers, there is a risk of structural damage to the aircraft if you attempt a landing in this scenario. So is this risk factored into the decision-making process as to whether you should land back at the departure airport? If you had a medical emergency immediately after take-off, might the Commander have to decide whether getting that person on the ground as quickly as possible is more important than potentially damaging the aircraft in an overweight landing? If so, rather you than me!

I don't mean to over-dramatise, but this seems to be a consequence of what you have said?

Nick

[Northbeach]

Some airplanes are built that have a provision for the pilots to dump fuel, spray it into the surrounding atmosphere where it evaporates, to get down to the landing weight under the conditions you describe.

My Boeing 737 does not have the ability to dump fuel. If I must land at a weight above my maximum certified landing weight there is a procedure I follow. Simply put the overweight landing procedure tasks me with landing very gently, a maximum of 360 feet per minute sink rate (descent) at the moment of touch down (landing). I note the sink rate, write it up in the maintenance log book after landing and turn the jet over to our maintenance department.

Now that I have an overweight landing in the maintenance record, the mechanics have an approved inspection already designed by the manufacturer and detailed in the jet's maintenance manuals that must be performed. Based on the results of the inspection the jet will be released after noting no damage, or if I have bent, busted or twisted things those damaged components will need to be repaired prior to the next flight.

And away we go.......................

Take the time to read the link provided by MarkerInbound. The Boeing article does an excellent job explaining various considerations of an overweight landing. I think you will like it!

(For simplicity I have omitted talking about calculating landing distance for the greater weight, declaring an emergency for operating outside of the limitations section of the flight manual, and the details of the various checklists, the 360 fpm is from memory if it serves me correctly. As usual in aviation there is always a little bit more to the subject, somebody else out there knows a lot more, has more recent and relevant experience.)

[redsnail]

These things are considered and the options weighed.
That is, what is the threat to the aircraft & other occupants versus the risk to the person have a medical problem.

If there is a real threat to the continued safe operation aircraft (such as both fuel lights coming on - Hawker) is greater than the risk of damage landing over weight then the landing will be done ASAP.

If the touch down can be kept to less than 600fpm, there should be minimal or no damage.

However, the aircraft will need an inspection and thus it's grounded.

[Northbeach]

Markerinbound,

Thank you for the link. The report is only 5 pages with lots of "pictures", it was a good read. Thank you for the resoucre!

Respectfully,

Northbeach