What is the reason for fuel dumping system?

[Piper19]

Outsider, then please clarify why they tell engineers lies and explain the real reason so we also know. As far as I know the automatic dump system in e.g. a B777 will only dump till max landing weight not max go-around or something.
And on some a/c it's indeed an option, because the max landing weight is the same as max takeoff weight. Which means the gear is strong enough to take the weight.

[mutt]

I dont know of any airliner where the MTOW is the same as the MLW. The Fuel Jettison system is there to comply with FAR/JAR Approach / Landing Climb regulations. As you said, on some aircraft its an option which you have to purchase, before doing so you should assess if there is an impact on your takeoff weights due to the inability to land within 15 minutes after takeoff.

The B777 system will automatically set fuel to remain to the MLW, however this can be adjusted by the FUEL TO REMAIN selector. It isnt a limitation!

Now who exactly is lying to the engineers????

Mutt

[jcbmack]

HAHA

Thank you for your post Mutt.

[krujje]

A couple of points:

* Although civil aircraft can land at weights above MTOW, they are neither designed nor certified for that.

* Landing gear impact is a very complex event in which horizontal speed relative to the ground is important. Landing gear and backup structure are designed principally to three cases which occur during the touchdown: maximum vertical reaction, spin-up and spring-back. The spin-up and spring-back cases are influenced by the horizontal speed and do design certain parts of the structure. Manufacturers consider a range of landing speeds (including 0!) when deriving design load cases.

[mutt]

Although civil aircraft can land at weights above MTOW, they are neither designed nor certified for that

How can they land at a weight higher than they can takeoff? Remember that MTOW does mean MAX TAKEOFF WEIGHT!

Mutt

[cessnapuppy]

How can they land at a weight higher than they can takeoff off? Remember that MTOW does mean MAX TAKEOFF WEIGHT

I'm guessing...HITCHHIKERS!

[jcbmack]

Thermo...

Quote:
How can they land at a weight higher than they can takeoff off? Remember that MTOW does mean MAX TAKEOFF WEIGHT
I'm guessing...HITCHHIKERS!

A magical place where thermodynamics does not apply at all, and combustion means you get more fuel produced for use, from compression and not less...

[Piper19]

I know you can manually adjust the fuel to dump on a 777, but the pilot normally hasn'tvto use that. A 757 has no fuel dump, so can land at MTOW. (I know procedures and certifications can differ, but the gear is strong enough to take the MTOW on landing).
Now still, if anybody can explain that approach climb to me, and why we have to check gear struts if it isn't the overweight that decides if a jettison system is there or not.

[mutt]

but the pilot normally hasn'tvto use that

Au contaire, the pilot must ensure that it isnt limiting, if it is limiting then he has to adjust the fuel dump quantity.

If you look at the B777 QRH it states:

In the event an overweight landing is required and the fuel dump system is unavailable, landing climb limit weights should be checked if a Flaps 30 landing is planned.....At weights exceeding Flaps 30, a Flaps 20 landing should be performed.

So the concern is getting the aircraft on the ground, the only consideration given to the required maintenance actions is the instruction to land with approx 6-fps rather than 12-fps.

If we look at the B744, Fuel Jettison is shown in the DITCHING checklist, I doubt that has anything to do with maintenance checks

In the MD11, its shown in the DRIFTDOWN checklist, but this aircraft is interesting as the FCOM also states:
Maximum takeoff weight may be limited by:

• Second Segment Limiting Weight
• Weight Limited By Fuel Dumping
• Maximum Tire Speed Limiting Weight
• Weight Limited by Obstacles......

Hang on, did that really say that takeoff weight is limited by Weight Limited by Fuel Dumping?? Why? And this is where the whole thing comes back to FAR 25.1001.......

NOTHING TO DO WITH AFTER LANDING MAINTENANCE CHECKS!

Mutt

[FE Hoppy]

Now still, if anybody can explain that approach climb to me, and why we have to check gear struts if it isn't the overweight that decides if a jettison system is there or not.

approach climb = 2.1% or 2.5% single engine, go around flap, gear up, Vac
landing climb 3.2% all engines, landing flap, gear down, Vlc(which may not be less than Vref)

overweight landing checks have to be carried out whenever the aircraft lands above the certified max structural landing weight. Regardless of the weight required to acheive approach and/or landing climb performance.

[Mad (Flt) Scientist]

Originally Posted by mutt

How can they land at a weight higher than they can takeoff? Remember that MTOW does mean MAX TAKEOFF WEIGHT!

Mutt

Well, test aircraft do perform takeoffs at values above what will be the certified MTOW, in order to be able to conduct in-flight tests at, or very close to, MTOW, after having attained a safe test altitude. And it's all done legally, by means of special dispensation for the test aircraft.

If, in one of those overweight takeoff cases the aircraft were to, say, suffer an uncontrolled fire and (like many of ours) have no fuel dump system, the crew would (have to) perform an immediate landing, which very well could be above MTOW.

There's no technical reason why it wouldn't work - there's no discintinuity in the actual engineering of the gear and structure at MTOW or MLW, you just don't meet specific margins and test requirements.

And just for additional info, I don't think any of our (Part 25) aircraft have MTOW equal to MLW and the only one which has a fuel dump system has it as much for cg control in the event of fuel feed problems as anything else.

We also have an aircraft where MTOW is lower than max in-flight weight, just to add to the amusement.

[jcbmack]

Mad Flight Scientist

Thank you for your informative post.