What is the reason for fuel dumping system?

[QuEsT147]

Hi guys

Have a quick question about fuel dumping system. I always thought that it is there to decrease the landing weight in an emergency, but now I know that overweight landings are possible, so why is it necessary, does it have any other reason?

I read somewhere, that it has something with performance capabilities of the aircraft but cannot find any reference, could someone please shed some light on this for me?

Thanks for advance:)

QuEsT147

[One Outsider]

It it there so you can comply with approach or landing climb requirements.

[QuEsT147]

Thanks for answer, could you explain it more for me, what these approach and landing climb requirements mean?

I am wondering, because, if you are able to continue the takeoff with one engine inop, even at MTOW, how is it, that you would not be able to comply with landing climb requirements, in landing phase your landing weight should be surely less than takeoff weight?

Thanks for help

QuEsT147

[Rubber Dog]

Fuel Jettisoning Conditions
JAR 25.1001 Subpart A FAR 25.1001 Subpart E
“JAR/FAR 25.1001
A fuel jettisoning system must be installed on each aeroplane unless it is shown that
the aeroplane meets the climb requirements of Approach Climb gradient and Landing
Climb gradient at maximum take-off weight, less the actual or computed weight of
fuel necessary for a 15-minute flight comprised of a take-off, go-around, and landing
at the airport of departure with the aeroplane configuration, speed, power, and thrust
the same as that used in meeting the applicable take-off, approach, and landing climb
performance requirements of this JAR-25.”
When the Maximum Takeoff Weight (MTOW), less the weight of fuel
necessary for a 15-minute flight (including takeoff, approach, and landing at the departure airport) is more than the maximum go-around weight, a fuel jettisoning system must be available.

[QuEsT147]

Thanks much

where can I find these approach and landing climb gradients? Are they higher than normal takeoff climb gradients?

Thanks

QuEsT147

[One Outsider]

Have a look here (http://www.britflight.com/wingfiles/performance/gettingtogripswithaircraftperformance.pdf) from page 117 onwards.

[Wizofoz]

Whilst that is the regulatory requirement, it is not always safe and prudent to land above max landing weight (or even AT max landing weight) even if climb gradient requirements can be met.

A number of scenarios, both normal and non-normal may make it prudent to reduce landing weight by jettisoning fuel.

[purgatore75]

Hi all,
it may happen that the max landing weight in some aircrafts is lower than the take off weight that's why you have to install a fuel dumping system.
From an operational point of view (pilots): in case of an air turn back the pilot is able to fly an aircraft that is more maouvreable in landing with the CG advanced (case of fuel dumped ) due to lower weight. The more you add fuel the more CG is moving back decreasing in manouvreability.
From a maintenance point of view: an overweight landing can be possible but you have to verify some structural items in order to verify if the technician is facing only an overweight case or an hard landing case, in order to maintain aircraft airworthiness. As a matter of fact, an aircraft after an hard landing is un-airworthy so is grounded and you have to verify structurally if the energy dissipated during the touch down has bent (permanent deformation) the wings or the fuselage or the undercarriage (or wathever).
In both cases remember always that the TOTAL ENERGY involved during landing is directly linked to the Linear Momentum ( mass * velocity) of the aircraft in changing its movement (also called Inertia) so, to minimize landing damages you have to be as lighter as possible.
Hope I explained right what you requested.
Purgatore75.

[Storminnorm]

And another thing!
The only time you have too much fuel is when you're on FIRE!!!

(But never use the fuel dumps if you ARE on fire!!!)

[B-HKD]

And it makes the beancounters cry :}

[Wizofoz]

Purgator-

Yes it explained, but unfortunately almost entirely incorrectly!!

There is no hard and fast rule as to whether fuel is loaded in front or behind the CG. Most designs put it as near as possible to ON the CG. The aircraft HAS to be able to stay within CG limits throughout the flight, so has to be able to use any fuel on board whilst still being in balance. CG is not an over-riding consideration in terms of fugal jettison or otherwise.

Additionally, conventionally laid out aircraft become more maneuverable as the CG moves AFT, not FORWARD.

Overweight landings do often require maintenance action as for the same VERTICAL acceleration, there is an increase in the amount of KE absorbed by the structure (KE= 1/2Mv^2. Momentum = M*V) HORIZONTAL velocity makes little difference to the strains on the airframe.

[411A]

The more you add fuel the more CG is moving back decreasing in manouvreability.


In many types, the more fuel you add (center tanks full) the further forward the CG resides.

[Poodles in flight]

In the type I used to fly there was an engine failure brief pror to takeoff , from the Captain at high weights, that included the phrase directed at the FE (me) , "in the event of an engine failure bleeds and dumping will be automatic" , even at low level we would try to lighten the aircraft to improve performance on three engines as we were often pretty tight . With a gut load of fuel 10 dump pumps got rid of a lot in a short time .

Obviously if the wing was on fire we wouldn't dump .

At lower weights the FE's engine out brief include the phrase " Bleeds and Dumping are available" to remind the Captain that we could improve our performance.

Personally I resolved that I would rather dump fuel on someones house than dump an aeroplane on their house.

[Piper19]

quote from Wizofoz: "Overweight landings do often require maintenance action as for the same VERTICAL acceleration, there is an increase in the amount of KE absorbed by the structure (KE= 1/2Mv^2. Momentum = M*V) HORIZONTAL velocity makes little difference to the strains on the airframe. "

Correct, that's the only reason why jettison systems are needed. I often do inspections on overweight and hard landings. Both require to check the gears and visual damage. If found, you're in big trouble because a phase 2 inspection has to be performed, which can include disassembling whole gears, struts and flap to wing connections. Landings are more stressfull on a gear than takeoffs, let alone hard ones.

JAR 25.1001 Subpart A FAR 25.1001 Subpart E explains that a jettison system is needed if MTOW is higher than max go-around weight.
The difference in max takeoff weight in this is that they include higher flap settings, thus less climb performance.

[One Outsider]

Correct, that's the only reason why jettison systems are needed.
That is just nonsense. It might be what engineers are told, but it is still nonsense.
*
It is purely due to approach or landing climb requirements. On some AC is is even an option.

[aviationluver]

Followup,

Are the approach and go-around scenarios that you are talking about with both engines or single-engine? My belief is that single-engine go-around at max gross weight is what the regulation is written for.

[Milt]

The F-111 fuel dump and burn has cancelled out the SOP not to dump jet fuel if you have a fire.

It became necessary to determine the effects of a fuel dump with the F-111 because the fuel exit nozzle nestles closely in between both engine exhaust nozzles which readily ignite the fuel when engine afterburning. As TAS reduced it was observed that the flame front of the dumping fuel came closer to the dump nozzle but at no time ever caught up with the aircraft. A little of that raw fuel being dumped has to have time to combine with enough air to become burnable.

So over to the Mythbusters to show that you can dump fuel safely airborne in the presence of a fire.

[FE Hoppy]

Flame arresters are fitted in the dumping nozzels of the l1011. We were of the opinion that if things were so bad that you were returning at max gross with a fire then dumping was a viable option as it couldn't really make things worse unless the flame arrestors didn't work in which case why were they get fitted to the jet. In this scenario missed approach climb gradients were not a consideration but stopping on the runway and not braking the wings on touchdown was.

Note: This was on the mill tanker version where we had more fuel than you could shake a big stick at on board.

[jcbmack]

Higher weights still mean a higher approach speed requirement and slower acceleration. A 10% increase in weight creates a 10% or so increase in landing distance, with a higher speed of approach, which in turn creates more momentum. A 10% increase in weight increases the stall speed by about 5%

For takeoffs where the liftoff speed is about 15% above the stalling speed, say we have a 10% increase in weight; this increase in weight would translate to roughly 20% increase in the take off run.

Keep in mind that as air density decreases both engine and aerodynamic performance decrease; (now, at optimum altitudes, engines do operate more efficiently as per performance limits of specific aircraft; an increase in air density means a lower density altitude) this is specifically applicable to take offs, where at higher altitudes it takes more runway to take off. Density altitude will also affect aircraft performance, and this includes on approach.

An uphill slope will increase the take off ground run and a downhill slope will increase landing ground run. The weight of the aircraft is therefore a very important factor to consider. Sure you can land at maximum landing weight, but it is not advisable to have more weight than you really need and have calculated for. Of course grass or snow can increase ground roll, due to less effective brakes, despite increased rolling resistance, so your weight is still a practical and critical thing to consider.

[jcbmack]

many aircraft do not have the ability to fuel jettison, so in that case, a faster than usual approach and a slightly rougher landing will take care of it; I still think erring on the side of caution and having a fule jettison system is wise.

Performing a holding pattern (or circling around) in some cases is fine too.

[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]

Thank you for your post Mutt.:8

[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 :confused::confused::confused: 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! :ok:

[jcbmack]

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! http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/thumbs.gif


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...:yuk:

[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]

:confused::confused::confused: 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]

Thank you for your informative post.