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 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]

High Landing Weights:

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]

Other Than That:

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.