Firefighting Aircraft - Maximum Drop Weight Limitation(?) 1

[mikero]

Would anyone here know a good reason why a maximum drop weight limitation would be imposed on an aircraft modified for firefighting role (by means of a tank installation under the fuselage belly)?

--> The aircraft max drop weight limitation, imposed for some reason by the tank installer, is significantly less than the aircraft's max take-off weight.

What this results in is that if the aircraft takes off at max take-off weight to survey an area and the crew suddenly come across a situation where a drop is requested immediately, they cannot legally comply until they have burnt off a fair amount of fuel, or dump the fuel first.

I can't figure out why this limitation might be imposed. Any good ideas? Thanks in advance.

 

[MikeHalloran]

I'd >>>GUESS<<< that it's because there is a load associated with the drop proper, e.g. aero load from drag of the door opening, or impact load from limiting its motion, that will overstress one little part, somewhere, if the door is opened with a full fuel load.

Which begs the question: "If they could get down to the max drop load by dumping fuel, could they also get down to the max drop load by dumping water at the same mass flow rate as the fuel dumps?"



Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[aviat]

There were three accidents resulting in wing separations, one of which was a C130 in California, was caught on film and shown on the news. The NTSB gave several recommendations following these, and the FAA has implemented some of them such as reduced loads.

http://www.ntsb.gov/Recs/letters/2004/A04_29_33.pdf

 

[MikeHalloran]

That's a sobering report.



Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[mikero]

Good discussion points so far - and thanks for that report link.

But I'm still left wondering how dropping the water/retardant with less fuel in the wings will result in less load on the aircraft structure than dropping the same amount of water/retardant with more fuel in the wings (which would result in this limitation). There's no difference in how much, or at what rate, water/retardant is dropped in either case.

The aircraft, by the way, is a converted L-188 Electra.

As an aside, this maximum drop weight limitation has been in effect for several years, well before the C-130A incident.

 

[SWComposites]

Could it be that there is a dynamic loading issue with dropping the water with a full fuel load in the wings?

 

[Sparweb]

Mike, the video of the C-130 losing its wings is even more sobering.

The CG of the aircraft usually shifts as the retardant is being dropped. The control inputs that kept it in balance before the drop can make it climb. Actually, the aircraft is often already in a climb, and maybe faster than V_A. It is also getting lighter, very suddenly. The wings are still at a high angle of attack (supporting the previous heavy load) so it wants to float up even more. The g-forces can be very high. Limit load has always been intended as an "occasional" occurrence, not a daily procedure. Herc's already have lots of fatigue problems with their spar roots. This mission just makes things worse.

Steven Fahey

 

[GregLocock]

I think it comes back to static versus dynamic analysis.

Say you are standing on a bridge, and jump up and down on it.

No problem.

Now load the bridge up with a static weight to within 1 engineer's weight of breaking, and repeat.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[btrueblood]

A good visual picture, Greg.

 

[mikero]

The aircraft's take-off weight is not reduced. Yet to drop the load (water/retardant) that the aircraft is carrying, a significant amount of fuel must first be burned off or dumped. Why?

I suspect, as does SparWeb it seems, that this may have something to do with the CG shift that occurs during the drop. What I don't understand is how this situation is improved by first getting rid of fuel before dropping; or put another way, how does having more fuel in the wings during the under-belly drop make it less safe than with less fuel?

 

[btrueblood]

There are a couple posts above that give good reasons (the amount of mass in the wings will directly affect how they respond to sudden changes in g-loads, and the stresses that result) which are probably the necessary & sufficient reasons for the restriction. My only addition would be to consider that aircraft controllability (roll rates and roll acceleration rates) in highly turbulent air occurring over fires is reduced/hampered by mass in wing tanks. Belly tank masses, being closer to c.g., affect roll rates much less.

 

[Sparweb]

Mikero, a potential source of information for you would be a Flight Manual and the Supplement written for the modification for firefighting service.

Compute the weight shifts starting at the extremes of the flight envelope and you may find excursions when the retardant is dropped.

If memory serves, many Electra conversions have been done by Aero Union, including firefighting. See what they say.
This question still sounds worth asking. There's sometimes the possibility that the answer will lead to a solution allowing greater utility.

Steven Fahey, CET

 

[O320]

Hi
I think the reason might be acceleration loads on other parts than wings and spars. Imagine the wing lifting an a/c whose weight is suddenly reduced by a significant amount. The wings still produce the same lift so the result will be an upward acceleration imposing great loads on engine supports and other structural parts. Fuel in the wing tanks will most likely exert a large pressure on the bottom skins. I could go on, but I think you see what I mean.
Dan

 

[rb1957]

i do hope you're not dumping the fuel on the fire ?

 

[O320]

One more comment like that and we will...

 

[LtDanM]

The fuel weight of the P-3 is concentrated in the wings. Tanks 1-4 are the primary tanks. Tank 5 is in the wing center section and in the fwd lower fuselage. Tank 5 is only used for flights over 5 hours duration.

The weight in the wings is called a "relieving" weight. That is, the weight is located where the lift is generated. That means fuel weight does not increase the bending moment on the wings.

The water bomber conversion locates all the water/retardant along the fuselage bottom. That weight DOES increase the bending moment on the wings, as the lift is generated out along the wings.

The gross weight limitation on the fire bombers is lower than the P-3 due to this loading configuration. Equivalent weights do not result in equivalent loads on the structure.

Dan
P-3 Structures Engineer