Path: utzoo!attcan!uunet!husc6!mailrus!ames!amelia!eos!steve From: steve@eos.UUCP (Steve Philipson) Newsgroups: sci.space.shuttle Subject: reasons for fuel dumping [was: Re: shuttle landing sites] Message-ID: <1914@eos.UUCP> Date: 14 Nov 88 23:17:48 GMT References: <1185@client2.dciem.dnd.ca> <786@dinl.mmc.UUCP> Reply-To: steve@eos.UUCP (Steve Philipson) Organization: NASA Ames Research Center, Calif. Lines: 116 We've been seeing a lot of speculation on the net recently on why jets dump fuel before an emergency return-to-airport landing when that event occurs shortly after takeoff. Almost all of the speculation was wrong. Here are excerpts from the posts: dkrause@orion.cf.uci.edu(Doug Krause) starts off with the question: >I can't quote weights of either craft, but I know that if a 747 going >out of LAX has to abort the flight, it has to dump most of it's fuel >into the Pacific before returning to land. Now I don't know if the >weak part is the landing gear or the runway, but a loaded 747 is ob- >viously VERY heavy. dtynan@sultra.UUCP (Der Tynan) writes: >It seems to me, that the reason for fuel dumping, is safety. Of course, I >don't fly 747's very often :-), so this isn't much more than idle speculation. >However, that said, I wouldn't like the idea of being on a 747 during the >most difficult phase of the flight, when the fuel tanks (which contain VERY >flammable fuel) are full, but again, this is just an opinion... mears@hpindda.HP.COM (David B. Mears) writes: >I would hazard a guess that dumping the fuel has nothing to do with the >weight of the aircraft, but is done for fire safety reasons. ... billa@cvedc.UUCP (Bill Anderson) writes: >I suspect the reason for dumping the fuel has much more to do with >reducing the risk/severity of fire in the event of a crash than it has >to do with reducing the weight of the aircraft. eriks@cadnetix.COM (Eriks Ziemelis) writes: With regards to the 747 landing/dumping fuel: Being an ex-Boeing employee, I can atest to the fact that fuel is dumped for safety reasons. ... Finally, bgm@client2.dciem.dnd.ca (Bruce Matthews) writes: >Not correct. The maximum landing weight of most wide body airliners is a >good deal lower than the maximum take-off weight. The fuel dump is >required to bring the aircraft weight down to the max landing weight. The >limiting factor is the landing gear. Landing a 747 at close to max >take-off weight would run a serious risk of landing gear collapse. The >resulting fire would then be another problem. Good job, Bruce. To support that post, I present the following data: Variant Max T/0 Wt. Max landing wt. 747-100SR 600,000 lbs 525,000 747-100B 750,000 564,000 747-136 710,000 564,000 747-200B,C 833,000 630,000 747-300 833,000 630,000 747-400 870,000 630,000 Thanks to John R. Gersh and Hon Wah Chin for looking up the numbers. John also made the following observation: The common max landing weight for the last three variants also seems to argue for a structural consideration for some component(s) (landing gear?) common to all three. Indeed, landing gear and its supporting structure are the main factor in the common max landing weights. As for fire risk, the risk is lower with full tanks than partially filled tanks. The problem in post-crash fires is not raw fuel, but rather fuel vapor. A carburetor on an engine is there to mix fuel and air in precise ratio to optimize combustion. Tanks with lots of vapor in them are thus much more likely to explode when punctured (and sparks are generated) then those that have little or no vapor. Sure, you don't add "fuel to the fire", but the objective is to not start a fire in the first place. Most returns for landing are precautionary in nature; a crash is NOT anticipated. There is another important reason to dump fuel. holroyd@dinl.uucp (kevin w. holroyd) observes that they dump fuel even when they won't be landing on the gear: >They also dump the fuel if the landing gear doesn't come down. Obviously >at this point overloading the landing gear no longer is a factor. It is >to reduce the risk of fire, that they dump fuel. In addition, the plane >must be at or below max landing weight. The observation is correct, but the reasons are not. Max landing weight is a structural consideration for the gear, so it is not pertintent for a belly landing. What IS important is that the touchdown speed be minimized, both to minimize the length of the landing roll (or slide, in te case of a gear-up landing) and to minimize impact and deceleration force. A jetliner carries a large percentage of its maximum weight in fuel. The higher its weight, the higher its stall speed and landing speed. Dumping most of its fuel would allow a jet to decrease its landing speed by about 20 to 30 knots (very approximate figure here). Reducing aircraft weight by dumping fuel has a few other important effects. Reduced weight results in an immediate increase in climb performance. This would be considered critical if there is terrain to climb over. Stall speed is reduced, so if there are configuration problems (stuck flaps, slats, etc.) a greater speed margin is available over stall speed. Lighter weights also reduce the wheel braking power required to stop the aircraft. In most cases, the operating procedures are not set up to minimize post-crash fires, but to eliminate crashes. Lower weights reduce the demands placed on the flight crew and aircraft components, making a successful emergecny landing (with no additional damage or fatalities) much more likely. -- Steve (the certified flying fanatic) steve@aurora.arc.nasa.gov