Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Airliner engineering questions

  1. Apr 24, 2017 #1
    Do fuel lines in airliners need motor pumps or does the fuel flow occur by gravity?

    If motor pumps.. don't they get out of order in mid-air?

    If an airplane is hit with an EMP (let's say from atmospheric nuclear detonations).. I think the turbojet can still turn but won't the motor pump be affected?
     
  2. jcsd
  3. Apr 24, 2017 #2

    Nidum

    User Avatar
    Science Advisor
    Gold Member

    The fuel storage and distribution system in a modern airliner is both complex and sophisticated . There are usually several different tanks all interconnected . Pumps are used to transfer fuel from any chosen selection of tanks to the engines and to transfer fuel from one tank to another .

    The pumps on the engines can be mechanically driven or electrically driven . Most older designs certainly had mechanical drive .

    The fuel pumps in the airframe which service the tanks are almost always electrically driven .

    All critical systems on a modern aircraft have at least one level of backup available in event of main system failure .

    Airbus fuel system

    Concorde fuel system 1

    Concorde fuel system 2
     
    Last edited: Apr 24, 2017
  4. Apr 24, 2017 #3
    I know oxygen is needed for combustion.. but is the reason the aviation tank in the airplane doesn't explode is because the fuel is pumped out to the turbojet under pressure? Let's say the airplane is parked with the turbojet not on. Then you open the turbojet fuel nozzle and lit the fuel line tube with a cigarette lighter.. would the fuel tank explode?
     
  5. Apr 25, 2017 #4

    Baluncore

    User Avatar
    Science Advisor

    A flame cannot burn inside a pipe that is full of liquid fuel because the air cannot get in while H2O and CO2 combustion products are getting out.

    Fuel is sprayed as a mist into a turbojet combustor where it is combined with compressed air and so burns continuously. When there is a possibility of an air fuel mix in a fuel line, flame traps such as wire mesh screens are fitted.

    If there is any possibility that the air-fuel mix in a fuel tank might be ignited, an inert gas such as nitrogen should be used instead of air to replace the volume as fuel used.
     
  6. Apr 25, 2017 #5
    Whenever I fill up my car gasoline tank a thousand times already.. I always get concerned of any cigarette butts nearby.. does the car typical gasoline tank has enough oxygen to explode or not?
     
  7. Apr 25, 2017 #6

    Nidum

    User Avatar
    Science Advisor
    Gold Member

    Most fuel tank explosions are two stage events . Some initial fire - maybe due to leakage after a crash - heats the tank until fuel starts to vaporise rapidly . If the vapour can vent fast enough then you get a blowtorch flare . If it can't then tank explodes and you get a fireball explosion .

     
    Last edited: Apr 25, 2017
  8. Apr 25, 2017 #7
    I wasn't referring to a crash.. but simply filling up gasoline at gas station... if say cigarette butt was thrown inside, would the tank really explode?
     
  9. Apr 25, 2017 #8

    Baluncore

    User Avatar
    Science Advisor

    It is not that simple. If the gasoline vapour has saturated the air in the tank, which is probably the case while refilling a tank, the air-fuel mixture will be too rich to propagate a flame so it should not explode. A gasoline engine will not start if it is flooded, because the air-fuel ratio is too rich and cannot be ignited by the spark.
    http://www.mathesongas.com/pdfs/products/Lower-(LEL)-&-Upper-(UEL)-Explosive-Limits-.pdf
     
    Last edited: Apr 25, 2017
  10. Apr 25, 2017 #9

    russ_watters

    User Avatar

    Staff: Mentor

    As said, it is unlikely. But fires from the escaping vapor, ignited by static electricity are common. You can Google them and watch a few on YouTube.

    My ex boss once told me that cigarettes are not hot enough to ignite gasoline and says he extensively tested it, but I wouldn't consider that gosphel.
     
  11. Apr 25, 2017 #10

    russ_watters

    User Avatar

    Staff: Mentor

    Oxygen and an ignition source are both needed. There is no danger with flames propagating back through a fuel line due to the lack of oxygen. Millions of applications for thousands of years show this conclusively. I have half a dozen in my house right now.

    For planes, this is a concern if there is a potential ignition source and air in a tank. That's likely what brought down TWA 800; a short circuit providing the spark.
     
  12. Apr 27, 2017 #11

    CWatters

    User Avatar
    Science Advisor
    Homework Helper

    It won't usually explode but it can burn...




    We virtually eliminated this problem in the UK some decades ago by removing the latch on the nozzle so you cannot let go of the handle. You have to keep holding it for fuel to be dispensed.
     
  13. Apr 27, 2017 #12

    CWatters

    User Avatar
    Science Advisor
    Homework Helper

    YouTube to the rescue..



    But I certainly wouldn't try that at home.

    I once poured gasoline over a pile of damp tree branches I was planning to burn. The gas dripped down through the pile evaporating as it did so until I accidentally created a fuel-air bomb. Diesel is safer.
     
  14. Apr 27, 2017 #13
    I did that once too. A terrifying scene when I lit it, it seemed to suck all of the air out of my yard. I will never do that again. Ever.

    That's right. Like kerosene lighter fluid for your charcoal. It burns, but you don't get that "whoomph" when you set a match to it. One reason a lot of boats run diesel engines; no building up explosive gasoline vapors in the bilge.
     
  15. Apr 27, 2017 #14
    A1 jet fuel is basically ultra filtered kerosene.
    Kerosene will not ignite with out a some form of surface area amplification.
    You can point a blow torch at a bucket of A1 and the only consequence will be getting yelled at. :mad:
    The theory of general relative boss comprehension is absolute
    It's written as brass=ignorance*pride cubed or B=i*p3
     
    Last edited: Apr 27, 2017
  16. Apr 27, 2017 #15
    Thanks for the very helpful tips.
    Prior to this thread. I thought gasoline is like TNT explosive where any flame can ignite the whole thing that is why whenever I stopped by gas station. I always told my passengers to remove seatbelt and put their fingers on the door unlock button in case the gas tank explodes from someone throwing a cigarette butt into the gas tank and we can jump out of the car. So it would just cause fire and have enough time to run.

    Have you guys heard about Liquified petroleum gas.. https://en.wikipedia.org/wiki/Liquefied_petroleum_gas
    Some cars use it as fuel.. and I heard news some houses explode because of leaking LPG gas.. why does it explode when oxygen is needed to ignite the inside?

    And to make this not off topic and connected to airlines. I'd like to ask something about the turbojets in airplanes. Someone explained turbofans rotate due to the aviation fuel and not because it is one big motor. So does this mean when an EMP (electromagnetic pulse) hit a commercial plane.. it would still fly and only all the controls are lost from fried circuits? or would the turbofans stopped rotating too?
     
  17. Apr 27, 2017 #16

    You ask some good questions. As to the fuel issue I do not believe that it is 100% but most airliners are nitrogen blanketed. This started several years ago when forensic review of a crash revealed that a spark in one of the tanks caused an explosion. After that they now carry compressed nitrogen and keep the tanks blanketed so oxygen is excluded.

    On the issue of EMP It is possible to shield components. Critical systems sometimes are built that way. EMP is very rare but lightning strikes are known to happen occasionally. They have many of the same effects and builders practice due diligence.
     
  18. Apr 27, 2017 #17

    Baluncore

    User Avatar
    Science Advisor

    The fire would be vapour outside the tank. It would not burn inside the tank.
    The compressed gas in the cylinder does not explode. An LPG leak from a cylinder inside the building makes a flamable mix which is then ignited by a spark or flame. The building explodes, not the cylinder.
    The control systems for the motor require electronics to be operating. It is possible that an engine would keep running, but without any flight control, instruments or engine control.
     
  19. Apr 27, 2017 #18
    This.
     
  20. Apr 28, 2017 #19
    what do you mean by blanketed?
     
  21. Apr 28, 2017 #20

    Nidum

    User Avatar
    Science Advisor
    Gold Member

    Google ' inerting aircraft fuel tanks '
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Airliner engineering questions
  1. Engineering question? (Replies: 1)

Loading...