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Landing gear bays in modern commercial aircraft have fire detectors installed, but there is no fire extinguishing/suppression system in this area.

What is the logic behind having fire detection in this area when we don’t have extinguishing?

Why doesn't this area have fire a extinguishing system like other unpressurized areas like engines and APUs.

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    $\begingroup$ I wonder if it has to do with the fact that the engines and APU have a supply of flammable liquids going to them. Where as the landing gear bay only has the less flammable rubber and fire-resistant hydraulic fluid. If the rubber catches fire, you can extend it away from the aircraft body, exposing it to a 200 knot wind. $\endgroup$
    – Dean F.
    Sep 16 '20 at 12:29
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    $\begingroup$ When your landing gear is on fire you are most likely on the ground, at which point it's more practical to wait for ground crew to come and extinguish the fire, although you need to know a fire has started in order to call for help. $\endgroup$ Sep 16 '20 at 22:14
  • $\begingroup$ @user3528438 Wheel well fire detection is about a fire in the enclosed space of the wheel well itself, i.e. with the gear retracted. It wouldn't detect hot brakes on the ground unless they were thermo-nuclear hot. Temperature sensors on the brakes would detect that condition, but that's separate from wheel well fire detection. $\endgroup$
    – Ralph J
    Feb 27 at 17:25
  • $\begingroup$ @RalphJ not necessarily. A leaking hyd tube on the top part of the gear may cause a fire to propagate in the wheel well. And some aircraft are very low: cdni.rt.com/files/news/1f/1f/90/00/9.jpg . But yes, most of the times with gear extended the fire is outside the gear bay. $\endgroup$
    – ocirocir
    Feb 27 at 20:29
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Cost/Weight/Complexity, the usual reasons why you avoid to add another systems. But let's analyse if you really need it.

Easy to prevent

From the data of this Boeing document:

  • at least 62% of the wheel well fires are caused by applying excessive grease
  • at least 11% of the wheel well fires are caused by hydraulic leaks
  • at least 3% of the wheel well fires are caused by incorrect application of solvents

From the fire protection standpoint the landing gear is a quite simple device: maintenance staff should pay attention where and how much grease they put, pay attention where solvents are put, and the only flammable liquid is the hydraulic fluid. An engine is much more complex, you have many flammable fluids (fuel, hyd, oil), many pipes and extreme forces, which make the maintenance error and engine damage more probable. Then, causes can also be external (like bird strikes). This makes an engine fire event probability much more higher.

Moreover, most of all events of wheel well fires appear after landing (i.e., after braking), which is still a serious problem, but not as serious like in-air fire.

Rare

Wheel well fire are 10x less probable than engine fires and 1.5x less probable than APU (source):

fire probabiliyu

There were only 2 fatal accidents related to wheel well fires: Propair Flight 420 and the Nigeria Airways Flight 2120. In both cases pilots were unaware of the fire and they didn't take any countermeasure.

Easy to solve

Just put the landing gear down. The >= 200 kts air does the rest. If it doesn't work, at least the most flammable materials (rubber, hyd pipes) and the heat source are now outside the aircraft frame and it will take more time to reach the wheel well, so you have more time to land.

Since wheel well fires are caused by high brake temperatures, the procedure is even usually applied before the fire actually exists. For example, the HOT BRAKES procedure of Airbus requires to lower the landing gear (and turn on the brake fan, if available) as soon as the brakes become overheated, potentially preventing in this way the fire event.

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    $\begingroup$ Your article is talking about brake fires, not fires in the wheel well itself, which is what the OP is asking about. Either way, though, "put the gear down" is the solution. Which would also, to the OP's question, disperse any extinguishing agent across the sky, removing its utility for fire protection. $\endgroup$
    – Ralph J
    Feb 27 at 17:28
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    $\begingroup$ @RalphJ Fire detection is there for fires caused by brakes (the only thing can reasonably be a source of ignition in the wheel well). Clearly, any other component of the aircraft can be cause a fire in extreme situations, but you don't have detectors/extinguishers everywhere, the OP was asking related to fire detectors in the wheel well. $\endgroup$
    – ocirocir
    Feb 27 at 18:03
  • $\begingroup$ The article doesn't really contribute to an answer -- it's all about "how to prevent" rather than what to do when they occur. Most of the brake fires occur after landing (and some before takeoff), where wheelwell detection wouldn't matter. To the extent that you get a fire in the wheelwell, it was most likely caused by dragging brakes, which the article omits from their discussion, since its entire focus is on maintenance practices. The really germane part of the answer is the last part: put the gear down. $\endgroup$
    – Ralph J
    Feb 27 at 19:53
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    $\begingroup$ The OP didn't ask "what to do when they occur"... Deciding whether a protection system is added or not on the aircraft is a combination of factors during the design phases, that take in account maintenance procedures, costs, event probability, and alternative solutions (i.e.., lowering the landing gear). $\endgroup$
    – ocirocir
    Feb 27 at 20:09
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There is no fire protection for wheel wells because any extinguishing agent would be immediately dispersed into the atmosphere as soon as the most important step for a wheel well fire, lowering the landing gear, is accomplished.

It's more important to get the tires outside the aircraft immediately and avoid containing the fire in a small space where it can do damage. Instead, let the airstream remove heat & any spraying hydraulic fluid.

Gear wells aren't sufficiently air-tight that allowing the fire to consume all the O2 is an effective response.

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