Some aircraft do have this detection system (many Boeing aircraft), some do not. Why? What's the main purpose of the wheel well temperature detection system, even though there are brake temperature sensors.
What's the main purpose of this detection system even there are brake temperature sensors.
Far more often than not, the need for brake temperature sensors is after landing to tell you how hot the brakes are. If you had to use heavy braking upon landing, they're going to be hot. How hot they are tells the captain in part how he needs to taxi and whether he should advise ground personnel to stay away from the sides of the tires where the thermal plgus are and possibly to use cooling equipment (typically a huffer) to cool them down.
Wheel well temperature sensors are needed after takeoff. The potential problem here is that you need to keep an eye on how warm the wheel well is getting from the hot tires you have retracted and enclosed in the wheel well.
As an example, let's say you have a 747 freighter loaded to a max takeoff weight of 825,000 lbs or more at Honolulu and taking off on RWY 8R. That's 20,000 feet of taxiing and another 10,000 feet plus of takeoff roll. The flexing of the tires during that taxi and takeoff generates a lot of heat. Once the wheels are in the wells, that heat transfers into the wells and eventually leaves the airplane. However, if for whatever reason, the tires already had a lot of extra heat before the taxi started or a lot of braking was done during the taxi, or the airplane was stuck down low in warmer air after takeoff, whatever, the wheel well temperature might exceed the safe limit.
The solution is usually simple, drop the gear and cool the tires. If you don't you risk a wheel well fire.
Some aircraft do have this detection system (many Boeing aircraft), some do not. Why?
The short answer is that they have been designed so that they don't need it, which begs the question: why aren't all aircraft designed such that they don't need it. Essentially every aircraft design includes compromises. It's all part of tailoring the design to best accomplish what the aircraft's primary task is to be. Using the 747 example, you could, for example, install a cooling system in the wheel wells to take care of the heat, or you could use heavier tires that didn't flex as much and thus didn't heat up as much, or you could have additional structure able to resist the heat or dissipate it quicker. But whatever means you used, it would mean hauling around more weight and the attendant costs in fuel and loss of payload capability when you didn't need it most of the time. Far better to avoid those high costs, but in the interests of safety install a much lighter system that will alert you to the need to cool off the wheel well by dropping the gear.
Insofar as potential 747 wheel well overheats are concerned, baring mechanical failure, it's generally a problem only when the aircraft is used in a manner that it wasn't designed for. For example, the 747 was designed as a long-haul transport. As such, there would be plenty of time for the wheels to cool down in the wheel wells. As I remember we were told in ground school that 5.5 hours was the cool down time. So, if you have a short leg and then a quick turn with high weights, you might be taking off with tires that have more heat in them than the system was designed to handle.
When JAL decided to use the 747 for high-density, short hauls within Japan, Boeing came up with the 747SR (Short Range) which included a redesign of the undercarriage as well as several other things. The design goal was that that any commuter airliner needs: short legs, frequent takeoffs and landings, and it made sense to optimize the design for that criteria, and that optimization required a decrease in range and payload as I remember.
Cargo, engine and wheel well fire detection is considered essential for many a/c. If a high friction wheel assy. Is taken up into the enclosed environment of a wheel well any ensuing fire would be directly under the pax. Compartment a simple loop detection sys. would enable the crew to lower the gear before catastrophic failure.