I recently saw an Avro RJ100 on approach and I wondered why the Avro RJ100 and also the RJ85 have the air brakes at the rear of the aircraft, and not on top of the wings like most airliners?
What is the advantage you get from this, and/or what made it impossible to mount the air brakes on top of the main wings?
Note: the BAe 146 / Avro does have wing spoilers.
Air brakes are in most cases mounted directly on the fuselage. Forward mounted air brakes would disrupt the airflow to the wings and engines, so the designers are left with three choices: above, under, or at the rear / on the sides.
Under, like on the Aero L-39, works well1 if the air brakes aren't big. Above is usually if all else fails, like on the F/A-18 Hornet and F-15.
Rear-mounted—inline with the center-of-mass—is the most convenient, aerodynamically and for maintenance. Like the Avro, F-16, or F-86.
Now, as to why the Avro—as most military jet aircraft and few jetliners—has it:
The Avro RJ lacks thrust reversers "due to their perceived reduced effectiveness in anticipated conditions." The reduced effectiveness is because of the low landing speed and the closeness of the engines to each other on each side. Using reverse at low speed risks FOD ingestion and stalling of the nearby engine.
It reasoned that since the 146 would have a touchdown speed of only 90 kt, and because reversers are not normally used below about 60 kt, their effectiveness would be limited. Reverse thrust also would have subjected each engine to an extra cycle/flight.
Maximum descent rate achievable with the airbrakes is 7,000 ft/min and at speeds of less than 250 kt and below 10,000 ft, there is still 4,000 ft/min available. By comparison, a typical airliner descent rate on the approach is around 2,000 ft/min. BAe 146 operators may also use airbrakes during the landing roll.
(Flight, 1981)
One advantage that came out of having air brakes is the ability to fly steep approaches into airports like London City.
[For] several years the BAe 146 was the only conventional jet aircraft capable of flying from London City Airport.
Because of the Avro's high thrust-to-weight ratio, having air brakes deployed on approach makes it easier to control the speed. The thrust-to-weight ratio for the RJ100 is 0.28:1, compared to 0.16:1 for the comparable Boeing 717. It's even higher for the smaller RJ's.
Having a high thrust-to-weight ratio also allows operations from short runways and runways within cities, as the airplane can climb more steeply, thereby reducing the noise impact and also allows high payload.
Air brakes differ from spoilers in that air brakes are designed to increase drag while making little change to lift, whereas spoilers reduce the lift-to-drag ratio and require a higher angle of attack to maintain lift, resulting in a higher stall speed.
The Avro still sports wing spoilers as shown above, those also help in braking as they transfer more weight to the wheels, and help reduce the likelihood of a bounced landing.
1 Belly-mounted air brakes on the L-39 help the pilot configure the plane for landing. Half-flaps + air brakes require the same trim/power as full-flaps and landing gear down (hence the under positioning). The pilot would use the air brakes to slow down, put down half the flaps, and trim the aircraft. The pilot can then on final approach put down the landing gear and full flaps, retract the air brakes, all without having to re-trim or change the engine power setting (which is narrow-ranged on the L-39).
Related: Why does this regional jet have its air brakes wide open before touchdown?
The Avro RJ100 does have something on its wings- three spoilers on each, though these act exclusively as lift dumpers after touchdown (and another two for roll control).
It is perfectly possible to make the lift spoilers act like airbrakes- as in other aircraft; in case of 146, having exclusive airbrakes serve a number of purposes:
The airbrakes increase the descent rate of the aircraft, more than doubling it compared to other airliners. This allows for steeper descents (like in London city).
The large area of the airbrakes also eliminates the need for complex thrust reversers once on the ground; this reduces maintenance requirements and cost.
The airbrakes also help in speed control. The use of airbrakes enables appraoch at higher thrust settings, enabling a better option of go-around rather than relying only on the throttle controls. Unlike the spoiler, the airbrake doesn't affect lift, enabling its use in such scenarios.
The airbrake is effective in all operational speeds, enabling faster descent in case of an emergency, though I'm not sure if it was a design consideration.
They have them mounted at the rear fuselage because they work better, and because there isn't a set of stabilisers mounted there: the stabilisers are mounted on top of the vertical tail, in a T-tail arrangement.
The Fokker 100 has the same arrangement with air brakes as the RJ100 has. My office used to be at the airport where they were built, and I could see the line-up of landing aircraft coming in, all nose high - except for the little Fokkers which had their nose pointed at the runway allowing the pilots a clear vision unimpeded by a nose cone sticking up.
Wing spoilers acting as air brakes brake very efficiently as well of course, but they also destroy lift. In order to retain the amount of lift required to fly, the aircraft angle of attack needs to increase. That can be unsafe in a landing situation, so the manufacturer needs to mount leading edge slats which allow for higher AoA.
Air brakes such as the RJ100 has have only upsides, since they influence drag only, not lift. They're not used everywhere because the bit of fuselage where they are mounted usually houses horizontal stabilisers: deploying the air brakes would distort the air flow around these, which is never a good thing.
