What prevents aircraft with a tailcone APU from using tail-mounted airbrakes? The BAe 146 and Fokker F-28, which are virtually alone among jetliners in lacking thrust reversers, compensate by using large, tail-mounted airbrakes to help slow the aircraft during descent and landing:



(Image by Adrian Pingstone at Wikimedia Commons.)

According to this answer to a previous question about why we don’t see this on more (and bigger) jetliners, the BAe 146 and F-28 are only able to use tail-mounted airbrakes because, unlike most jetliners, they lack an APU in the tailcone:


  ...Air brakes such as the RJ100 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 an APU.


(Emphasis added.)

But I don’t see why having an APU in the tailcone would preclude the use of tail-mounted airbrakes; the APU is situated in the middle of the tailcone and exhausts out the tip of the tail,1 while tail-mounted airbrakes are mounted on the sides of the tail and deploy to the left and right:



(Image by dtom_cro at Photobucket.)

As such, having a tailcone APU shouldn’t necessarily preclude having airbrakes on the sides of the tail; in the 146, the space between the airbrakes is basically empty, with only a light truss supporting the rear collision beacon, but it'd be easy to have something more substantial - such as, for instance, an APU-and-other-machinery-housing structure - in between the petals:



What am I missing here?



1: As @ymb1 points out in their comment, the BAe 146 does have an APU, but its APU obviously does not exhaust out the tip of the tail; that statement regarding APU exhaust placement refers to its location in the vast majority of jetliners in general.2

2: That last statement obviously does not apply for jetliners where that real estate is taken up by a full-blown engine (727, Trident, Tu-154, Yak-40, L-1011, Yak-42, etc.).
The airlines prevent them or: the airlines won't buy them if they were offered

The photos you have already show an APU in the tail section (exhaust on the starboard side).

Adding air brakes requires increased structural integrity (weight; money) and additional system connections (weight; money).

When they are of no use, they're simply not added. Why are they of no use?

Well, pure air brakes (as opposed to speed-brakes/spoilers) have 1–2 functions (for jetliners):


Compensating for the lack of thrust reversers
Performing steep approaches when the thrust-to-weight ratio is high (small plane that has too much power). As I noted here: 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.


Neither affect most jetliners (note that the babybus (A318) can be certified for steep approaches, and it doesn't need such an air brake).

The airlines would rather carry payload (paying load) and not bother paying for maintenance to regularly check and fix a novel item – not all airports are London City: high thrust-to-weight ratio is good for taking off from short runways and performing steep climbs from in-city airports for noise reduction.