I was looking at a detailed picture of a plane while following its path on Flightradar24.com. Here is the Avro RJ45 I am talking about:
I wonder why its aerobrakes at the empennage seems wide opened before touchdown. What I remember from all the planes I took is flaps are at maximum before touchdown and aerobrakes (sometimes automatically) opens when landing gears touch the runway.
Is it due to special constraints for this particular aircraft?
(Source) F-15 landing with airbrake extended.
(Source) Airbrakes on the Fokker 70.
From Wikipedia:
The engines [on the Avro] lack thrust reversers due to their perceived reduced effectiveness in anticipated conditions. Instead, the plane features large airbrake with two petals below the tail rudder at the rear of the fuselage, which has the advantage of being usable during flight and allowing for steep descent rates if required.
... also...
Airbrakes on the Fokker 70's tail section – similar to that found on the BAe 146 / Avro – allows it to conform with the 5.5° glide slope at London City Airport.
Some aircraft are overpowered with high power-to-weight ratio. The thrust lever range alone won't be enough to hold the approach 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.
Low power and the plane loses speed, some power and the plane gains speed. So, more power is used in conjunction with airbrakes, such as the F-15 shown above.
Once on the ground, they help slow down the plane, same as spoilers.
(Source) Spoilers.
Spoilers have the added benefit of spoiling the lift–transferring more weight to the tires for increased braking capability.
Related: Why do some aircraft (e.g. Avro RJ85) have rear-mounted air brakes?
Primary reason is speed control. The thrust output of a jet engine does not have a linear relationship to the gas core or fan speed and the most effective thrust range is somewhere between 75-100% N1. Commercial jets and military fighters tend to be quite 'slippery' and require drastic throttle reductions to slow down without the use of speedbrakes. This puts the aircraft at considerable risk at lower altitudes or on short final where a large thrust increase may be needed to correct for a gust factor, wind shear or rapid descent. Unfortunately it takes a period of time for the gas core and fan of a jet to 'spool up' and start providing the needed thrust. This slow response time of early jet engines in critical power situations has produced horrifying results, as was the case here on July, 1955 with a fatal rampstrike of an F7U-3 Cutlass coming aboard USS Hancock (CV-17). See at 5:43 into the video.
It can also happen even with the most modern engines as this F-18 rampstrike proves.
To compensate for the thrust response problems, pilots will fly the pattern and approach at a high thrust lever setting, then maintain speed and descent control using application of speed brakes. This gives good energy management and control but allows the pilot to quickly get thrust if he needs it to return to glideslope, climb or go around altogether.
The Fokker F-28/F70/F100 and the BAe 146/Avro RJ are the only two airliners with rear mounted air brakes because they were the only two airliners not designed with thrust reversers.
It is Standard Procedure to use the rear mounted air brakes on every landing because they add drag and reduce the landing distance.
Other airliners don't have this type of air brake simply because they don't need them.
These two types also employ wing spoilers for use on touchdown just like other large aircraft. Wing spoilers are only used after touchdown, or at higher altitudes, and are not used during the final approach and flare.
