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: Avro RJ45

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?

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    $\begingroup$ I think you are confusing speed brakes with spoilers. Speed brakes are deployable in the air, usually up to Vne. Spoilers destroy lift and are usually interlocked with the squat switches so that they aren't inadvertently deployed in the air. $\endgroup$ – Ron Beyer Mar 19 '17 at 20:49
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    $\begingroup$ @RonBeyer, spoilers often are deployed in the air and act as air brakes, though usually they don't extend all the way then. They are not used at low speed though as they increase the stall speed, which speed brakes on other parts of the plane than wings don't. $\endgroup$ – Jan Hudec Mar 20 '17 at 18:58

enter image description here
(Source) F-15 landing with airbrake extended.

enter image description here
(Source) Airbrakes on the Fokker 70.

1. It allows steeper approaches

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.

2. It allows better speed control

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.

enter image description here
(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?

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    $\begingroup$ During a go-around retracting the speed brake is like instant thrust. $\endgroup$ – radarbob Mar 19 '17 at 22:18
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    $\begingroup$ It also make a go around safer since the speed brakes can retract much faster than the engine can increase power. $\endgroup$ – David Schwartz May 25 '17 at 18:07

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.

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    $\begingroup$ In the USS Hancock video, what's the reason for everyone landing with an open canopy? Visibility? $\endgroup$ – Flambino Mar 20 '17 at 1:49
  • $\begingroup$ Dunno. It's a good question. $\endgroup$ – Carlo Felicione Mar 20 '17 at 3:01
  • $\begingroup$ Just posted it as a proper question :) $\endgroup$ – Flambino Mar 21 '17 at 0:14
  • $\begingroup$ It used to be common for Navy pilots to fly propeller driven aircraft as well as early jets open cockpit during low speed flight. The earlier canopy designs permitted this. I don't think it was done for any specific reason save maybe they like the feel of flying that way. $\endgroup$ – Carlo Felicione Mar 21 '17 at 0:39

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.

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    $\begingroup$ Hi Mike, Fokker 70/100 have thrust reverse. $\endgroup$ – ymb1 Mar 20 '17 at 11:27
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    $\begingroup$ Yes but the Fokker 70/100 are developments of the F28 which had no thrust reversers. That is the only reason it retains the air brake. $\endgroup$ – Mike Sowsun Mar 20 '17 at 15:11

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