L6 - Performances25-11-2005 | |
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L6 - Performances25-11-2005 | |
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This lesson is about how approach controllers can deal with the traffic according to aircraft performances and use of speeds. If you are unfamiliar with IAS, TAS, GS and other speeds, read L5-Speeds before. 1 - Aircraft performances : Here are some general specifications (intervals) for controllers to deal with the traffic according to the type of aircraft.
2 - Arrivals - Usable speeds : a) Vmo (maximum operation speed) : - when flying at the Vmo, an aircraft has the highest rate of descent (the higher figure of the above intervals). - when flying below the Vmo, the rate of descent is average. - use of aibrakes : whereas the aircraft is flying at the Vmo or not, the rate of descent with airbrakes out may be 2 or 3 times (even more) the rate with airbrakes in. b) 220 KIAS : a very common speed to be used on approach control is 220 KIAS for almost all aircraft (except general aviation) can use it (see approach speed column above). Thus, it is more convenient for the controller to deal with traffics all flying at the same speed. The advantages are : - for most aircraft, 220 KIAS is higher than the minimum clean speed and do not induce a significant change in fuel burn (except for some heavies which minimum clean speed is about 230-250 KIAS). Remember most pilots are quite reluctant to have slats/flaps down too early. - 220 KIAS is often the max speed to be applied into holding patterns. - at 220 KIAS, aircraft can easily accelerate if requested because flaps and slats are still up (except for heavies). - at 220 KIAS, aircraft can easily reduce too because flaps/slats may often be lowered immedialtely without waiting for the speed to slow down and then it doesn't take too long a time to reach the final approach speed. c) Minimum clean speed : when willing to use a speed below 220 KIAS without penalizing the pilot (mainly as far as fuel burn is concerned), the controller may ask the pilot to reduce to minimum clean speed (which is quite variable, depending on the type of aircraft or even airlines directives - particularly, as said above, heavies have a minimum clean speed higher than 220 KIAS). d) 180 KIAS : as you can see above, the final approach speeds go from 110 to 170 KIAS (general aviation excluded). If for any sequencing regulation you need an aircraft to keep a high speed on final, 180 KIAS should be the maximum requested before the OM (outer marker) or before the FAF/FAP. After these points, it is better to leave the pilot choosing his/her normal final speed. Note : the pilot may refuse to start the final with a high speed. e) Minimum approach speed : unlike the previous case, you may have to ask an aircraft to reduce to the minimum approach speed. This shouldn't be requested more than 25 miles inbound the airfield because the aircraft will stay for a long time close to its stall speed and the handling of the aircraft is quite uncomfortable. In addition, this significantly increases the fuel burn. Note : use this speed with caution; some aircraft have a very low minimum approach speed (below 80 kt). Having an aircraft at 10 NM inbound at 70 KIAS could cause some troubles for the following traffics. 3 - Indicated airspeed and air traffic control at mean and high altitude : As you know, depending on the altitude, the speed shown onboard (IAS) is quite different from the one shown on radar scopes (GS). Also, for a given IAS, there could be a big difference in TAS. Thus, the IAS could appear as of no interest for controllers. However, in case of a conflict, the aircraft are flying more or less at the same altitude. For a given IAS at a given level, the TAS almost remains the same within +/- 2000 ft from this level. In such a case, the aircraft IAS are comparable and the controller can use them for regulation because it is mainly a problem of relative airspeeds between aircraft. 4 - Speed reduction : a) Normal reduction : if the controller just requests a speed reduction, without any other information, the pilot will just reduce the thrust, waiting to reach the requested speed. This could be long : a reduction from 320 to 220 KIAS takes about 10 NM at 10 000 ft and 7 NM at 5000 ft. b) Quick reduction : if the controller needs a quick reduction, it is better to inform the pilot and give the reason : "XXXX expedite reducing to....(reason)". The pilot will then reduce the thrust and use the airbrakes and/or spoilers. When reaching the requested speed, the pilot switches the airbrakes/spoilers in and apply the relevant thrust. For information : - airbrakes : generate drag. - spoilers : generate drag and decrease lift. - both expedite the speed reduction. c) Important note : speed reduction and descent are somewhat uncompatible. When asked to reduce speed when descending, most pilots will stop the descent (or keep a very low rate of descent) until they reach the requested speed and then they will resume a normal rate of descent. The controller has better to make a choice : - ask the pilot to reduce speed first and then start the descent. - ask the pilot to descend first and reduce speed when steady. 5 - Bank angle : The normal bank angle is 25° to 30° (clean configuration). More than 30° is considered as uncomfortable for passengers. Into holding patterns, at the minimum clean speed or maximum holding speed indicated on the chart, the bank angle shouldn't be lower than 25°. With flaps down, the bank angle is reduced to 15°, mainly because the airspeed is closer to the stall speed and altitude is low. Also, at a given bank angle, don't forget the higher the speed is, the higher the radius of turn is. And at last, at a given speed, the higher the bank angle is, the lower the radius of turn is. This could be important when an aircraft is requested to hold over a fix, the size of the holding pattern may vary a lot, according to the speed and bank angle of the holding aircraft. | ||||||||||||||||||||||||||||||||||||||||||
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