ground speed for navigational aspects: and IAS in the descent and climb phase. The relationship between height. TAS. density. temperature. Mach number and IAS is shown in Fig. 1. The airframe flight limiting parameters discussed so far enable a simple Flight Performance Envelope (FPE)tobedrawnasshowninFig. 2. This envelope and its optimum operating point only apply for a particular set of flight conditions. The area around the operating point contains the safety margins: they comprise speed reserves (increase in speed towards high speed buffet; reduction in speed towards stall buffet) and a 'G' reserve or buffet margin, for maneuvering forces and/or air gusts with vertical components. (The effect of the latter is proportional to TAS. which we have shown to be high in the cruise.) Not only does such a gust increase the loading on the wing, but the ·resultant velocity' of the airflow takes the wing nearer to its critical Mach number, which in turn further reduces the margins. If an aircraft experiences turbulence it has to adopt a speed that reduces the potential forces. whilst still leaving sufficient margins to deal with the buffeting. The rough-air speed is usually around 280klAS (knots IAS). or a Mach number of perhaps M 0. 7 (M 0.82 for the faster big jets). The above theory therefore shows that the pilot is in an airframe kept airborne by reference to IAS (EAS) and his whole world is largely governed by this. i.e. his airframe stalls at an IAS.
and it overspeeds at an IAS (which at the higher flight levels and/ or speeds is itself largely supplanted by compressibility limitations referenced to the speed of sound). As already mentioned, the radar controller's display is a true airspeed picture: it is the controller who has to bring the effect of the two speeds (TAS and IAS) together in his brain. and he/she eventually acquires the knack by experience, mainly · on the job'.
aircraft is behaving normally and flying at a constant IAS, the TAS will begin to fall. (Since the speed of sound is still increasing. the Mach number will continue to fall.) Finally, TAS will reduce to the same figure as IAS at sea level (for practical ATC purposes anyway). The changeover point. or more precisely. the· changeover flight level'. between Mach limitation and IAS limitation is subject to many variables. However. some sort of changeover reference is necessary for ATC The Technicalities of Speed mental effort and generally FL245 can Control be taken as a suitable point. This Descending aircraft close up on changeover point (flight level) tends to each other (the leading one is lower vary with the cruise speed: e.g. 8737, and therefore slower): climbing air- M 0.72, changeover FL245; 767, craft move apart. This assumes similar 757. Tristar. 747. M 0.8. FL300, flight profiles and speeds (rare in real BAe 146, M 0.6, FL210. life), and starting from similar heights. Generally 250/ 290 klAS is flown This is why the following paragraphs for descent. and 2801330 klAS in the concentrate on the descending en route climb after departure due to phase; everything is coming together the extra weight. Most modern jets both horizontally and vertically! can be pushed to descent at 350 klAS Consider an aircraft arriving in the but this gives unnecessarily high rates controller's airspace and ready for of descent and also wastes fuel. descent. It will be flying to a Mach The practical aspect of achieving number. and will start its descent any necessary speed control is made more or less at the same Mach num- easier with the aid of one or two rules ber; as the aircraft descends the TAS of thumb. Consider two aircraft, may increase slightly as the speed of descending line astern. 6 nm apart. sound increases (this is of no practical and both descending at the same significance): at the same time the air optimum speed (IAS): then. since the density is increasing and thus the IAS leading aircraft is bound to be lower will be increasing until at a particular than the succeeding one, probably by flight level (say around FL2 70) it 4000ft, the TAS of the leading aircraft reaches the required IAS operating will be less; therefore the two aircraft value. are closing up. ATC must therefore From this point onwards the air- apply a speed differential (IAS) to craft will be IAS limited instead of these aircraft. The rule of thumb for Mach limited. and assuming that the this (at these higher levels and speeds)
DESIGN OPTIMUM OPERATINGPOINT 37,000ft
Mmo
'FL245' FLIGHT PERFORMANCE ENVELOPE &;
HEIGHT
TOO FAST
~
~ SEA LEVEL------,-6..,.5-kt
Vs ______
3_1_0.._k_t ____
4....,20._k_t _______
..____
TRUE AIR SPEED
Fig. 2 Note: Vmo/Mmo Maximum Operating Speed (k!AS and Mach number)
22
THECONTROLLER/DECEMBER1987