Imaginary situation. An aerodrome requires a Continuous Descent Approach technique; the FAF is at 8 DME and 2500ft agl. Usual Speedywings Airlines straight in approach plan is to maintain 250KIAS at the SLP until 2 miles to the FAF (10 DME), then decel as rapidly as possible to begin the final approach.
On a particular day, the controller advises 'due traffic, decel to 220 knots by 18 DME please' on the same straight in approach.
From the 18 mile point, how much longer do you think it'll take to cart your bus load of shell-suited oiks and their harpies at 220 instead of 250?
18-10=8; 8 miles at 250 KIAS takes 1.92 minutes; 8 miles at 220 takes 2.18 minutes. A whole 15.7 seconds longer.
When teaching pilots to fly Air Transport flights, we regularly discussed the perceived time saving which resulted from 'high speed approaches' - and how little it actually was. In fact more could be saved from stand-to-stand by planning the roll-out point and taxiway exit carefully than from keeping 'pedal to the metal' until just before the FAF. We developed a low-drag approach technique for the coal-fired old rust-bucket I used to fly as the Vickers FunBus is hardly a whisperjet and we really needed to avoid upsetting folk under the flight path with unreasonable noise - but the idea wasn't to save time, just noise.
So, although the extra 15.7 secs might be perceived as being annoying, if the traffic ahead is a lumbertub flat out at 180, the difference between closing on it at 40 rather than 70 kts might make the difference between achieving separation minima and busting it.
I doubt whether ATC ever intend to mess people about; they are as aware of your commercial pressures as anyone else. An appreciation of how little extra time an early decel will cost might perhaps be useful to some?
Last edited by BEagle; 12th Jun 2004 at 08:46.