Planning the final-approach commences at the end of it … NOT the start!
Planning the final-approach commences at the end of it…
From the time that a student pilot ‘escapes‘ the familiar bounds of the circuit area of his or her ‘home‘ airfield, a new challenge (among many others) is to understand how to plan the re-entry to the circuit, either at ‘home‘, again, or at a new an unfamiliar airfield, when starting to venture ‘cross-country‘. No two airfields are alike, for many reasons.
The generally-standard circuit entry tracking and altimetry procedures for both ATC-controlled and uncontrolled airfields and aerodromes are well documented by aviation authorities’ publications: But what about the considerations applying to the re-configuring of the airplane with landing gear and flaps and the finer points of airspeed control? Has your flight instructor offered any advice, or referred you to a written explanation?
Have you ever been made aware of a universal technique – one that can be applied in any situation: Whether VFR or IFR? Small airplane or large? High performance or not? Many pilots haven’t and resort to just ‘winging it‘ (no pun intended) on each and every approach. That makes it hard to be safe, efficient and graceful.
There is such a simple and well-proven technique; however, interestingly, the planning starts at the END of the approach … NOT the start. It may prove quite helpful before and, perhaps, even after you gain further flying experience. We approach airfields in 3-dimensions, so why not add altitude into the mix?
For EVERY approach, in ANY airplane, the simple key is to decide on a ‘Height above (runway)threshold‘ (elevation) – (HAT), by which the airplane should be fully re-configured for landing, and fully stable: That is, established on the runway centreline, on a standard 3º approach path angle – to the correct visual aim point for the subject airplane – and stabilised on the correct approach airspeed (Vapp). This ideal condition is often referred, colloquially, as being established ‘in the slot‘.
(Note: Vapp is based on Vref for the landing weight and landing flap setting, plus additives for variations in airfield elevation, ambient temperature and wind velocity. In turn, Vref is based on 1.3-times the stall speed for that landing weight and landing flap setting.)
This height will vary for each airplane size, weight and approach speed. For example, many airlines use 1000ft HAT, when in instrument meteorological conditions (IMC) and 500ft HAT, for visual meteorological conditions (VMC). Furthermore, the pilot not flying (PNF) makes the standard call, “1000 STABLE” or “500 STABLE“, as appropriate, or “500 GO-AROUND“, if the approach is assessed as unstable and, therefore, unsafe to continue the approach.
For much smaller RA and GA aircraft, figures of, say 300ft HAT (VMC) and 800 or 1000ft (IMC) may be considered suitable.
The point is, that this HAT becomes the basis of the approach sequencing, BACK from which the pilot computes when to make each successive flap and landing gear selection and reduce airspeed accordingly, until finally configured and stable by the selected HAT.
The following example illustrates the technique:
My former employer, Qantas Airways Ltd had a standard operating procedure (SOP), among many, that stipulated 500ft HAT (VMC) and 1000ft HAT (IMC) as the target height at which the aircraft had to be stabilised, with landing gear extended, flaps 30º or 40º (whichever was decided) and stabilised ‘in the slot‘ and on the selected approach speed. In the interests of simplicity – and safety, to avoid making a rushed approach, it was my practice to assume 1000ft HAT for all landings.
Now, in order to allow time for the flaps to run from 25º to 30º or 40º and for the airspeed to reduce and to stabilise on VApp, it was appropriate to select flaps 30º or 40º at 1200ft HAT. Experience on type proved that 500ft height intervals between the successive configuration changes were ideal. Working backwards up the approach, shows how the sequence plan is unfolded.
Note: It can be seen that the ‘end of descent‘ was located, nominally, at approximately 13nm from the runway threshold, at 3700ft, at Vref 40º +70 KIAS, at which point the first selection to Flaps 1º and Vref 40º +50 KIAS could be made, to be stabilised at 3500ft and so on.
It should be noted, also, that the technique is not restricted to straight-in approaches. A further advantage offered by the 500ft stepped configuration changes technique is that it lends itself readily to ‘bending‘ around a circuit or instrument approach procedure.
Similar flap extension schedules can be developed for other airplane types. Here’s an example for the Cirrus SR 22:
There are major benefits of this HAT-sequencing technique. The first is that it can be tailored to any airfield, simply by adding the runway threshold elevation to the preferred HAT figures. (If the runway threshold elevation was, say 430ft AMSL, then that figure could be rounded to 500ft and added to each ‘gate‘. So, in the Cirrus SR 22 example, above, flaps 50%/16º would be selected at 1500ft QNH, with the airplane stabilised at 85-90 KIAS by 1300ft QNH. Then, flaps 100%/32º would be selected at 1000ft QNH and stabilised at 80-85KIAS by 8ooft QNH.
This intelligent technique can be applied to both a straight-in approach situation, or ‘bent‘ around the applicable legs of a circuit. A further benefit is a smooth and stable re-configuring schedule that is comfortable for fare-paying passengers (remember them?) and un-hurried in its execution, improving flight safety, in the process. Finally, the 500ft intervals in the sequence provide some predictable ‘space‘, to re-assess, to execute landing checklists and to receive, consider and respond to radio calls.
May we suggest that you consider the above in reference to your airplane and then give it a go: It works.
Wishing you many safe landings
Captain David M Jacobson FRAeS MAP
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