It is often said that a good landing can only come after a good approach. This is true and a poor approach can lead to bad landings, loss of control and accidents.
This was recognised by the airline industry in the 1950s and 1960s. Whilst investigating a number of undershoot and overshoot accidents, the recurring theme was that there were significant deviations from the final approach speed and glideslope, the aircraft was not fully configured for landing until late in the approach, or the aircraft had a high rate of descent. In other words, the final approach was not stable. In response to this, the concept of the stable approach was put forward. By mandating a stable approach, the number of landing accidents drastically reduced.
What is a stable approach?
The aircraft can be described as flying a stable approach when all of the following criteria are met:
Final approach speed is maintained and there are no significant fluctuations
Only small changes in heading and pitch are required to maintain the correct flight path
The aircraft is in the landing configuration and the landing checklist has been completed
The rate of descent is stable and not excessive
Small fluctuations are inevitable and do not constitute an unstable approach.
What constitutes an unstable approach?
The precise details will vary depending on the aircraft. In the context of microlights, it is my opinion that the approach is unstable if:
the airspeed is more than 5kts above the final approach speed, or 3kts below
large changes in heading or pitch are required to attain the correct flight path
the aircraft is not in the landing configuration
the landing checklist has not been completed
there are large fluctuations in the rate of descent, or the rate of descent is excessive.
By which point should I be flying a stable approach?
In the airline world, they will typically require an approach to be stabilised by 1,000ft if operating in IMC, or 500ft if operating in VMC.
Due to the smaller circuits that microlight and light aircraft fly, these figures may be unreasonable. It is suggested that for the typical microlight, you should be stable by 200ft above ground level.
Why does it matter?
A significant proportion of accidents occur in the approach and landing phase. The pilot's workload is already high and a stabilised approach will minimise the extra tasks and corrections required. For example, having the airspeed nailed early in the approach will mean one less thing to worry about at the later stages of approach.
When a pilot continues an unstable approach, they will have an increased workload and they may become overwhelmed or simply unable to place the aircraft in the correction position, speed or attitude for landing. The aircraft may arrive at the threshold too high, too fast, out of alignment or incorrectly configured for landing.
How can I fly a stable approach?
Ensure that you fly a circuit large enough to give you a suitable sized final approach. You will need a varying amount of time to configure the aircraft for landing depending on the aircraft type and your experience levels. Do not fly a smaller circuit than you can handle. Plan ahead so that you have completed your landing checklists before you turn on to final approach where possible. Ensure your airspeed in the circuit is not too high. Bear in mind that you will need to decelerate to final approach speed after the turn to final so leave enough room to do so. Ensure that you do not turn on to final approach with excessive height, as this will leave you in a situation where you need to dissipitate energy and will likely lead to an unstable approach.
How could an approach become unstable?
There are a number of reasons why an approach would become unstable. For example:
windshear or strong turbulence
wake turbulence
ATC pressure to approach faster due to traffic behind
configuration changes late in the approach
poorly executed approach
What should I do if my approach is unstable or becomes unstable?
Consider setting yourself a target height or distance from the runway, beyond which, you will commit to a go-around if the approach is, or becomes unstable. I suggest a ball-park figure of 200ft above ground level, but it will depend on the aircraft type and pilot's experience level.
Shouldn't I just try to 'fix' it?
The ability to quickly correct for problems and deviations comes with experience and familiarity with the aircraft type. An experienced pilot may have no problem losing an extra 10kts of airspeed in the last 200ft, but an inexperienced pilot could significantly destabilise their approach trying to do so. It is ultimately for the pilot to decide what is acceptable and at which point they will go-around. It is worth considering that a lapse in judgement at this point could result in an accident. For this reason, the airlines mandate a go-around if the approach is unstable below their set height.
It certainly helps to think about your approach before landing. What is your go-around criteria? It seems to me that a lot of pilots do not consider a go-around at the late stages of the final approach. A short self briefing, perhaps on the downwind leg will bring your options to the front of or your mind and make it more likely that you will execute them.
"If the approach is unstable, or a successful landing is in doubt, below XXXft, I will go-around."
Video: An unstable approach in a 737.
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