A stabilised approach is a fundamental factor in reducing the occurrence of both hard landings and runway excursions. Within their Operations Manual, most airlines define the criteria for a stabilised approach and mandate that the approach must be abandoned and a go around executed if the stabilised criteria have not been achieved by a specified height above the touchdown zone elevation (TDZE). This height can vary by airline or may have multiple values based on IMC/VMC criteria or might have multiple height "gates" in which the higher is a "should" gate which may only be passed if evidently correcting towards meeting stabilisation criteria by a lower "must" gate. The most common height values in use are 500 and 1000 feet but some operators have elected to mandate stabilised criteria at a greater height above TDZE.
Responsibility for achieving stabilised parameters rests with the flight crew and it is solely their decision to execute a missed approach at the appropriate height should those parameters not be achieved. However, air traffic control clearances and instructions, be they vectors, altitude constraints or speed restrictions, especially in combination with adverse wind conditions or non-standard approach path intercepts, can make achievement of the stabilisation criteria difficult or even impossible and could well lead to a controller induced go around. To enhance awareness of these issues, the French Direction des Services de la Navigation Aérienne (DSNA) has produced two videos which are targeted at their controllers.
The French DSNA has produced two videos to help make controllers aware of the increased cockpit workload and greater risk of an unstable approach that are associated with speed-restricted tailwind operations and with short vectoring that leads to an aircraft intercepting a glide slope from above.
Non Stabilised Approach (NSA)/Non Compliant Approach (NCA): Short Vector and Interception from Above
In this video, an arrival aircraft is provided vectors to shorten its route to the final approach. Note that the controller does not offer, and the flight crew does not request, track-miles-to-run information. Ultimately, the vectors result in the aircraft being above the glide slope at the point of localiser interception. Glide slope capture from above is successful but the crew is not able to achieve stabilised aircraft criteria and a go around is executed.
Controllers, when viewing the video, should keep in mind throughout that:
- using the automation of a modern large jet to achieve glide slope capture from above requires that a specific, seldom used, sequence of events be actioned by the pilot flying
- not all operators provide capture from above training to their crews and even those that are trained have little opportunity to practice the manoeuvre outside the simulator
This event can be rightfully classified as a controller induced go around. However, the pilots, in that they accepted the shortened route and did not anticipate the potential need for glide slope capture from above, are also partially at fault. Actions that might have been taken by the controller to help the pilots achieve a successful landing outcome include:
- providing track-miles-to-run information - provides situational awareness and, in part, the basis for clearance acceptance
- modifying descent profile - assuming minimum vectoring or sector safety altitudes are not compromised, an earlier descent would have reduced the likelihood of (the need for) glide slope capture from above
- providing early speed reduction - builds in time to allow for checklist and briefing completion and enables earlier selection of landing configuration
Non Stabilised Approach (NSA)/Non Compliant Approach (NCA): Tail wind on Final and High Speed
It is common practice, especially at busy airports, for ATC to impose speed restrictions on arrival traffic thus maintaining appropriate separation between aircraft whilst optimising runway capacity. Clearances such as "maintain 160 knots (IAS) until 4 miles" or "maintain 170 knots until the final approach fix" are common. However, it is imperative that the speed restrictions be reasonable and that they be tailored to allow adequate distance (time) for the aircraft to achieve stabilised criteria prior to reaching the 1000' (or 500') gate.
The following video demonstrates the consequences of a typical speed assignment clearance that has not been modified to compensate for adverse wind conditions.
This event can also be classified as a controller induced go around but, once again, the pilots accepted the clearance without question. The video clearly demonstrates why both pilots and controllers need to be aware of, and make adjustments for, both runway wind and the winds on approach to ensure a successful, stabilised approach.
For a 3 degree glide path, the 1000' stabilisation gate is located just over 3 miles from the runway threshold. Between that gate and the termination point of the speed restriction, there must be sufficient distance (time) for the aircraft to slow to approach speed and become compliant with all other stabilisation criteria. Whilst it is convenient to think of speed reduction in terms of knots lost per nautical mile flown (and many aircraft manufacturers provide information in those terms), this is an over simplification. In reality, it is actually knots lost per unit of time with the nominal time interval being that required to fly a mile while decelerating in no wind conditions. With that in mind, a speed reduction in headwind conditions will take less distance over the ground (than in still air) whereas in a tailwind, the ground distance will be increased, sometimes substantially. Controllers must either adjust the requested speed or the point (location) to which that speed is to be maintained (or both) in circumstances of strong tailwinds on approach. Controllers can determine the existing wind by pilot report or by comparing reported IAS with the ground speed of the aircraft.
A stabilised approach is a fundamental factor in reducing the occurrence of both hard landings and runway excursions. Responsibility for achieving stabilised parameters rests with the flight crew and it is solely their decision to execute a missed approach should those parameters not be achieved. Air traffic control clearances and instructions, especially in non standard or adverse conditions can make achievement of the stabilisation criteria difficult or even impossible.
Controller induced go around can be prevented by issuing appropriate clearances and instructions that have been formulated with full consideration given to prevailing conditions. The DSNA videos are a valuable tool in helping to reduce controller induced, failure-to-stabilise, go around occurrences.