As pilots, most of us are quite pleased when Air Traffic Service (ATS) offers a direct routing, advises us to "maintain best forward speed", tells us "you are number one for the airport", or provides vectors that reduce total track distance. However, during the approach phase, these actions can potentially lead to being left too high and/or too fast and, consequently, result in an unstable approach, the necessity to capture the glideslope from above, or the need to abandon the approach and conduct a go around.
A stabilised approach is a fundamental factor in reducing the occurrence of both hard landings and runway excursions. Within their operations manuals, most operators 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 operator, 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 air navigation service provider Direction des Services de la Navigation Aérienne (DSNA) has produced two videos which are targeted at their controllers but convey important information for pilots as well.
In an effort to help make their controllers aware of both the increased cockpit workload, and the greater potential for an unstable approach, that can occur when aircraft are vectored or speed restricted, the DSNA has produced two videos. One video examines short vectoring that leads to an aircraft intercepting a glideslope from above and the other addresses speed-restricted operations under tailwind conditions. As these videos are controller focused, some written, pilot perspective, commentary has been provided to make the videos more meaningful to flight crew.
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 glideslope at the point of localiser interception. Glideslope capture from above is successful but the crew is not able to achieve stabilised aircraft criteria and a go around is executed.
Some pilots, when viewing the video, might consider the commentary on the "risk" and "stress" caused by the event as overstated. However, 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 actions must be completed 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
- The video is targeted at air traffic controllers, not at pilots
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 to achieve a successful landing outcome include:
- refusing clearance - acceptance of a short route clearance is a judgement issue and should be made based on crew experience, aircraft capability and distance to run
- requesting track-miles-to-run information - provides situational awareness and, in part, the basis for clearance acceptance
- requesting early descent - assuming minimum vectoring or sector safety altitudes are not compromised, an early descent would reduce the likelihood of (the need for) glide slope capture from above
- requesting early speed reduction - builds in time to allow for checklist and briefing completion and enables early selection of landing configuration
- early selection of landing configuration - puts aircraft into a power against drag configuration facilitating both glide slope capture from above and achievement of stabilised criteria
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 opportunity for the aircraft to achieve stabilised criteria prior to reaching the 1000' (or 500') gate.
The following video demonstrates the consequences of a higher than normal speed assignment and not adjusting the speed restriction criteria during adverse wind conditions,
This event can also be classified as a controller induced go around. However, once again, the pilots accepted the clearance, this time a speed restriction, 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 distance 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. Pilots of modern aircraft generally have both wind and ground speed information available and, although one does not fly based on ground speed, they should be aware of the values and use the information to determine whether or not a speed restriction is appropriate and acceptable. If it is not, they must be prepared to refuse the clearance.
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 thus leading to a controller induced go around. To reduce the likelihood of such events, flight crew must evaluate an ATC clearance in the context of prevailing conditions, crew experience, and aircraft capability, and be prepared to refuse the instructions if an unstabilised approach is the most likely outcome.