Unstabilised Approach: Lack of or Wrong Information About Distance to Touchdown
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|Category:||Toolkit for ATC - Stabilised Approach|
Failure to initially advise and /or periodically update track estimated track miles to go when providing radar vectoring onto final approach or the passing of incorrect track miles without prompt correction.
Effect of Action on Flight Crew and Their Potential Response
Uncertainty in respect of track miles when being vectored, including being taken off a STAR for vectoring, creates uncertainty which crews may attempt to resolve by guessing rather than by a request to ATC for the information they need.
The consequences may raise crew workload close to safe limits and in the case of close-in reductions in track miles not obvious to the crew or specifically advised by ATC can result in an inability to establish a stabilised approach and lead to the approach being discontinued.
Misinformation on track miles prevents crews from managing their descent effectively and can where the actual track miles are less than advised can lead eventually to circumstances conducive to an unstabilised approach.
If radar vectors or shortcuts are provided, that bypass part of the planned route or the STAR track, the flight crew will need to accommodate the management of the descent profile to achieve a timely deceleration and configuration set-up. This may be challenging and add induced-pressure and workload to the flight crew.
The reduction of distance to fly, as in short cuts, imposes significant constraints on the aircraft’s optimal descent and deceleration profile.
It is worth remembering that distance to touchdown information enables pilots to manage their descent profile and also energy efficiency. This is especially important when there is a deviation from the STARs during vectoring and cleared shortcuts.
When computing the top-of-descent (TOD) point, the flight crew - and the FMS - assume a typical descent profile ( that is typically a 3 ° / 5 % / 300 ft/nm descent gradient) followed by the published STAR to the expected landing runway.
As an example, at a final FL 330 cruise altitude, the TOD will be located at 330 / 3 = 110 nm track-distance from the runway threshold.
Pilots expect accuracy of estimated distance from touchdown (DTD) to increase as distance to go reduces. Unexpected reduction in DTD can cause problems with energy management and an overly ‘ambitious’ offer might encourage some crews to ‘can-do’ themselves into an overload situation. Both these situations can easily effect in unstabilised approach. Crews usually expect to be advised of significant changes to DTD as soon as possible and, if tactical assessments change and speed control cannot resolve matters, the solution should be to increase distance rather than reduce it.
As per PANS-ATM (Doc 4444):
“126.96.36.199 b) when an aircraft is given its initial vector diverting it from a previously assigned route, the pilot shall be informed what the vector is to accomplish, and the limit of the vector shall be specified (e.g. to ... position, for ... approach).”
The phraseology for reporting DTD is:
188.8.131.52 Vectoring for ILS and other pilot-interpreted aids
b) YOU WILL INTERCEPT (radio aid or track) (distance) FROM (significant point or TOUCHDOWN).
Flight crew uncertainty about distance to touchdown affects the optimal profile management and can result in aircraft being high and fast on final approach.
ATC Options to Avoid the Action
- Always give distance to touchdown when vectoring an aircraft towards final approach.
- Be aware of what track miles are generally reasonable for height above aerodrome level for jet and for turboprop aircraft respectively.
- When corrective action is needed because an aircraft is too high in relation to estimated track miles, then it should be noted that turboprop aircraft are usually able to ‘go down and slow down’ at the same time more easily than jets because their normal descent forward speed is achieved with significant power even when in clean configuration.
- If more than a nominal reduction in planned distance to touchdown appears tactically advantageous within less than 15nm to go then this should be offered to the flight crew not simply instructed.
- Take care not to give wrong distance to touchdown estimates, especially underestimates and communicate any proposed or instructed changes promptly.
ATC Options to Manage the Consequences
- Ensure that the flight path of aircraft which accept distance to touchdown close to what are considered minimum are monitored carefully. If such heightened surveillance cannot be provided then do not offer ‘demanding’ distance to touchdown clearances.
- Correct any errors as soon as possible and offer additional track miles if this seems sensible or the crew request it.
- Recognise that a crew which reaches their limiting workload are likely to focus on the priority order ‘aviation, navigate, communicate’.
- Stabilised Approach
- DGAC (France) Publications on Non-Stabilised Approaches
- CFIT Precursors and Defences
- Continuous Descent Final Approach (CDFA)
- Non-stabilised Approach After ATC-Requested Runway Change (OGHFA SE)
- Runway Overrun After Unstabilised Approach (OGHFA SE)
- Runway Excursions - An ATC Perspective on Unstable Approaches
- Avoiding Unstable Approaches - Important Tips for ATCOs
- Unstable Approaches – ATC Considerations, January 2011