Unstabilised Approach: Late Runway or Approach Type Change
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|Category:||Toolkit for ATC - Stabilised Approach|
The change of instrument approach without adequate prior notification at any time after an aircraft has left the higher of cruise altitude or FL100 in descent to destination.
A change of landing runway which requires positioning onto another extended centreline below 2000 ft above aerodrome level after establishing visual contact with the initially-assigned landing runway.
Effect of Action on Flight Crew and Their Potential Response
The effect of a change of instrument approach during descent varies according to the circumstances but the necessary re-brief and FMS/radio set up requires time. Time will be increasingly difficult to find as the distance to go reduces.
If there is a change from a precision approach to a non precision one other than a change from a full Instrument Landing System (ILS) to a LLZ only to the same runway, then the change may be accompanied by an offer of delaying action to allow the crew to prepare for the new approach.
Visual manoeuvring to a closely aligned or parallel runway not below 1500 ft above aerodrome level by day in favourable weather conditions may be a realistic proposition for an experienced crew returning to their home base. For others, it may be an influence to an unstable approach even though the challenges are likely be aircraft handling rather than a lengthy re-brief and set up requirement.
Runway changes that result in more track miles remaining are generally easier to cope with as they provide additional time for the setup and briefing. With more track miles suddenly remaining the aircraft will be low on the ‘new’ profile which can be compensated for by reducing the rate of descent.
The most difficult runway change involves a change to a runway that results in less track miles remaining. As an example, a late change from 09 to 27 for an aircraft coming in from the east will probably result in a rushed setup and an abbreviated briefing and will, assuming the aircraft was on profile in the first place, suddenly put the aircraft high on the ‘new’ profile. In these instances additional track miles will probably be required.
The crew typically conducts approach briefing before reaching top-of descent. Changing runway, especially when reducing the distance to fly, induces significant crew workload.
It is worth highlighting that the sequence of aircraft deceleration and configuration set-up is entirely different for an ILS approach as opposed to a non-precision approach, as described below:
An ILS approach is usually flown at 3º and allows most aircraft to fly a large part of the approach on autopilot. It also requires relatively little manipulation of the autopilot during the approach; it is mainly speed reductions that take place to allow flaps and gear to be deployed.
For an ILS approach, after capturing the localizer in level flight, the aircraft is usually configured with approach flaps and gear up until reaching the G/S; when intercepting the G/S, the landing gear is selected down and the go-around altitude is selected on the FCU / MCP ; when the gear is down and locked, the flaps are selected to the landing configuration and the speed is reduced to VAPP; then, the landing checklist is accomplished and - time permitting - the main elements of the go-around and missed approach are rebriefed. The vertical navigation can be ensured by the autopilot with almost no intervention.
During an ILS approach the aircraft will continue to slow down and configure after becoming established on both the localiser and the glideslope. This means that the aircraft can maintain a relatively high speed until late in the approach. However, it is very difficult for a modern aircraft (A330 Family, B-737-800, etc) to descend on the glide slope and slow down simultaneously; therefore the controller must allow for deceleration to final approach speed when the aircraft is approaching the glide slope interception point. Flight crew requests for a slower airspeed during the final portion of the approach should be approved to the maximum extent possible.
For a non-precision approach ( e.g. a VOR-DME approach ), the aircraft may be required to reach the FAP / FAF in level flight, aligned with the final approach track and already fully configured and ready for landing ( i.e., with gear down, full flaps, VAPP, landing checklist and briefing complete ).
Non-precision approaches result in a much higher workload as in many cases, the crew must manually control rate of descent or altitude. Although it can be flown on autopilot, more manipulation of the autopilot is required as the crew still have to continuously adjust the rate of descent. The autopilot must usually be disconnected earlier compared to a precision approach although the decision point for visual transition (Minimum Descent Altitude) is higher and the soecifcation of te required visual reference is often slightly higher. As with ILS final approach tracks, non-precision approach tracks may also be offset from the runway which requires alignment with the runway cenreline during the visual segment below MDA. As these approaches are not usually flown or practiced as frequently as precision approaches, the level of crew proficiency may be less too.
Non-precision approach implies higher workload and less possibility for deceleration during the final approach compared with an ILS approach. When radar positioning is provided, controllers should aim to position the aircraft on the FAT at a greater distance than woukd be used for vectoring to the ILS.
ATC Options to Avoid the Action
- Keep the forecast and any actual trend in wind velocity under active review.
- Where landing runway is only advised at first crew contact, ensure that the runway advised is only subsequently changed exceptionally.
- Do not offer the option of a change of approach or landing runway just to achieve a reduction in ground delay reduction to aircraft below FL100. Aim to consider this aspect of the arrival when the approach runway is first advised.
ATC Options to Manage the Consequences
- Expect some crews to accept a change for which the necessary response may take them to the limit of their ability to operate safely.
- Monitor the flight path of any aircraft which accepts late changes with particular care so as to promptly detect any direct or indirect consequences of the additional crew workload.
- Non-stabilized Approach After ATC-Requested Runway Change (OGHFA SE)
- Runway Overrun After Unstabilised Approach (OGHFA SE)
- DGAC (France) Publications on Non-Stabilised Approaches
- CFIT Precursors and Defences
- Continuous Descent Final Approach (CDFA)
- Runway Excursions - An ATC Perspective on Unstable Approaches
- Avoiding Unstable Approaches - Important Tips for ATCOs
- Unstable Approaches – ATC Considerations, January 2011