Stabilised Approach

Description

Most airlines and other aviation organisations specify minimum acceptable criteria for the continuation of an approach to land. These vary in detail but the following summary published by the Flight Safety Foundation is one view of the important considerations.

Their Approach-and-landing Accident Reduction (ALAR) Briefing Note 7-1 suggests that "all flights must be stabilised by 1000 feet above airport elevation in IMC and 500 feet above airport elevation in VMC. An approach is stabilised when all of the following criteria are met:

  • The aircraft is on the correct flight path
  • Only small changes in heading/pitch are necessary to maintain the correct flight path
  • The airspeed is not more than VREF + 20kts indicated speed and not less than VREF
  • The aircraft is in the correct landing configuration
  • Sink rate is no greater than 1000 feet/minute; if an approach requires a sink rate greater than 1000 feet/minute a special briefing should be conducted
  • Power setting is appropriate for the aircraft configuration and is not below the minimum power for the approach as defined by the operating manual
  • All briefings and checklists have been conducted
  • Specific types of approach are stabilized if they also fulfil the following:
    • ILS approaches must be flown within one dot of the glide-slope and localizer
    • a Category II or III approach must be flown within the expanded localizer band
    • during a circling approach wings should be level on final when the aircraft reaches 300 feet above airport elevation; and,
  • Unique approach conditions or abnormal conditions requiring a deviation from the above elements of a stabilized approach require a special briefing.

An approach that becomes unstabilised below 1000 feet above airport elevation in IMC or 500 feet above airport elevation in VMC requires an immediate go-around."

Other applications of the Stabilised Approach principle used outside North America do not necessarily distinguish between VMC and IMC approaches, which makes it possible to track compliance using OFDM.

Some Operators also specify aircraft status at a 'should' gate ahead of the 'must' gate envisaged by the FSF system. This is typically 500 feet above the 'must' gate, for example a 'should' gate at 1000ft agl followed by a 'must' gate at 500ft agl. Failure to satisfy the former requires that corrective action is feasible and taken whereas failure to satisfy the latter requires a go around.

Effects

Continuation of an unstabilised approach to land may result in an aircraft arriving at the runway threshold too high, too fast, out of alignment with the runway centre-line, incorrectly configured or otherwise unprepared for landing. This can result in aircraft damage on touch-down, or runway excursion and consequent injury or damage to the aircraft or airfield installations.

Defences

The existence of an appropriate procedure which allows flight crew to determine whether an approach is sufficiently stabilised to allow it to be continued at specified 'gates' with strict observance confirmed by automated tracking using the Operator's Flight Data Monitoring (FDM) Programme. Note that if the Flight Safety Foundation recommendation that there should be different 'gates' for IMC and VMC is followed, then such tracking becomes impossible.

Typical Scenarios

  • An aircraft on approach to land is not stabilised after a late clearance to reduce speed. SOPs require the aircraft to go-around in the event of an unstabilised approach but the pilot continues the approach because of a desire to complete the flight on schedule, thus creating a signficant risk of consequential mishap affecting both the aircraft and its occupants.

Contributory Factors

  • Adverse weather (e.g. strong or gusty winds, wind shearturbulence).
  • ATC pressure to maximise number of movements (e.g. high approach speed).
  • Late change of runway.
  • Commercial pressure to maintain schedule.

Solutions

  • Strict compliance with the stabilised approach principle by pilots.
  • ATC awareness of factors within their control which can contribute to an unstabilised approach.

Accidents and Incidents Involving Unstabilised Approaches

On 28 January 2019, an Airbus A320 became unstabilised below 1000 feet when continuation of an ILS approach at Muscat with insufficient thrust resulted in increasing pitch which eventually triggered an automatic thrust intervention which facilitated completion of a normal landing. The Investigation found that having temporarily taken control from the First Officer due to failure to follow radar vectors to the ILS, the Captain had then handed control back with the First Officer unaware that the autothrust had been disconnected. The context for this was identified as a comprehensive failure to follow multiple operational procedures and practice meaningful CRM.

On 19 October 2015, an ATR 72-600 crew mishandled a landing at Ende and a minor runway excursion occurred and pitch control authority was split due to simultaneous contrary inputs by both pilots. A go around and diversion direct to the next scheduled stop at Komodo was made without further event. The Investigation noted that the necessarily visual approach at Ende had been unstable with a nosewheel-first bounced touchdown followed by another bounced touchdown partially off-runway. The First Officer was found to have provided unannounced assistance to the Captain when a high rate of descent developed just prior to the flare. 

On 3 May 2019, a Boeing 737-800 significantly overran the wet landing runway at Jacksonville Naval Air Station at night when braking action was less than expected and ended up in shallow tidal water. The Investigation found that although the approach involved had been unstabilised and made with a significant tailwind and with only a single thrust reverser available, these factors had not been the cause of the overrun which was entirely attributable to attempting to complete a landing after touching down on a wet runway during heavy rain in conditions which then led to viscous aquaplaning.

On 14 October 2019, a Sikorsky S92A manoeuvring below low cloud in poor daylight visibility in an unsuccessful attempt to locate the intended private landing site flew north towards rising ground approximately ¾ mile east of it, coming within a recorded 28 feet above terrain near to occupied houses before making an emergency climb and over-torqing the engines followed by an unstable but successful second approach. The Investigation found relevant operator procedures absent or ineffective, an intention by the management pilot in command to reach the landing site despite conditions and uncertainty about the applicable regulatory context for the flight. 

On 26 July 2017, a Hawker Beechcraft 850 left wing sustained extensive damage when it contacted the runway at Mykonos during a 2.7g touchdown after an unstabilised approach in benign weather conditions had been continued. The Investigation found that the aircraft was airworthy prior to a temporary loss of control at touchdown which occurred after stick pusher activation due to the airspeed being more than 20 knots below the applicable reference speed and only three knots above the applicable stall speed. The monitoring of the First Officer’s approach by the Captain was minimal and late with few alert calls given.

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Related Articles

Further Reading

DGAC (France) has published three documents in the English language related to non-stabilised approaches.

Flight Safety Foundation

Airbus Safety Library

CANSO

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Flight Data Services Case Study

IATA

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