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 4 October 2017, an Airbus A320 slightly overran the end of runway 22 at Surat during an early morning daylight landing. A temporarily displaced landing threshold meant the runway length was only 1,905 metres rather than the 2,905 metre full length. The aircraft remained on a paved surface and was undamaged. Its crew did not report the excursion which was only discovered when broken runway lighting was subsequently discovered. The Investigation found that the non-precision approach made was unstable and that a prolonged float in the subsequent flare meant that only 600 metres of runway remained ahead at touchdown.

On 2 September 2016, an ATR72-600 cleared to join the ILS for runway 28 at Dublin continued 800 feet below cleared altitude triggering an ATC safe altitude alert which then led to a go around from around 1000 feet when still over 5nm from the landing runway threshold. The Investigation attributed the event broadly to the Captain’s inadequate familiarity with this EFIS-equipped variant of the type after considerable experience on other older analogue-instrumented variants, noting that although the operator had provided simulator differences training, the -600 was not classified by the certification authority as a type variant.

On 13 September 2016, a Boeing 737-300 made an unstabilised approach to Wamena and shortly after an EGPWS ‘PULL UP’ warning due to the high rate of descent, a very hard landing resulted in collapse of the main landing gear, loss of directional control and a lateral runway excursion. The Investigation found that the approach had been carried out with both the cloudbase and visibility below the operator-specified minima and noted that the Captain had ignored a delayed go around suggestion from the First Officer because he was confident he could land safely as the two aircraft ahead had done.

On 29 January 2015, a Boeing 737-800 crew attempting to fly an NDB approach to Bergerac, with prior awareness that it would be necessary because of pre-notified ILS and DME unavailability, descended below 800 feet agl in IMC until an almost 1000 feet per minute descent when still over 8 nm from the runway threshold triggered an EGPWS ‘TERRAIN PULL UP’ warning and the simultaneous initiation of a go-around. The Investigation found that the PF First Officer was unfamiliar with NDB approaches but had not advised the Captain which resulted in confusion and loss of situational awareness by both pilots.

On 26 August 2019, an Airbus A320 attempted two autopilot-engaged non-precision approaches at Birmingham in good weather before a third one was successful. Both were commenced late and continued when unstable prior to eventual go-arounds, for one of which the aircraft was mis-configured causing an ‘Alpha Floor’ protection activation. A third non-precision approach was then completed without further event. The Investigation noted an almost identical event involving the same operator four months later, observing that all three discontinued approaches appeared to have originated in confusion arising from a slight difference between the procedures of the aircraft operator and AIP plates.

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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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