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.
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.
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.
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.
- Adverse weather (e.g. strong or gusty winds, wind shear, turbulence).
- ATC pressure to maximise number of movements (e.g. high approach speed).
- Late change of runway.
- Commercial pressure to maintain schedule.
- 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 27 January 2020, an MD83 made an unstabilised tailwind non-precision approach to Mahshahr with a consistently excessive rate of descent and corresponding EGPWS Warnings followed by a very late nose-gear-first touchdown. It then overran the runway end, continued through the airport perimeter fence and crossed over a ditch before coming to a stop partly blocking a busy main road. The aircraft sustained substantial damage and was subsequently declared a hull loss but all occupants completed an emergency evacuation uninjured. The accident was attributed to the actions of the Captain which included not following multiple standard operating procedures.
On 28 April 2018, a Dassault Falcon F900B came into close proximity with a Beech B36T Bonanza at the uncontrolled VFR-only aerodrome at Bremgarten during its tailwind approach to runway 23 made without contacting the designated Flight Information frequency as the other aircraft was on approach to runway-in-use 05 and in contact with Flight Information. The Beech pilot took avoiding action by turning north and climbing in order to avoid a collision. The Falcon 900 crew had not prepared for the approach which was then unstabilised with late gear extension and multiple EGPWS ‘SINK RATE’ warnings annunciated.
On 15 January 2023, an ATR 72-500 positioning visually for an approach to Pokhara was observed to suddenly depart normal flight and impact terrain a few seconds later. All 72 occupants were killed and the aircraft destroyed by impact. A Preliminary Report published by the Accident Investigation Commission has indicated that a stall warning and subsequent loss of control was preceded by an apparently unintentional and subsequently undetected selection of both propellers to feather in response to a call for Flaps 30. The Training Captain in command was supervising the Captain flying during familiarisation training for the new Pokhara airport.
On 29 November 2017, a Boeing 737-900 on an ILS approach at Atlanta became unstable after the autothrottle and autopilot were both disconnected and was erroneously aligned with an occupied taxiway parallel to the intended landing runway. A go-around was not commenced until the aircraft was 50 feet above the ground after which it passed low over another aircraft on the taxiway. The Investigation found that the Captain had not called for a go around until well below the Decision Altitude and had then failed to promptly take control when the First Officer was slow to begin climbing the aircraft.
On 1 January 2020, an Airbus A350-900 made an unstabilised night ILS approach to Frankfurt in good visual conditions, descending prematurely and coming within 668 feet of terrain when 6nm from the intended landing runway before climbing to position for another approach. A loss of situational awareness was attributed to a combination of waypoint input errors, inappropriate autoflight management and communication and cooperation deficiencies amongst the operating and augmenting flight crew on the flight deck.
DGAC (France) has published three documents in the English language related to non-stabilised approaches.
Flight Safety Foundation
Airbus Safety Library
Flight Data Services Case Study