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 13 November 2020, a Boeing 727-100 configured for cargo operations veered partially off the landing runway at Kigali after a late touchdown on a wet runway before regaining it approximately 1,000 metres later. The Investigation concluded that viscous hydroplaning after touchdown which occurred a significant distance left of the runway centreline had been contributory but absence of a prior go-around was causal. It was also found that the flight crew licences were invalid and that there were significant discrepancies in respect of the aircraft registration, the status of the operator and the validity of the Air Operator Certificate.
On 12 September 2020, an Airbus A318 was seriously mismanaged during a largely autopilot-controlled ILS glideslope capture from above and despite being unstabilised after the crew had intentionally ignored required approach management procedures, the flight was continued without hesitation to a landing. The Investigation found that the operator’s oversight of operating standards relating to unstabilised approaches was systemically flawed and also insufficiently supportive of their ‘Evidence Based Training’ method used for pilot training. It was also noted that the Captain involved had stated to the Investigation that “he considered this flight as a non event”.
On 16 May 2013, a DHC6-300 on a domestic passenger flight made a tailwind touchdown at excessive speed in the opposite direction of the of 740 metre-long runway to the notified direction in use and, after departing the runway to one side during deceleration, re-entered the runway and attempted to take off. This failed and the aircraft breached the perimeter fence and fell into a river. The Investigation identified inappropriate actions of the aircraft commander in respect of both the initial landing and his response to the subsequent runway excursion and also cited the absence of effective CRM.
On 8 February 2019, a Piper PA46-350P overran the landing runway at Courchevel and collided with a mound of snow which caused significant damage to the aircraft but only one minor injury to a passenger. The Investigation noted the Captain's low level of experience but the investigation effort was primarily focused on the risk which had resulted from a commercial air transport flight being conducted without complying with the appropriate regulatory requirements for such flights and without either the passengers involved or the State Safety Regulator being aware of this.
On 2 May 2016, a Boeing 737-800 veered off the 2,500 metre-long landing runway near its end at speed following a night non-precision approach flown by the Captain. It then stopped on grass having sustained damage to both the left engine and landing gear. The Investigation noted that a significant but allowable tailwind component had been present at touchdown and found that the approach had been unstable, the approach and touchdown speeds excessive and that touchdown had occurred beyond the touchdown zone after applicable operating procedures had been comprehensively ignored in the presence of a steep authority and experience gradient.
On 7 August 2020, a Boeing 737-800 making its second attempt to land at Calicut off a night ILS approach with a significant tailwind component became unstabilised and touched down approximately half way down the 2,700 metre-long wet table top runway and departed the end of it at 85 knots before continuing through the RESA and a fence and then dropping sharply onto a road. This caused the fuselage to separate into three pieces with 97 of the 190 occupants including both pilots being fatally or seriously injured and 34 others sustaining minor injuries. Significant fuel spillage occurred but there was no fire.
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, Indonesia, 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.
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