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.
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
A320, Khartoum Sudan, 2005 - On 11 March 2005, an Airbus A321-200 operated by British Mediterranean Airways, executed two unstable approaches below applicable minima in a dust storm to land in Khartoum Airport, Sudan. The crew were attempting a third approach when they received information from ATC that visibility was below the minimum required for the approach and they decided to divert to Port Sudan where the A320 landed without further incident.
B737, Fort Nelson BC Canada, 2012 - On 9 January 2012, a Boeing 737-700 being operated by Enerjet on a passenger charter flight from Fort St. John to Fort Nelson overran the dry landing runway 03 at destination by 70 metres after an unstabilised visual approach had been flown in day Visual Meteorological Conditions (VMC). None of the 118 occupants were injured and there was no damage to the aircraft.
CRJ2, Barcelona Spain, 2011 - On 16 July 2012, a Bombardier CRJ200 being operated by Air Nostrum on a scheduled passenger flight from Badajoz to Barcelona with the First Officer designated as PF for the sector accepted a visual approach at destination after the offer of this by ATC due to adverse weather affecting the instrument approach procedure. The visual approach subsequently became unstabilised but was continued with a high rate of descent to a landing which was sufficiently hard to be likely to have caused structural damage to the aircraft.
D328, Mannheim Germany, 2008 - On 19 March 2008, a Dornier 328-100 being operated by Cirrus AL on a scheduled domestic passenger flight from Berlin Tempelhof to Mannheim made a long landing on the dry surface of runway 27 at destination in benign daylight weather conditions following a non precision approach before leaving the paved surface and colliding with an earth bank about 50 metres beyond the end of it.
GLF4, Teterboro NJ USA, 2010 - On 1 October 2010, a Gulfstream G-IV being operated by General Aviation Flying Service as ‘Meridian Air Charter’ on a corporate flight from Toronto International to Teterboro made a deep landing on 1833m-long runway 06 at destination in normal day visibility and overran the end of the runway at a speed of 40 kt to 50 kt before coming to a stop 30m into a 122m long EMAS installation. The aircraft suffered only minor damage and none of the 10 occupants were injured.