Runway Undershoot
Runway Undershoot
Definition
An event occurring during an approach to landing that results in an inadvertent landing or contact with the ground or an object short of the runway or intended landing area. Normally undershoots occur due to misjudgment of distance, speed, and/or altitude on final approach. It does not include occurrences in which the aircraft could not have reached the intended landing area; i.e., after loss of engine power. The undershoot distance is described by the longitudinal distance by which the aircraft undershoots the intended runway approach end.
Source: ACRP Report 3: Analysis of Aircraft Overruns and Undershoots for Runway Safety Areas
Description
Runway undershoot is an event that happens when an aircraft touches down before reaching the runway threshold. Such events usually have much more kinetic energy compared to overruns and therefore greater potential for damage or injury. The most common scenario is caused by some form of power loss. In such situations, the likely pilot response is a nose up input. This normally results in loss of speed, which turns into increased loss of altitude and could result in terrain impact. The lower the aircraft, the less opportunity there is to trade altitude for airspeed.
It must be noted that situations where the runway is unreachable (due to e.g. fuel starvation/exhaustion, engine failure, etc.) are not considered undeshoots even if the aircraft impacts terrain shortly before the threshold.
Effects
Touching down short of the runway could result in various negative outcomes, such as:
- Casualties or injuries to crew and passengers, especially in situations where there is substantial damage to the fuselage or a fire has occurred.
- Aircraft damage. Depending on the circumstances (such as rate of descent, speed, obstacles, surface type and conditions etc.), this could vary from minor damage (such as deflated tyre) to a hull loss (due to e.g. uncontrollable roll after ground impact or touching down in water).
- Aerodrome damage. This could include damage to elements of the aerodrome lighting system (e.g. threshold lights) or the ILS (localiser antenna).
- Impact on aerodrome operations. Depending on the damage to the aircraft and the aerodrome it is possible that a runway could be rendered out of service for a prolonged period of time.
Causal Factors
The following is a (non-exclusive) list of causal factors for runway undershoot events:
- Unstable approaches can result in a number of undesired outcomes such as a hard landing, tail strike, runway excursion or runway undershoot.
- Attempt to land too close to the runway threshold, e.g. when the landing distance available is only slightly longer than the required.
- Continuing the approach below MDA/DA even though required visual reference is not obtained or is lost
- Human error, including mismanagement of the flight controls or loss of situational awareness due to e.g. a visual illusion or runway misidentification.
Contributory Factors
The following factors, while not normally being the leading cause, could aggravate the situation depending on the circumstances:
- Poor visibility due to fog, rain, snow, etc. leading to insufficient visual references (or lack thereof).
- Wind shear sometimes causes sudden loss of altitude. If this happens shortly before touchdown, the result could be an undershoot.
- Runway lights or VASI failure at night can cause confusion as to the vertical position of the aircraft.
- Non-standard runway dimensions may give appearance that runway is further away or closer.
- Displaced threshold. The crew might not be aware of the fact, e.g. in case of runway misidentification (the crew believes they are landing on a different runway with no displaced threshold) or because the runway threshold has been temporarily displaced and the crew missed this during the pre-flight preparation.
- High rate of descent can make it more difficult to adjust the point of touchdown or prevent the crew from successfully executing a go-around.
- Pressures (either real or perceived) could influence flight crew's decision making into continuing the approach even though it is not stabilised or the required visual reference is not available.
Defences
The following measures could either reduce the risk or mitigate the consequences of a runway undershoot:
- Flying a stabilised approach and executing a go-around in case of unstable approach or insufficient visual references
- Flying a precision final approach generally reduces the likelihood of an undershoot due to the vertical guidance.
- Appropriate pre-flight briefing could identify risk factors that could lead to an undershoot.
- Correct maintenance of the RESA could mitigate the effects of an undershoot. While the RESA is primarily designed to mitigate runway excirsions (where the speeds are usually lower), this surface has often enabled aircraft to reach the runway threshold without sustaining substantial damage in minor undershoots.
- Making the aerodrome lights frangible is a standard defined in ICAO Annex 14 and reduces the damage to aircraft.
Accidents and Incidents
On 12 January 2023, an Airbus A330-300 touched down at Amsterdam at night 11 metres short of the runway after the captain manually flew below the visual ILS glidepath indication once below minimum decision height. This happened without comment by the other two junior pilots. The undershoot was apparently not recognised by any of the pilots, and debris on the runway was only discovered two hours later. The crew were unfamiliar with the relatively short runway. It was concluded that having incorrectly perceived the overrun risk as greater than an undershoot, they had prioritised speed control over maintaining the glide path.
On 4 October 2013, a Boeing 747-200 touched down short of the intended landing runway at Sokoto after the Captain opted to reduce track miles by making a direct visual contact approach in dark night calm wind conditions rather than continuing as initially cleared towards an ILS approach in the reciprocal runway direction. The Investigation was hampered by an inoperative FDR and failure to preserve relevant CVR data on the grounded aircraft and concluded that the decision to make a visual approach rather than an ILS approach when the VASI was out of service for both runways was inappropriate.
On 5 August 2019, a Cessa 560XLS touched down in runway undershot at Aarhus whilst making a night ILS approach there and damage sustained when it collided with parts of the ILS LOC antenna caused a fuel leak which after injury-free evacuation of the occupants then ignited destroying most of the aircraft. The Investigation attributed the accident to the Captain’s decision to intentionally fly below the ILS glideslope in order to touch down at the threshold and to the disabling of the EGWPS alerting function in the presence of a steep authority gradient, procedural non-compliance and poor CRM.
On 29 November 2018, a Let 410 landed on a temporarily closed section of the runway at Dubrovnik after a visual approach in benign weather conditions. The Investigation found that the flight crew had not carried out a sufficient pre-flight review of current and available information about a major multi-phase runway reconstruction there which they were familiar with. The opportunity for better advance and real time communication with aircraft operators and their flight crew and the benefit of the recommended ‘X’ marking at the beginning of any temporarily closed part of a runway, omitted in this case, was noted.
On 9 May 2008, a Boeing 737-800 made a low go around at Perth in good daylight visibility after not approaching with regard to the temporarily displaced runway threshold. A second approach was similarly flown and, having observed a likely landing on the closed runway section, ATC instructed a go around. However, instead, the aircraft flew level at a low height over the closed runway section before eventually touching down just beyond the displaced threshold. The Investigation found that runway closure markings required in Australia were contrary to ICAO Recommendations and not conducive to easy recognition when on final approach.
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