This article is based on an operational safety study of one of the Top 5 Network Manager operational safety priorities for 2013 and 2014 - “Landing without ATC clearance”. The Study was conducted by the EUROCONTROL Safety Improvement Sub-Group (SISG) and it is intended to serve as a reference for the Network actors in case they undertake operational safety analysis and improvement activities for landing without clearance.
Despite checklists, recurrent training and other defences, a number of pilots and flight crews have failed to obtain clearance to land from air traffic control (ATC) before landing their aircraft at controlled aerodromes. Whatever the causal factors, these high-severity incidents create an inherently high risk of a runway incursion accident.
If such a landing occurs, but an accident does not result, some say that only ‘providence’ prevented the accident, i.e. that by coincidence no other aircraft, vehicles or people were in the path of the landing aircraft.
Evidence-based mitigations could reduce the risks of landings without clearance.
Typical Reasons and Scenarios
The most common reasons for landing without clearance are:
- Loss of Communication – e.g. due to equipment failure or mistuning the next (TWR) frequency. In some cases, e.g. VFR flights, the tower controller might not even be aware that an aircraft is approaching and is intending to land.
- Runway confusion – e.g. at aerodromes with more than one runway or if two separate aerodromes are close to each other.
- Communications misunderstanding – e.g. an instruction to continue the approach and/or to expect landing clearance might be interpreted by the crew as a clearance to land.
- Absence of clearance overlooked by the pilot(s) – a landing clearance was not received but for various reasons the crew believed that they had (e.g. they were used to receiving it several minutes before landing).
- Deliberate – the crew were aware that they had not received a landing clearance but after assessing the circumstances decided that the best (safest) course of action was to land.
Similar to other occurrences (e.g. loss of separation), the safety impact is also determined by the traffic situation. There are three main scenarios for landing without ATC clearance:
- The runway was not occupied and no clearance was given (safest scenario);
- The runway was not occupied (yet) but a clearance was given to either enter or cross the runway; the controller or the person in the other aircraft or vehicle had a chance to prevent the conflict;
- The runway was occupied by another aircraft or a vehicle.
Hazards and Effects
The hazards and effects of landing an aircraft on a runway without clearance are essentially the same as for a runway incursion. Increased risk of collision for one or more aircraft on the ground is the predominant concern. When a collision occurs on the runway, at least one of the aircraft involved often is moving at high speed, which increases the probability and severity of injury, fatalities and major aircraft damage. The aircraft landing without clearance also can be at extreme risk of colliding with one or more vehicles travelling along the runway or crossing the runway, or striking people or objects.
Investigations of accidents and serious incidents also reveal secondary risk factors for the flight crew or pilot of the landing aircraft. In the moments after realizing their lack of landing clearance, some have aborted the landing (including after touchdown) and initiated a go-around maneuver. These and other unbriefed/unexpected actions in some cases exacerbated risk factors or caused consequences such as a tail strike or the loss of separation with another aircraft in the aerodrome traffic circuit, EUROCONTROL has noted.
Moreover, the startle factor produced by the pilot’s or flight crew’s sudden realization of the error can contribute to loss of control in flight. Hesitation to respond, inappropriate flight control inputs or unsafe over-compensation for the error have occurred — potentially leading to a runway excursion or to controlled flight into terrain (CFIT).
The EUROCONTROL report on landing without clearance cites contributory factors from a relevant 1994 report by researchers at the U.S.National Aeronautics and Space Administration (NASA). Causal factors from 37 events reported to NASA’s Aviation Safety Reporting System (ASRS) included cases of pilots who attempted to communicate with ATC on the wrong frequency; pilots who did not realize they lacked the clearance to land until the aircraft was on the runway (or turning off the runway and changing to the ground control frequency; pilots who said high workload had contributed to failure to obtain landing clearance; and pilots more likely to forget to contact the tower controller for landing clearance if ATC instructed them to do so at a relatively long distance from the runway (i.e., at the outer marker or as far as 20 nm [27 km] from the threshold).
There are two sets of barriers to prevent an occurrence from developing into an accident. Prevention barriers are used to avert the event and mitigation barriers are intended to soften the situation when an undesirable event has happened.
The most common prevention barriers are:
- Triggers for pilots to check landing clearance:
- An SOP to select the landing/taxi lights on only when a landing clearance is received;
- The inclusion of an item in the landing check-list;
- An SOP to do so at an existing fixed point in an approach such as at a height-defined stabilisation gate.
- Introducing an SOP which requires that two-way contact is established on each new frequency before the pre-select frequency is changed;
- Procedures and/or visual aids to help correct runway identification by pilots:
- Making sure that the correct runway and/or approach procedure are entered into aircraft on-board systems and that the information is displayed to both pilots;
- Pilot positive visual identification of the correct runway (except in Cat 2/3 conditions);
- Signs and markings to clearly indicate closed runway.
- Procedures and/or visual aids to alert pilots and controllers of runway availability and landing clearance status:
- Provision of an automatic alert to the pilot when a runway is occupied (e.g. FAROS);
- Other means for controllers to alert pilot to the absence (or existence) of a landing clearance (e.g. the availability of a selectable visual alert illuminated close to touchdown);
- The availability of an effective controller memory aid to annunciate whether landing clearances have been issued or not;
- System supported ATCO detection of aircraft about to land without clearance or on the wrong runway with one (e.g. ASMGCS and RIMCAS).
- Controller visual detection:
- Of an aircraft about to land without clearance;
- Of an aircraft about to land on a runway other than the one for which clearance has been given.
- Specific go-around policy in case of pilot awareness of no landing clearance which is effectively monitored for compliance.
The barriers that may mitigate the consequences of landing without clearance can be combined in two groups:
- Conflict detection (the risk of conflict was identified early enough and proper action was taken):
- Visually by the controller;
- With a system support tool by the controller;
- Visually or while monitoring the audio channel by the flight crew/vehicle driver.
- Conflict resolution (the conflict could not be prevented but was detected and properly resolved):
- By the controller after visual detection;
- By the controller with the help of a surveillance system;
- By the controller after detecting the risk with a safety net (e.g. RIMCAS);
- By the flight crew/vehicle driver after visual detection;
- By the flight crew/vehicle driver after detecting it while monitoring the frequency;
- By the flight crew/ground crew after detecting it with system support.
Factors that May Affect Barrier Effectiveness
Each aerodrome has its own specific set of features that may affect the prevention and mitigation barriers (some of these may also vary with time):
- Radar guided approaches (affect the possibility of communication loss being detected when transponder code 7600 is used);
- Meteorological conditions (in-flight/ground visibility affect the pilots’/controllers’ ability to detect potential threats in good time);
- Runway status (active, inactive, closed);
- Clearance conditions (multiple clearances, conditional clearances, procedures for issuing landing clearances);
- Physical visibility (control tower position, aerodrome layout, temporary obstacles).
- Final Approach Runway Occupancy Signal (FAROS) is an FAA-sponsored concept, which is now being deployed for operational evaluation in the USA. The concept is to inform the approaching aircraft (using PAPI or VASIS) that the runway is occupied.
- Runway Status Lights (RWSL) is developed and deployed at major airports in the USA. It is a fully automatic advisory safety system which provides direct alerts to both vehicles and pilots independently of the normal traffic control system operated by ATC. When RWSL indications contradict clearances, pilots and vehicle drivers are expected to prioritize response to the status lights. The absence of RWSL indications however does not equate to an ATC clearance to proceed.
- Advanced Surface Movement Guidance and Control System (A-SMGCS) is a modular system consisting of different functionalities to support the safe, orderly and expeditious movement of aircraft and vehicles on aerodromes under all circumstances. Higher levels of implementation deliver safety nets, conflict detection and resolution as well as planning and guidance information for pilots and controllers.
- Runway Awareness and Advisory System (RAAS) is designed to improve flight crew situational awareness, thereby reducing the risks of runway incursion, runway confusion and runway excursions. It uses airport data stored in the EGPWS database, coupled with GPS and other on-board sensors, to monitor the movement of an aircraft around the airport. It also provides audio or visual advisories and warnings depending on the aircraft position.
- Enhanced Vision System (EVS) is a technology which incorporates information from aircraft based sensors (e.g., near-infrared cameras, millimetre wave radar) to provide vision in limited visibility environments. Safety is enhanced, especially during approach and landing in limited visibility. Obstacles such as terrain, structures, and vehicles or other aircraft on the runway that might not otherwise be seen are clearly visible on the IR image.