Description

Landing without clearance occurs when the pilot/flight crew of an arriving to land aircraft fails to obtain clearance to land from air traffic control (ATC) before landing their aircraft at a controlled aerodrome.

This article gives an in-depth description of the most common factors which may lead to landing without ATC clearance occurrences in line with identified patterns. It also provides an overview the most effective barriers against such occurrences and describes good practices that could be used by pilots/crews and air traffic controllers to prevent landing without clearance occurrences.

Hazards and Effects

The hazard(s) associated with and the possible 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 (landing surface) is the predominant concern. 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. 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:

1. The runway was not occupied and no runway clearance was granted to other traffic (safest scenario);
2. The runway was not occupied (yet) but a clearance had been issued to other traffic to either enter or cross the runway and the controller or the person in the other aircraft or vehicle had a chance to prevent the conflict;
3. The runway was occupied by another aircraft or a vehicle.

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 realising the lack of landing clearance, some have aborted the landing (including after touchdown) and initiated a go-around manoeuvre. These and other unbriefed/unexpected actions in some cases exacerbated risk factors or caused consequences such as a tail strike, loss of separation with another aircraft in the aerodrome traffic circuit. Moreover, the startle factor produced by the pilot’s or flight crew’s sudden recognition of the error (landing without clearance) 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).

Contributing factors

The factors listed below, either combined or on their own could "assist" the build-up of a situation where an aircraft (almost) lands without receiving an ATC clearance to do so:

• Failure to call as instructed - sometimes aircraft are transferred too early or too late (after landing) to the Tower frequency. At busy aerodromes, it is customary to instruct pilots to call again later (e.g. at 4 miles from touchdown, when overflying the outer marker or other radio aid, etc.) in order to obtain a landing clearance. The period of time may be long enough for the pilots to forget they had to make a call, especially given the higher workload situation. Another common failure scenario is the pilot/flight crew remaining on the approach frequency instead of switching to the Tower.
• Loss of Communication ;– e.g. due to equipment failure, 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.
• Expectation bias - it is a part of human nature to believe something expected has already happened. Pilots, especially under high workload, may easily assume that, among the other things said and done, the landing clearance has been received. The final approach is one of the busiest phases of flight. With so many things to do (especially in adverse weather conditions), it is possible that pilots forget to obtain a landing clearance. The Tower controller, on the other hand, may also be too busy (or may be waiting for a call from the aircraft being unable to see it) and fail to issue the clearance timely (or a go-around instruction, as appropriate).
• Absence of clearance – a landing clearance was not received but for various reasons the crew believed that they had or discovered too late that they had none (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.

Prevention Barriers

Prevention barriers are intended to avert the event which in this context is to reduce or eliminate the risk forof landing without clearance occurrences. The advice and guidance provided hereafter are derived from common sense, best practices and lessons learned from relevant studies and initiatives such as the Global Action Plan for the Prevention of Runway Incursions (GAPPRI). It is not intended to supersede or replace regulatory requirements, local instructions and procedures.

Procedures and best practice for pilots/crews:

• Use of triggers to check or acknowledge the receipt of a landing clearance:
  • An SOP to select the landing/taxi lights on only when a landing clearance is received (see GAPPRI AO25, AO26);
  • If feasible, 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;
• Ensure potential for early or late landing clearance as well as late handover from approach to tower frequency is included as part of the TEM Briefing. Consider effective mitigation strategies, e.g. early configuration and optimum sequencing of cockpit tasks during approach to relief the Pilot Monitoring (PM) from high workload and possible task saturation (see GAPPRI AO27). Consider nudging ATC for handover or issuance of landing clearance, if required.
• 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;
• Ensure pilot/crew positive visual identification of the correct runway;
• Ensure pilot/crews are always mentally prepared for a go-around, rejected or baulked landing. 

Procedures and best practice for controllers:

• Visual runway inspection before issuing a clearance for runway entery or crossing – this may reveal a (possibly unknown or forgotten) aircraft that is about to land without a clearance.
• Visual observation of all traffic to confirm all clearances are properly complied with. The probability that an aircraft (with a valid landing clearance) may attempt to land on a wrong runway or taxiway should always be considered.
• Maintenance of situational awareness is critical. Forgetting about an aircraft being on the final is a common reason for landing without clearance. Establishing and following procedures for strip handling (e.g. placing marks or moving the paper strips) can be a simple and effective solution;
• Coordination with adjacent units (especially approach units) is essential. If the situation permits, the Tower should be advised that an aircraft is about to call. If this is not possible due to congested traffic a procedure for electronic handover/takeover may be considered.
• Relying only on pilot reports in congested traffic situations is sometimes not advisable. The pilots may forget that they need to make a position report or they may not be able to do it due to the high frequency occupancy. In some cases, assistance from the approach could be helpful if the situation permits (e.g. the approach controller advises the Tower via hotline when an aircraft is over a specified position). Surveillance systems such as ASMGCS might also be a viable option;
• Timely aircraft handover to the tTower (e.g., not later than FAF/FAP) as workload and possible task saturation for the PM might lead to forgetting frequency switching or check-in with Ttower (see GAPPRI Annex 3 data findings).
• Strict adherence to standard phraseology and readback/hearback procedures is essential for preventing misunderstandings and detecting expectation bias. This is especially true for “expect landing clearance” instructions.
• The overall use of the phrase “expect landing clearance” might need to be considered. Generally, a pilot approaching an aerodrome is expecting a landing clearance anyway and will continue to expect it regardless of whether a specific instruction for this is received. It may be a good idea to use the phrase on special occasions only (e.g. if a training or an airworthiness flight is intending to perform a go-around or a touch-and-go). Avoid issuance of late landing clearance. Busy frequency and high workload may contribute to controller being unable or forgetting to issue the landing clearance.  Criteria should be decided locally (e.g., not below 1,000ft above ground level).

Controller Memory Aids

Controllers memory aids are used to assist the controller in maintaining situational awareness. These require manual input. The most common memory aids are:

• Flight strip positioning rules – placing flight strips in specific locations according to the clearances issued;
• Electronic or handwritten marks on the flight strips (e.g. a tick when a landing clearance is given);
• Switches – a simple on/off switch with a corresponding sign (e.g. “runway occupied”) might be useful for maintaining situational awareness and the controller remembering that a clearance has been given to the (approaching) aircraft.

Automated Tools and Systems

Automated tools and systems act as safety nets and do not require controller’s input to operate. Most of these systems (FAROS, RWSL, RAAS, EVS) are targeted at the flight crew and provide additional layers of protection against hazardous landings. Surveillance systems (e.g. SMR, A-SMGCS) and safety nets (e.g. RIMCAS) on the other hand provide the controller with means to enhance their situational awareness, in particular:

• In poor visibility conditions;
• If parts of the manoeuvring area are not visible from the tower due to obstructions;
• When an unknown aircraft that has not checked in on the frequency approaches the runway threshold (e.g. entering the CTR from uncontrolled airspace).

Also, surveillance systems can be very helpful in mitigating human error, e.g. in the case of:

• Aircraft not communicating with the Tower due to various reasons, e.g.:
  • Crew transferred from approach to tower too early or too late;
  • Crew tuning a wrong frequency;
  • Crew tuning the correct frequency but not checking in.
• Aircraft approaching the wrong runway due to loss of positional orientation;
• The controller forgetting about an aircraft on final.

Mitigation Barriers

Sometimes aircraft land without a clearance even though all ATC procedures are strictly followed and all memory aids are used correctly, causing a runway incursion. In such cases, mitigation barriers are intended to reduce the severity the consequences.

Two types of mitigation barriers can be distinguished: conflict detection barriers (the risk of conflict was identified early enough and proper action was taken) and conflict resolution barriers (the conflict could not be prevented but was detected and properly resolved).

The most effective mitigation barriers are:

• ATCO prevents the conflict after detecting it with system support before or with an intended runway entry clearance.
• Crew/driver prevents the conflict after detecting it, based on traffic monitoring (listening to R/T or visually), before or with an intended runway entry clearance. This barrier is relatively weak but offers the benefit of a “double” opportunity for conflict detection – visual and listening to the R/T.
• Runway conflict resolution by flight crew/driver after visual conflict detection. This barrier is relatively weak but offers the benefit of limited delay (no need of an ATC-crew communication loop) for crew action in case of conflict detection.

If the controller spots an aircraft that has (almost) touched down without having received a landing clearance, the following factors need to be considered:

• Instructing the flight crew to go around or even informing them that they have no valid landing clearance might worsen the situation. The crew (being startled by the fact) may execute an abrupt manoeuvre resulting ultimately in loss of control, tail strike, or runway excursion.
• A go-around instruction might sometimes be a safer option if there is imminent collision danger (e.g. there is another aircraft or vehicle on the runway and it would not be able to vacate in time).

Factors affecting Efficiency

As shown above, there are a number of safety barriers intended to reduce or mitigate the risk of landing without clearance. Their effectiveness often depends on local circumstances. The most notable factors that affect the safety barriers are:

• Surveillance coverage – the level of ATC surveillance coverage may differ. Radar guided approaches affect likelihood for detecting an aircraft bound for landing and the existing situation of loss of communication when the transponder Mode A is set to squawk 7600.
• Meteorological conditions affect the possibility of detecting potential threats in good time. The in-flight visibility and time of the day may be such as to allow pilots to:
  • Recognise potential threats in good time.
  • Prevent the recognition of potential threats in good time.
• Runway status – runways can be in use or not for traffic purposes and there may or may not be obstructions. The runway statuses that can influence the efficiency of barriers for the different scenarios are:
  • Active Runway;
  • Inactive Runway;
  • Closed Runway.
• Clearance conditions – variation in local procedures and practices for delivering a landing clearance. The way landing clearance is delivered at different airports may vary, including whether or not:
  • Multiple landing clearances are used;
  • Conditional landing clearances are issued;
  • There are specified minimum distances from the runway threshold (or landed/departing traffic) by which a landing clearance must be issued.
• Physical visibility (visual surveillance capability from the TWR) - the view (direct or using CCTV) of the relevant part of the aerodrome and its vicinity from the ATC Tower may be restricted by:
  • The location, height, design, equipment of the tower;
  • The airport layout or obstructions;
  • The temporary presence of aircraft or vehicles.
• 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.

Further Reading

EUROCONTROL

• Operational Safety Study: Landing without ATC clearance, Edition 1, May 2014
• Analysis of reported runway incursion serious incidents - European data sample of 2013 – 2015, Feb 2017
• Analysis of investigated runway incursion accidents and serious incidents - worldwide data sample of 2006 – 2016, April 2017
• Final Report to Flight Safety Foundation: Go-Around Decision-Making and Execution Project, Flight Safety Foundation, March 2017