Relations between pilot’s final authority and TWR’s responsibilities in conditions of poor or impaired visibility

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Relations between pilot’s final authority and TWR’s responsibilities in conditions of poor or impaired visibility

26TH ANNUAL CONFERENCE, Nairobi, Kenya, 27-30 April 1987

WP No. 82

Relations Between Pilot’s Final Authority and TWR’s Responsibilities in Conditions of Poor or Impaired Visibility


Did we all forget the Tenerife crash (March 1977) At that time many legal considerations were made and Lengthy discussions took place about the possibilities of aircraft manoeuvres on Aerodromes affected by poor visibility conditions, the Aerodrome facilities, and, mostly the relations between the final authority of the piIot-in-command and of the ATC unit (TWR) were examined. Possible conflicts and the lack of adequate international rules resulted.


It has to be noted that, as far as visibility is concerned, no international rules define minima below which ground operations are prohibited. ICAO has not decided if such minima shall be established by the State authority, by the State where the aircraft is registered or by the concerned airline.

Annex 6 Part I chapter 4 para 4.1.3, only state that, “ subject to their published conditions of use, aerodromes and their facilities shall be kept continuously available for flight operations during their published hours of operations, irrespective of weather conditions “. Apparently States are not obliged to issue any minima, either for domestic or for international flights, although a number of States do. Additionally, the majority of airlines have published their own specific minima.

Aerodrome facilities, the technical service and the runway’s structure are essential to classify the Aerodromes and to establish limitations of use, thus the possibility of ground operations and the lack of facilities (such as ASMI radar), extremely useful but not mandatory, has no legal consequence, since it is up to the pilot-in-command to take decisions based on operational conditions and clearances received. Anyway the most interesting problem is represented by the interrelation between the pilot’s authority (concerning the final decision) and the requirement for him to comply with the instructions received from ATC.

ICAO Annex 2, para. states that “an aircraft operated on a controlled aerodrome shall not taxi on the manoeuvring area without clearance from the aerodrome control tower and shall comply with any instructions given by that unit”. Correct information, and its correct use, plus the effectiveness of separation applied may be essential for the flight safety. Only the pilot-in- commend has the final authority, but there are circumstances where his decisions are based only on information not directly observed by him. In other words there are cases where the pilot-in- command, although having the “final authority”, must rely only on information received from ATC.

From a legal point of view a clearance based on a traffic information, unknown to the pilot but essential, cannot be considered as a “permissive act” (which doesn’t relieve the pilot of his responsibilities). Bearing this in mind, let’s examine how a TWR can perform its duties.

Aerodrome control towers shall issue information and clearances to aircraft under their control to achieve a safe, orderly and expeditious flow of air traffic on, and in the vicinity of, an aerodrome with the object of preventing collision(s) between:

(a)  aircraft flying in the aerodrome traffic circuits around an aerodrome;

(b)  aircraft operating on the manoeuvring area;

(c)  aircraft landing and taking off;

(d)  aircraft and vehicles operating on the manoeuvring area;

(e)  aircraft on the manoeuvring area and obstructions on that area.

(ICAO DOC 4444. RAC 501/10 Part V-1)

The techniques used by control towers are not subject to any regulations, either ‘local or international. ICAO DOC 4444, Part V1 does not specify the means to use to keep control; with reference to the visibility conditions. That’s why it’s generally considered that the aerodrome control is based on visual observation. The aerodrome control TWR and pilots-in-command should never loose the visual contact. At any moment the TWR should be able to verify the traffic situation and the pilot to assure the compliance with the principle ”to see and be seen”. Both are Iiable for preventing collisions.

What happens when visibility is poor or impaired? “Quid Juris”? Apart from the obvious suspension of VFR operations (in case of meteorological conditions deteriorating), IFR operations will continue, based only, on radio communications. In absence of any visual reference the pilot shall only comply with “clearances” received. ATC has to continue providing separation (also when sight is permanently obstructed): So, who is responsible and what means and criteria shall the TWR use?

ICAO rules do not include the use of the radar or procedural separation minima, to control traffic on the ground. They only remind ATC, for taxi procedures, “to issue concise instructions and adequate information to the pilot” (ICAO DOC 4444, RAC 501/10 Part V, para 4.1 also says “Aerodrome controllers shall maintain a continuous watch on all visible flight operations on and In the vicinity of an aerodrome, including aircraft, vehicles and personnel on the manoeuvring area, and shall control such traffic in accordance with the procedures set forth herein and applicable traffic rules.

If there are other aerodromes within a control zone, traffic at all aerodromes within such a zone shall be co-ordinated so that traffic circuits do not conflict”. What happens when the operations are not ‘visible’ ? IFR traffic taxiing shall be separated only by means of radio communications, of a correct compliance with the instructions received, of an exact identification of all moving vehicles, of a correct execution of the pre-flight duties. But the exact identification of the position of aircraft flying in the aerodrome circuit, of aircraft and vehicles and obstacles on the manoeuvring area, when the visibility is poor or impaired, is difficult both for the TWR and the pilot-in-command. Pilots may also be unfamiliar with that aerodrome.


The relation between the pilot’s “final authority” and the Aerodrome TWR’s authority in conditions of poor or affected visibility is not well defined by international rules. Many States, where possible, have found their own solutions to the problem.

IFATCA should endeavour to study problem, from the technical (SC1) and the legal (SC7) point of view, stressing the possible extension of the legal liability of TWR Controllers.


It is recommended that SC7 collect all relevant information concerning the liability of Aerodrome controllers with regard to flight operations in conditions of poor or restricted visibility.

Last Update: September 20, 2020  

December 1, 2019   640   Jean-Francois Lepage    1987    

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