AIR OPS Definitions
Low visibility procedures (LVP) means procedures applied at an aerodrome for the purpose of ensuring safe operations during lower than standard category I, other than standard category II, category II and III approaches and low visibility take-offs. (IR-OPS Annex I)
Low visibility take-off (LVTO) means a take-off with a Runway Visual Range (RVR) lower than 400 m but not less than 75 m. (IR-OPS Annex I)
Note that ICAO requires LVP for all departures below 550m RVR, not just LVTO.
Low visibility procedures exist to support Low Visibility Operations at Aerodromes when either surface visibility is sufficiently low to prejudice safe ground movement without additional procedural controls or the prevailing cloud base is sufficiently low to preclude pilots obtaining the required visual reference to continue to landing at the equivalent of an ILS Cat 1 DH/DA. It should be noted that in the latter case, surface visibility may be relatively good but the TWR visual control room may be in cloud/fog.
Low visibility on a Runway
On aerodromes where the ground marking and lighting is adequate, ground traffic at reasonable flow rates can often be sustained safely in reduced visibility. An aeroplane on the ground is at its most vulnerable during the landing and the take-off phases of flight when the options for avoiding action, if an obstruction is encountered, may be very limited. The aircraft is likely to be badly damaged or destroyed if it collides, at high speed, with any sizeable object.
Making the necessary transition to visual reference during the final stages of an approach to land in poor visibility is critical and certain requirements must be met to reduce the risk of a Runway Excursion. Low visibility take off also requires careful attention to correct runway alignment before the take-off is commenced; an ILS LLZ signal can be used for verification if available. If a rejected take-off (RTO) is carried out, pilots must maintain awareness of runway length remaining using whatever external visual cues are available; relevant runway lighting, signage or markings may be available.
As visibility deteriorates, the potential for runway incursions by aircraft, vehicles or personnel increases. The risk of inadvertent runway incursion by taxiing aircraft is greatest at aerodromes with complex layouts and multiple runway access points. This risk can only be managed adequately by the application of procedures that provide the pilot with clear, unambiguous guidance on routing and holding points or ground traffic patterns. Airport ground charts which show hotspots arising from frequent or opposite traffic, pilot reports of confusion or bad signage may improve pilots and controllers situational awareness.
The safe operation of airside vehicles depends upon drivers being adequately trained and thoroughly familiar with the aerodrome layout in all visibility conditions and on their compliance with procedures, signs, signals and ATC instructions. In low visibility conditions, extreme vigilance is required and special procedures, including restrictions on normal access, may be invoked. All of this is an essential product of the Airport Operator Safety Management System (SMS).
Controller ability to detect manoeuvring area conflicts may be reduced in poor visibility conditions, as the controllers may be unable to visually confirm whether their clearances are properly complied with.
Aerodromes that wish to continue operating in poor visibility or are available for instrument approaches in conditions of low cloud are required to develop and maintain LVPs.
Aerodromes that provide precision instrument approaches which provide guidance below ILS Cat 1 or equivalent DA/DH are required to have additional procedures in place that ensure the protection of signals transmitted by the ground based radio equipment that is used for the approach.
The point at which LVPs are implemented may vary from one aerodrome to another depending on local conditions and facilities available. It will usually be determined by a specific RVR or cloud base measurement. Typically an RVR below 550 metres or a cloud base below 200 ft aal will trigger LVPs.
Adequate consideration should be given to the time taken to implement fully all of the measures required to protect operations in low visibility conditions. Provision should also be made for alerting airlines, and other organisations with movement area access, in good time prior to the introduction of LVPs. This is particularly important where companies exercise control over their own apron areas and maintenance facilities adjacent to the manoeuvring area.
Regulatory authorities offer guidelines in respect of LVP implementation and suspension. A typical example is found in UK CAP 168: Licensing of Aerodromes, Appendix 2B, which contains information on the subject. ICAO European Region guidance material on LVPs is available in ICAO EUR Doc 013 "European Guidance Material on All Weather Operations at Aerodromes".
Low Visibility Operations may only be conducted under strict conditions, which are described fully in IR-OPS Subpart E Low Visibility Operations (LVO) and associated Acceptable Means of Compliance and Guidance Material, and EU-OPS 1.440 - EU-OPS 1.460 and relevant appendices. Essentially these concern the following main areas:
Taxi-out for departure and taxi-in after arrival in low visibility conditions is one of the most demanding phases of all-weather operations. The following good practices should be considered for inclusion in the SOPs:
- A thorough and good briefing for the taxi-out or taxi-in phase (route) is extremely important; the brief of the taxi pattern should use headings and turn directions for better orientation;
- No paperwork whatsoever shall be done during taxi-out or taxi-in, all checks should be done at a standstill;
- All flight crew members must have the taxi chart available during all ground operations during LVP;
- If there is any doubt about the position of the aircraft whilst taxiing before take-off or after landing, the flight crew should stop the aircraft and inform ATC immediately; early verbal intervention or even taking over control for braking may be needed by the pilot monitoring (PM) to prevent a safety relevant event.
- ATC should be asked for guidance in standard English phraseology. ATC can then immediately give the necessary urgent instructions to aircraft about to depart or land; to discontinue take-off or approach as applicable, before taxiing assistance and guidance is offered to the ‘lost’ crew;
- Lights can be helpful to make the aircraft visible to others;
- Never cross a lit red stop bar;
- The runway should be confirmed by both pilots before any take-off (a/c heading upon entering the runway must match the painted numbers on the runway);
- When rejected take-off is carried out the crew must maintain awareness of the runway length remaining using whatever external visual cues are available (relevant runway lighting, signage or markings, remaining runway indication on the Head Up Display and should bear in mind that:
- The aircraft is/may not by visible to the tower controller;
- Not all airports have Surface Movement Radar;
- It is important to inform the ATC tower once the rejected take-off is completed.
Installing Runway Awareness and Advisory System (RAAS) on the aircraft improves situational awareness both on the ground and when airborne.