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Visual References

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Article Information
Category: Controlled Flight Into Terrain Controlled Flight Into Terrain
Content source: Flight Safety Foundation Flight Safety Foundation (FSF)
Content control: EUROCONTROL EUROCONTROL

Description

The phrase 'Required Visual Reference' is used in relation to the transition from control of an aircraft by reference to flight deck instrumentation to control by reference to external visual references alone. Those visual references, including aids, should have been in view for sufficient time for the pilot to have made an assessment of the aircraft position and rate of change of position in relation to the desired flight path. In Category III operations with a decision height the required visual reference is that specified for the particular procedure and operation. (ICAO Annex 6, and PANS-ATM).

The establishment of visual references at the completion of an instrument approach is an important process which determines whether the approach may be continued to landing, or a go-around must be flown.

Note: the vertical or slant view of the ground through broken clouds or fog patches does not constitute an adequate visual reference to conduct a visual approach or to continue an approach below the applicable MDA/H or DA/H.

The section below headed "European Regulations" details what these visual references must be. The remainder of this article deals with the process of transition within the aircraft cockpit.

According to Flight Safety Foundation (FSF) Approach-and-landing Accident Reduction (ALAR) Briefing Note 7.3 — Visual References , "The transition from instrument references to external visual references is an important element of any type of instrument approach."

The briefing note points out that two common Task task-sharing philosophies are common:

  • "Pilot flying-pilot not flying (PF-PNF) task-sharing with differences about the acquisition of visual references, depending on the type of approach and on the use of automation:
    • Nonprecision and Category (CAT) I instrument landing system (ILS) approaches; or,
    • CAT II/CAT III ILS approaches (the captain usually is the PF, and only an automatic approach and landing is considered); and,
  • "Captain-first officer (CAPT-FO) task-sharing, which usually is referred to as a shared approach, monitored approach or delegated-handling approach.

"Differences in the philosophies include:

  • The transition to flying by visual references; and,
  • Using and monitoring the autopilot."

"The task-sharing for the acquisition of visual references and for the monitoring of the flight path and aircraft systems varies, depending on:

  • The type of approach; and,
  • The level of automation being used:
    • Hand-flying (using the Flight Director [FD]); or,
    • Autopilot (AP) monitoring (single or dual AP)."

The briefing note than proceeds to discuss task sharing and other considerations for different types of approach.

European Regulations

AMC1 to IR-OPS CAT.OP.MPA.305(e) and Appendix 1 to EU-OPS 1.430 define the required visual references for continuion of a precision approach or a non-precision approach as follows:

Non-Precision Approach A pilot may not continue an approach below MDA/H unless at least one of the following visual references for the intended runway is distinctly visible and identifiable to the pilot:

(i) Elements of the approach light system;
(ii) The threshold;
(iii) The threshold markings;
(iv) The threshold lights;
(v) The threshold identification lights;
(vi) The visual glide slope indicator;
(vii) The touchdown zone or touchdown zone markings;
(viii) The touchdown zone lights;
(ix) Runway edge lights; or
(x) Other visual references accepted by the Authority.

Precision Approach A pilot may not continue an approach below the Category I decision height ... unless at least one of the following visual references for the intended runway is distinctly visible and identifiable to the pilot:

(i) Elements of the approach light system;
(ii) The threshold;
(iii) The threshold markings;
(iv) The threshold lights;
(v) The threshold identification lights;
(vi) The visual glide slope indicator;
(vii) The touchdown zone or touchdown zone markings;
(viii) The touchdown zone lights; or
(ix) Runway edge lights.

Category II Operations A pilot may not continue an approach below the Category II decision height ... unless visual reference containing a segment of at least 3 consecutive lights being the centre line of the approach lights, or touchdown zone lights, or runway centre line lights, or runway edge lights, or a combination of these is attained and can be maintained. This visual reference must include a lateral element of the ground pattern, i.e. an approach lighting crossbar or the landing threshold or a barette of the touchdown zone lighting.

Category IIIA Operations For Category IIIA operations, and for Category IIIB operations with failpassive flight control systems, a pilot may not continue an approach below the decision height ... unless a visual reference containing a segment of at least 3 consecutive lights being the centreline of the approach lights, or touchdown zone lights, or runway centreline lights, or runway edge lights, or a combination of these is attained and can be maintained.

Category IIIB Operations For Category IIIB operations with fail-operational flight control systems using a decision height a pilot may not continue an approach below the Decision Height ... unless a visual reference containing at least one centreline light is attained and can be maintained.

Accidents and Incidents

The following events on SKYbrary involve lack of visual reference as a factor:

  • MD83, vicinity Paris Orly France, 1997 (On 23 November 1997, a McDonald Douglas MD 83 being operated by AOM French Airlines on a scheduled passenger flight from Marseille to Paris Orly made an unintended premature descent almost to terrain impact at 4nm from the destination runway in day IMC before a go around was commenced. A subsequent approach was uneventful and a normal landing ensued. There was no damage to the aircraft or injury to the occupants.)
  • A320, Khartoum Sudan, 2005 (On 11 March 2005, an Airbus A321-200 operated by British Mediterranean Airways, executed two unstable approaches below applicable minima in a dust storm to land in Khartoum Airport, Sudan. The crew were attempting a third approach when they received information from ATC that visibility was below the minimum required for the approach and they decided to divert to Port Sudan where the A320 landed without further incident.)
  • B733, vicinity Chambery France, 2010 (On 7 February 2010, a Boeing 737-300 being operated by Jet2 on a scheduled passenger flight from Leeds/Bradford UK to Chambery France was making an ILS approach to runway 18 at destination in day IMC when a Mode 2 EGPWS ‘Terrain, Pull Up’ Warning occurred. A climb was immediately initiated to VMC on top and a second ILS approach was then made uneventfully. Despite extreme pitch during the early stages of the pull up climb, none if the 109 occupants, all secured for landing, were injured.)
  • B738, Mildura VIC Australia, 2013 (On 18 June 2013, a Boeing 737-800 crew en route to Adelaide encountered un-forecast below-minima weather conditions on arrival there and decided to divert to their designated alternate, Mildura, approximately 220nm away where both the weather report and forecast were much better. However, on arrival there, an un-forecast rapid deterioration to thick fog had occurred with insufficient fuel to go anywhere else. The only available approach was flown, but despite exceeding the minimum altitude by 260 feet, no visual reference was obtained. A further approach with the reported overcast 100 feet agl and visibility 200 metres was continued to a landing.)
  • A320, vicinity Bahrain Airport, Kingdom of Bahrain, 2000 (On 23 August 2000, a Gulf Air Airbus A320 flew at speed into the sea during an intended dark night go around at Bahrain and all 143 occupants were killed. It was subsequently concluded that, although a number of factors created the scenario in which the accident could occur, the most plausible explanation for both the descent and the failure to recover from it was the focus on the airspeed indication at the expense of the ADI and the effect of somatogravic illusion on the recently promoted Captain which went unchallenged by his low-experience First Officer.)

Related Articles

Further Reading

  • ICAO Doc 4444: PANS-ATM;

Flight Safety Foundation

The Flight Safety Foundation ALAR Toolkit provides useful training information and guides to best practice. Copies of the FSF ALAR Toolkit may be ordered from the Flight Safety Foundation ALAR website