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Night Visual Approaches
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|Category:||Controlled Flight Into Terrain|
|Content source:||Flight Safety Foundation|
A visual approach flown during hours of darkness.
Although a visual approach is the first type of approach taught to student pilots, it has its own inherent risks and liabilities. Careful consideration should be given to all pertinent factors before flying a visual approach in preference to an instrument procedure, especially at an unfamiliar airport. The Flight Safety Foundation (FSF) Approach-and-landing Accident Reduction (ALAR) Briefing Note 7.4 makes the following recommendation:
"Accepting an air traffic control (ATC) clearance for a visual approach or requesting a visual approach should be balanced carefully against the following:
- Ceiling and visibility conditions;
- Darkness (light levels);
- Wind, turbulence;
- Rain or snow; and/or,
- Fog or smoke;
- Crew experience with airport and airport environment:
- Surrounding terrain; and/or,
- Specific airport and runway hazards (obstructions, etc.); and,
- Runway visual aids:
The specific focus of this article is on the risks and illusions associated with visual approaches at night.
While man-made obstacles in the vicinity of an airport such as buildings or towers are normally lit during the hours of darkness, natural obstacles such as hills or trees are not. As a consequence, unless there is exceptional illumination such as a full moon on new snow, natural obstacles will be largely invisible to the pilot during a night visual approach. Without due care, this factor greatly increases the potential of a CFIT accident. In fact, numerous CFIT accidents have occurred during visual approaches during hours of darkness.
A visual illusion known as "black hole effect" is another inherent risk of night visual approaches. Black hole conditions exist on dark nights (usually with no moon or starlight), when there are no ground lights between your aircraft and the runway threshold. The black hole illusion, sometimes called the featureless terrain illusion, fools pilots into thinking they are higher than they actually are, causing them to fly dangerously low approaches. Perception scientists disagree as to the exact cause of this illusion and probably no single theory fully explains the phenomenon as there are many factors involved. The most extensive study was conducted by Boeing researchers after a series of airline black hole accidents in the 1960’s. Using a flight simulator, experienced Boeing instructor pilots (with more than 10,000 hours each) conducted entirely visual approaches to runways in black hole conditions. The result was that without the aid of altimeter or glide slope information, most pilots flew excessively low approaches and crashed into terrain short of the runway.
Another significant night visual approach risk stems from the way we interpret visual cues. Consider the runway light illustrations below:
Which of the following statements would you suggest best describes the illustrations?
- panel B represents the sight picture for a normal approach to the runway, panel A shows the sight picture for an aircraft below optimum flight path and panel C the sight picture of an aircraft higher than the optimum flight path; or
- panel A is the sight picture for an aircraft on a standard glide path to a down-sloping runway, panel B for an aircraft on a standard glide path to a level runway and panel C for an aircraft on a standard glide path to an up-sloping runway; or
- panel B is the sight picture for an aircraft on approach to a standard width runway, panel A to a wider than standard runway and panel C to a narrower than standard runway.
In fact, any of the above choices could be correct. The issue at hand is that the sight picture of the runway lights alone is not always sufficient to ensure a safe visual approach path to the runway. Additional information or further visual cues can be required.
Unless the pilot is intimately familiar with the airfield, thorough pre-approach study and preparation is required. The list of potential questions is almost endless but the pilot must concern himself with the following issues before commencing the approach:
- What is the terrain like approaching and surrounding the airfield?
- Is there a published circuit altitude and is that above or below the minimum circuit height dictated by the Company Ops Manual?
- When can I safely begin a descent to circuit altitude based on any terrain or obstruction issues in the area surrounding the airport
- When joining the circuit, are there any published limitations to circuit direction (i.e., RH circuits only to runway XX)
- Is there a published maximum distance that I can extend my downwind leg before turning towards the runway (i.e., terrain penetrates a 3 degree glideslope 6nm from the threshold of runway YY)
- What runway lighting is available (is it ground or pilot controlled)
- Are there approach lights
- Is there visual approach path guidance (PAPI or VASIS)
- Is the runway standard width or is it wider/narrower than what I am used to
- Is the runway level or is there a slope
- If the runway is not level, is it up sloping or down sloping
- Are there navigation aids (such as DME) that I can use to help determine the position the aircraft
The illusions and inherent risks of night visual approaches can be made even worse in reduced visibility due to low cloud, mist or precipitation. The basic risk in poor visibility is that objects will appear to be further away than they actually are. Even in conditions of near perfect visibility, there are still risks. The bright lights surrounding a “city” airport could easily mask the obstruction warning lights on any mast or tower in the vicinity. Vigilance during night approaches, irrespective of the conditions, is therefore critical.
On a clear night, lights can be seen from a long distance and it is quite difficult to judge distance without the use of specific landmarks or reliance on electronic aids. Commencing descent for a straight in approach based solely on the runway lights might therefore occur prematurely and result in descent below a safe altitude thereby increasing the risk of a CFIT accident.
Pre-approach preparation and planning is the key to safe and successful night visual approaches.
Use all information and tools available to you. For example, a 3 degree flight path for all intents is 300’ descent per nautical mile. Therefore, from a 1500’ circuit altitude, the descent at 3 degrees will take approximately 5 track miles. From a 1000’ circuit altitude, the descent will take just over 3 miles. Similarly (and assuming still air conditions), at 60 KIAS, the descent rate associated with a 3 degree flight path will be 300 FPM, at 90 KIAS, 450 FPM, at 120 KIAS 600 FPM and so forth. If the rate of descent required to maintain the anticipated visual sight picture is greatly different from the computed value, you may be significantly closer to or further away from the runway than you thought.
When in doubt or unstable, go around and rejoin.
If there is any doubt about the ability to safely conduct a visual approach at a specific airport, revert to an instrument procedure if qualified to do so.
If not instrument qualified, divert.
- Controlled Flight Into Terrain
- Visual Illusions
- Visual Approach
- Accident and Serious Incident Reports: CFIT
CFIT Accidents & Incidents involving VFR flight at night
- EC55, en-route, Hong Kong China, 2003 (HF CFIT) (On 26 August 2003, at night, a Eurocopter EC155, operated by Hong Kong Government Flight Service (GFS), performing a casualty evacuation mission (casevac), impacted the elevated terrain in Tung Chung Gap near Hong Kong International airport.)