Visual illusions when landing in heavy rain
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Visual illusions when landing in heavy rain
Considerable international research has been carried out over the past 40 years into the visual illusionary effects experienced by pilots. This has included studies of visual errors induced by heavy rain on aircraft windscreens.
The research conclusions describe three effects of rain on windscreens. Firstly, a blurring of vision. Secondly, a prismatic refraction by the water pooling on the windscreen in a thicker layer at the top as the result of wind effect. This causes an apparent downward displacement of objects.
Finally – a light dispersion that causes perception of the visible horizon to appear as a gradient blur. The last two effects act in the same direction and cause the `horizon` to appear depressed as much as 1:2. This tends to cause a pilot to believe that he is higher than he is, and therefore descend the aircraft too low.
In November 1978 a DC-9-31 VH-TJP, landed 192 metres short of the threshold of Runway 27 at Melbourne Airport. The aircraft was conducting an ILS approach to Runway 27 in conditions of reduced visibility caused by heavy rain. During the ground roll to the threshold, the main landing gear of the aircraft struck and destroyed six lights in the high intensity approach lighting system serving the runway.
The aircraft was operating a RPT flight and there were ninety-one passengers and a crew of six aboard. No one was injured and the damage sustained by the aircraft was minor.
Read full report: https://www.atsb.gov.au/media/24850/197802603.pdf
The report indicates the pilot used excessive speed on final with the expectation of windshear caused by a front passing though the airport. Touchdown was 171 knots. Vref for the flap configuration was 132 knots.
On short final, visibility ahead in heavy rain was poor. Despite operation of the wipers, the rain on the windscreen blurred the pilots' vision and both stated the approach lights were badly diffused. Neither could see the runway ahead and they concentrated their attention on scanning for the row of green lights marking the threshold.
At an estimated 100 ft AGL, still without visual contact of the threshold lights, the captain stated he sensed the aircraft sinking rapidly. He attempted to counter this by pulling back on the control column, but did not increase power. Just before impact the First Officer said "You are going to hit the lights." Both pilots felt the landing gear strike the ground and the approach lighting. During the last 20 seconds of flight the rate of descent increased and averaged 1100 feet per minute
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This was a classic case of both pilots going heads up while in IMC in expectation of seeing the runway environment and thus excessive airspeed and rate of descent going un-corrected. Worse still, the presence of heavy rain on the windscreen meant both pilots forward vision could have been simultaneously affected by visual illusions of being higher than they actually were. When these weather conditions are experienced it is wise to have one pilot stay heads down on instruments until the other has positive sighting of the runway.
When CVR's were first installed in Australian airline aircraft, the Australian Federation of Air Pilots (AFAP) insisted that the information they contained should not be used in he investigation of air safety incidents or of any accident which the flight crew survived. In the interests of having this source of information available on other occasions, the then Department of Civil Aviation agreed to this demand for the time being. Consequently, the evidence in the cockpit audio record of VH-TJP was not available for use in this investigation.
The research conclusions describe three effects of rain on windscreens. Firstly, a blurring of vision. Secondly, a prismatic refraction by the water pooling on the windscreen in a thicker layer at the top as the result of wind effect. This causes an apparent downward displacement of objects.
Finally – a light dispersion that causes perception of the visible horizon to appear as a gradient blur. The last two effects act in the same direction and cause the `horizon` to appear depressed as much as 1:2. This tends to cause a pilot to believe that he is higher than he is, and therefore descend the aircraft too low.
In November 1978 a DC-9-31 VH-TJP, landed 192 metres short of the threshold of Runway 27 at Melbourne Airport. The aircraft was conducting an ILS approach to Runway 27 in conditions of reduced visibility caused by heavy rain. During the ground roll to the threshold, the main landing gear of the aircraft struck and destroyed six lights in the high intensity approach lighting system serving the runway.
The aircraft was operating a RPT flight and there were ninety-one passengers and a crew of six aboard. No one was injured and the damage sustained by the aircraft was minor.
Read full report: https://www.atsb.gov.au/media/24850/197802603.pdf
The report indicates the pilot used excessive speed on final with the expectation of windshear caused by a front passing though the airport. Touchdown was 171 knots. Vref for the flap configuration was 132 knots.
On short final, visibility ahead in heavy rain was poor. Despite operation of the wipers, the rain on the windscreen blurred the pilots' vision and both stated the approach lights were badly diffused. Neither could see the runway ahead and they concentrated their attention on scanning for the row of green lights marking the threshold.
At an estimated 100 ft AGL, still without visual contact of the threshold lights, the captain stated he sensed the aircraft sinking rapidly. He attempted to counter this by pulling back on the control column, but did not increase power. Just before impact the First Officer said "You are going to hit the lights." Both pilots felt the landing gear strike the ground and the approach lighting. During the last 20 seconds of flight the rate of descent increased and averaged 1100 feet per minute
............................................................ .......................................
This was a classic case of both pilots going heads up while in IMC in expectation of seeing the runway environment and thus excessive airspeed and rate of descent going un-corrected. Worse still, the presence of heavy rain on the windscreen meant both pilots forward vision could have been simultaneously affected by visual illusions of being higher than they actually were. When these weather conditions are experienced it is wise to have one pilot stay heads down on instruments until the other has positive sighting of the runway.
When CVR's were first installed in Australian airline aircraft, the Australian Federation of Air Pilots (AFAP) insisted that the information they contained should not be used in he investigation of air safety incidents or of any accident which the flight crew survived. In the interests of having this source of information available on other occasions, the then Department of Civil Aviation agreed to this demand for the time being. Consequently, the evidence in the cockpit audio record of VH-TJP was not available for use in this investigation.
Last edited by Centaurus; 13th Aug 2017 at 15:35.
