Rain on Planes

Heavy rain carries a host of dangers, including illusions that could lead to landing short


Very seldom does instrument training adequately prepare a pilot for flying in actual weather. While an instrument rating gives you the ability to fly in the clouds, the wisdom of knowing what to avoid is more difficult to earn. One weather situation that seldom comes up during training but that greatly increases the risk for the pilot is heavy rain.

The biggest risks of flying in heavy rain are the associated windshears, downdrafts and visual illusions. The problem can be particularly difficult during approach, where you have limited ability to divert unless you abandon the approach entirely.

Heavy rain showers, especially close to decision height, can be very distracting and further complicate a stabilized approach. Compounding the problem is that some approach navigation signals can be degraded when close to decision height, reducing their accuracy. Furthermore, heavy rain can produce visual illusions that distort the pilots perception of the runway location and distance.

Whether flying an airliner or a light aircraft, the effects are just as dramatic and just as serious. Airline crews are exposed to some of these illusions during simulator training. Established procedures regarding the use of instrument aids, visual approach light systems and cross-checking are used by the airlines to help prevent such accidents.

Its hard to determine just how often these factors have caused accidents in light aircraft, but the more-complete documentation of airline crashes has identified that the problem does exist. Each of the following airline accidents shows the visual illusions associated with IFR approaches in heavy rain.

In 1973, a Delta Air Lines DC-9 was cleared for an ILS approach to Chattanoogas runway 20 at 18:42:51. The special surface weather observation at 18:45 reported ceilings to be 400 feet broken, 1,100 feet overcast, visibility 2 miles, heavy rain showers, wind 160 at five knots, thunderstorm southeast moving east.

The crew joined the localizer and flew the ILS approach while performing normal call-outs and procedures according to FAA and Delta procedures.

At 18:50:05, the first officer, who was not flying, reported, I got the lights. The captain looked out of the aircraft and verified that the approach and runway lights were in sight and then returned to monitoring his instruments and the autopilot operation.

At 18:50:41, the captain ordered the windshield wipers be turned to the fast speed. At 18:51:26, the first officer reported, One hundred above minimums, and 2.5 seconds later the airplane reached the middle marker. The first officer reported, I got plus five, sinking to nine. (Translation: five knots above bug speed, sinking at 900 feet per minute.) Seconds later: Plus five sinking to 10.

A rapidly decreasing headwind, combined with a coupled ILS, caused an increase in airspeed, which required a nose-down pitch change to keep the aircraft on the glideslope. In addition, the glideslope signal induced a downward trend near the middle marker because the signal beam tended toward the lower tolerance limit.

The crew had been briefed on the restriction on the use of the glideslope and was prepared to fly the aircraft using visual cues from decision height to landing.

Nevertheless, when the pilot disconnected the autopilot the aircraft was established on a descent path that would result in touchdown short of the runway. At 18:51:42, the sounds of impact were recorded. The aircraft struck the approach lights 1,600 feet from the runway threshold and was destroyed.

Visual Illusion
The NTSB concluded that the captain disregarded the report of the first officer, possibly because of the influence of a visual illusion caused by the refraction of light through the heavy rain on the windshield. The excessive rate of descent, initiated by a wind shear condition and a glideslope that tended toward the lower signal limit, could not be arrested in time.

The crew of a Learjet 24 that landed on runway 20 six minutes before the accident stated, there must be a wind shear because were experiencing gusty conditions. Heavy rain was falling while the Delta jet approached the airport, with the National Weather Service reporting a rate of about 1.2 inches per hour.

Its clear that the combination of factors stacked up against this experienced crew. They werent the first and wont be the last to be overcome by this combination of risk factors.

While approaching Raleigh, N.C., in 1975, an Eastern Airlines Boeing 727 was flying through light to moderate rain on an ILS approach. The flight was uneventful until it approached the Raleigh-Durham area, where it made several deviations to circumnavigate heavy precipitation southwest of the airport.

The flight crew made the necessary altitude awareness calls and instrument cross-checks at 1,000 feet and at 500 feet. At 20:01:37, at 500 feet, the first officer reported ground contact. At 20:01:55, the first officer reported the runway in sight. The crew said that the approach lights, threshold lights, and runway lights were well defined and easily seen, without noticeable halo effect or backscatter.

About five seconds after reporting the runway in sight, the first officer called, VASI. Look a little bit low. About four seconds later he announced, Rate of descent too high, but later did not recall seeing a rate of descent greater than 700 feet per minute. The captain increased the thrust.

At 200 feet they encountered heavy rain. The flight engineer said his forward visibility went to nil and the crew lost all forward visibility as the windshield became opaque in heavy rain below decision height. At 100 feet, the captain described the situation as encountering a wall of water and as having the bottom fall out as he added thrust.

The high noise level of the rain striking the aircraft impeded the captains ability to understand the first officers alerting call. The captain said the intense rain, the loss of outside visibility, the increased thrust and the aircrafts contact with the ground occurred almost simultaneously. The flight data recorder showed descent rates as high as 1,400 fpm during the last 3.6 seconds of the flight. The aircraft struck the ground 282 feet short of the runway and slid to a stop.

The weather over the approach end of runway 23 deteriorated rapidly as the airplane progressed down the approach path for landing. The recorded rate of rainfall from 20:00 to 20:05 was more than two inches per hour.

The NTSB concluded that, as it descended below 200 feet, the airplane encountered heavy rain that is usually associated with a downdraft and a slight horizontal wind shear. But any horizontal wind shear was small in magnitude since only a two-degree heading correction to the left was needed to maintain the localizer and no significant airspeed changes occurred.

The NTSB further stated that it is unlikely that the aircraft actually encountered a large-scale downdraft. The actual increase in the rate of descent of about 500 fpm was shown on the aircraft FDR readout slightly before impact and simultaneously with the heavy rain encounter.

The board determined that when the Boeing entered the heavy rain it encountered downdraft and windshear activity that adversely affected the captains efforts to maintain a proper descent profile during the last portion of the final approach. But it also said that, because of the airspeed margin and thrust available, the forces exerted by these meteorological activities could have been overcome had the captain taken immediate action to recover.

The Visual Transition
Yet another accident shows how the effects of heavy rain can be dangerous even to experienced pilots.

A Pan Am flight in 1974 was on approach to Pago Pago in American Samoa. The runway was in sight when the aircraft was about eight miles from the threshold. The runway remained in sight during the entire approach, though a rainshower was at the far end of the runway and moving toward the landing threshold as the crew was executing the approach.

The captain was flying and did not intercept the glideslope smoothly. At the 6 DME fix, the aircraft was about 260 feet below the glideslope. The aircraft leveled off at about 1,750 feet, reintercepted the glideslope and followed a flight path that was roughly 100 feet above the glideslope.

As the aircraft approached 1,400 feet, the airspeed swung from 160 to 188 knots, with numerous power changes. At about 700 feet, 23 seconds before impact, the captain apparently went visual to complete the landing. Within three seconds of the captain going visual, the first officer said, Youre a little high. The rate of descent then increased from 690 fpm to 1,470 fpm and continued at that rate until impact.

The VASI was illuminated and operating satisfactorily at the time of the approach, though the first officer did not remember seeing the VASI lights at the time of the approach.

The NTSB determined the probable cause of the accident was the failure of the pilot to correct an excessive rate of descent after the aircraft had passed decision height.

The NTSB said the flight crew did not monitor adequately the flight instruments after they transitioned to the visual portion of the ILS approach. They did not detect the increased rate of descent and lack of crew coordination resulted in inadequate altitude callouts, inadequate instrument cross-checks and inadequate procedures monitoring. In addition, the board said visual illusions may have caused the crew to perceive incorrectly their altitude above the ground and their distance to the airport.

Eye, Eye, Eye
Of the many factors that can lead to visual illusions, there are several that could have caused the pilots to believe that they were higher than normal during the final approach. These include haloing and runway foreshortening. Numerous studies on the effects of the visual illusion phenomena have established they contribute to disorientation and faulty horizontal distance judgment by pilots.

A common factor in these accidents is the apparent misperception of the runway location. A pilot may be deceived because of illusions caused by the refraction of light through water on the windshield.

Numerous studies conducted on the effects of this phenomenon have established that faulty visual perception contributes to disorientation and erroneous judgment of horizontal and vertical distance. Windshields must be FAA-certified and are allowed to exhibit only a limited amount of refraction. However, certification requirements apply to a dry windshield.

The windshields on the Boeing 727 are relatively flat plates, thus having fewer distortion problems than the curving windshields found on most general aviation aircraft. Thus, any amount of yawing or crabbing on the approach can further distort the runway image in the average light aircraft.

You can see this on the ground by looking at a row of hangars or having a person walk from one side to the other. Youll notice how the image will distort in different spots. The same thing happens to the runways image during approach. While the distortions are kept to a minimum because of the FAAs certification standards, some amount of distortion still exists.

The most serious problem associated with water on the windshield is that the objects appear farther away than they actually are. The water on the windshield, the thickness of the windshield and the rain between the aircraft and the runway causes a refraction of the pilots line of sight to the runway in a downward direction. This bending of the light rays would cause the approach and runway lights to appear lower than their actual elevation. The pilot would believe that he is higher and farther away from his planned touchdown point than he actually is.

Rain can also affect the pilots perception of distance to the approach and runway lights by diffusing their glow (haloing) and thus causing them to appear less intense. This would lead the pilot to conclude that the lights were farther away than they actually were. On occasion, rain causes lights to appear larger (but not brighter) and the pilot believes he is closer than he actually is.

Another illusion, runway foreshortening, could have had an effect on the Pan Am crew. The heavy rain moving slowly down the runway toward the approach end, would have caused the physical dimensions of the runway to appear to decrease, thereby leading the pilots to believe that they were high on the approach.

Preconceived Notions
Pilots form mental impressions of the runway environment during the approach and use that memory on subsequent trips. These impressions are based on the visual appearance of the airport environment, light patterns, the apparent length of the runway and its projected shape.

The pattern of runway lights in the windshield and its relation to the other cockpit structures provides cues the pilot relates to heading, attitude and altitude. From these cues the pilot determines when the aircraft is positioned properly during an approach. Peripheral light cues from ground lights aid in filling in this mental impression of a correct approach. The loss of these peripheral cues because of rain-restricted visibility hinders the pilots ability to assess correctly the aircrafts position relative to the runway.

Pilots may believe that the hardest part of an instrument approach is tracking the approach course and vertical profile solely with reference to the instruments, and that the landing is assured as soon as the runway comes into sight. The truth is that the transition from the approach on instruments to the visual landing environment is so fraught with pitfalls that it has always been one of the most mistake-prone portions of the approach.

Procedures at the airlines require pilots to use precision approach aids (glideslope and localizer, if available) even during daylight visual conditions. Pilots are encouraged to choose a runway that is served by an ILS. Usually runways with precision approaches are also equipped with a VASI, further aiding a pilot during the final portion of the descent to landing.

If the runway is served by a non-precision approach without any vertical guidance, you can tell if you are close to a three-degree glide path by cross-checking your altitude over the approach lights. Your altitude should be about 200 feet above the landing zone when crossing over the approach lights.

Airline crews are also required to continue monitoring their instruments. Limits are placed on vertical velocity, engine spool-up, configuration and how much airspeed can vary. Deviations beyond a certain amount are grounds for immediately executing a missed approach. The single pilot in a light aircraft may want to think about doing the same. Unfortunately, a single pilot doesnt have the luxury of a copilot to back him up.

The illusions and weather phenomena associated with heavy rain are the biggest hazards that come with a downpour, but pilots should also beware of potential problems with runway contamination, engine flame-out due to water ingestion and aerodynamic degradation. Well have more on those issues another time.

A wonderful flight instructor teaching me how to fly a taildragger told me that the flight isnt done until youve got the taildragger tied down. The same thing applies to landing in the rain.

Even though you may have been knocked around by the turbulence and icing in the clouds, maybe given tight vectors by ATC to further contribute to your high workload, the truth is that when you break out on the approach, the risks can be just as high if a storms moving in.

Also With This Article
Click here to view “Seven reasons the A doesnt always belong in LLWAS.”

-by Patrick Veillette

Patrick Veillette is an ATP with more than 15,000 hours in airplanes from Cubs to tankers to Boeing jets.


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