LESSON 4: Aircraft Accidents caused by Winter Weather Hazards
|Welcome again to the fourth,
and final, lesson of the National Weather Association's "Winter Weather
and Flying" internet course in the public domain. In this lesson we
will look at how winter weather hazards have created unsafe situations for
Before we study individual accidents, please link to a special lesson on the impact of very cold weather on aircraft altimeters. There are significant deviations from the expected altitude clearances from very cold air. You need to know about it.
Cold Weather Altimeter Errors.
Note: If the Cold Weather Altimeter Errors is too big for your monitor, then you can either view a small version of the presentation (click here for smaller version) or set the size of your display to a larger width and height -- at least 1024 by 768.
And now on to aircraft accidents...
|Terry Lankford, author of numerous books on aviation and co-chairman of the NWA Aviation Weather Committee, contributed these insights from his experience studying aircraft accidents.|
Structural icing affects aircraft in three ways: flight and engine instruments, induction systems, and structural icing. An erroneous assumption is that icing only affects the IFR pilot. This is not true. Icing affects the VFR pilot as well as the IFR pilot. In fact, based on total numbers, VFR pilots are involved in more icing related accidents than IFR pilots.
Instrument icing has caused jet air carrier as well as general aviation accidents. A Boeing 727 was lost due to an iced over pitot tube. As static pressure decreased during the climb, the airspeed indicator showed speed increasing. The autopilot attempted to hold airspeed by increasing pitch, resulting in a stall. A Boeing 737 crashed because of an iced over engine power sensor—the airplane was simply not developing takeoff power, even though the instrument indicated so.
Structural icing accidents accounted for only about 40% of total accidents involving icing. The majority of icing accidents are attributed to carburetor or induction system icing, with less than 10% involving icy runways.
Here are three accident reports from the USAF's Flight Safety Center.
Cold- weather can bring serious hazards for the aviator who is not paying attention. Let’s look at one mishap that occurred that shows how the environment brings unexpected problems. This mission was a night, local training sortie flown in the local traffic pattern. During the planning and preflight phase of the mission the crew received a weather briefing which included information on the snowfall that had occurred that day.
Airfield operations concentrated their snow removal efforts on clearing the main runway in accordance with the airfield operations plan. Only the main runway was completely cleaned.
The aircrew set out on the night mission and after competing some transition work at another local airfield, they came back to the home field to accomplish additional training. After completing an approach and landing to the main runway, the crew set out to accomplish some short field landing training and flew an approach to the main runway with a circle approach to land on the cross runway. Because of men and equipment on the runway the flight crew could only accomplish a restricted low approach to about 500 ft AGL, on this attempt. Undaunted the crew again flew an approach to the main runway and circled to land on the cross runway. This time, the approach and landing to the cross-runway went off without a hitch. The mishap aircrew and their aircraft were able to handle the snow of the crossing runway without a problem. However, the snow removal operations on the main runway had left a snow berm across the runway they were using. That was a problem. With the lack of light due to nightfall, the aircrew did not see the mound of snow across their path until it was to late to take any evasive action. The aircraft hit the berm and received damage to the landing gear and underside of the aircraft.
How about another AF accident?
This mishap illustrates the point that cold weather provides unexpected hazards during all phases of airfield operations. The aircrew must be ready to deal with these hazards anytime and anywhere. Everyone thought they were doing their job, but some details were left out of the equation and maybe the comfort of flying in the local environment took its toll. The result of this was damage to an aircraft and the temporary loss of combat capability.
This mission was a return to home station with a departure from an overseas location. The aircrew received a weather briefing at the departure base which showed marginal conditions at home station. They complied with all applicable instructions for filing to a destination with marginal weather. Enroute they updated their weather information. They also got an update once they arrived in the local area of their intended landing field.
Conditions at the airfield had deteriorated to the point the aircrew could not fly an approach. Upon checking the alternate airfield weather, the crew decided to enter holding to see if the home weather would improve enough to allow them to fly an approach. While in the holding pattern they reviewed applicable instructions for landing the aircraft in adverse weather conditions. When the weather at the home station improved, they began an instrument approach to land. The aircraft broke out of the weather in time to allow the crew to land.
However, when the wheels touched down on the partially plowed runway, the aircraft began to hydroplane. The aircrew tried to correct back to the runway’s centerline but was unable. The mishap aircraft slid off the runway and damaged the landing gear and parts of the underside.
Once again, we see an example of the increased hazards that winter weather brings to aviation. This aircrew was at its home airdrome and familiar with local flying procedures. However, the weather required more from them then they expected.
Remember, you can always try to get a current runway condition report before you land.
How about the third and final accident for today.
Our final mishap was a combat support sortie for a transport aircraft. The aircrew received the preflight weather and the briefing form did not forecast icing for the enroute stop. About 45 minutes out from the destination, the crew received a PIREP from another transport aircrew that had left the enroute stop about an hour prior. Weather was reported as a solid deck from 300 ft to 15,000 ft and no report of encounters with icing or turbulence. Upon arrival at the enroute stop, the aircrew was required to fly a radar pattern, right in the heart of the solid deck, to complete the approach. It was a moonless night and the environment where they were flying did not permit the use of outside aircraft lighting. The aircraft ice detection systems indicated an icing condition but the automatic de-icing system turned on. Soon after a short period of time the detection systems indicated an ice-free situation. This indication cycled on and off which is normal for the environment the crew was flying in. When the pilot selected maximum reverse after landing, 3 out of 4 engines shut down. Severe clear ice had built up on the engine fuel control sensors, and when maximum reverse was selected the fuel sensor did not give the engine enough fuel to maintain its operation. This clear icing built up went undetected by the aircrew and its detection systems. This is a (no pun intended) clear example of understanding environmental weather conditions that are conducive to icing and thinking about what preventive actions to take when the aircraft enters this environment.
| It is
now my privilege to introduce the President of the NWA, Mr. John McLaughlin.
He is a broadcast meteorologist and fellow aviator.
Greetings fellow aviation and weather enthusiasts!
On behalf of the nearly 3000 members of the National Weather Association, I thank you for taking the time and effort to increase your knowledge of winter weather and the hazards the cold season can pose to flight operations.
Whether you fly for a living, or just enjoy taking a rental aircraft around the patch in VFR conditions, I trust the information contained in the winter flying course has been valuable to you. As an active commercial-rated pilot and flight instructor, I can assure you that while summertime convection may get all the attention, it is winter weather that can really sneak up on you and turn a routine flight into a situation that even multi-thousand hour aviators struggle to handle. One of my still vivid memories from flight training more than twenty years ago is asking another pilot about a aircraft dangling by the tail in a cluster of tall pine trees off the end of the runway. A small amount of snow on the runway and aircraft, coupled with some poor pilot judgment, resulted in what I saw.
I commend the NWA Aviation Weather Committee for their first class effort in assembling the training material, and a special thanks to all of the people and agencies providing time and resources for the winter flying course. Without their assistance, this course would not have been possible.
If you have not browsed the thunderstorm training course, also on the NWA website, I encourage you do take the time to do so. This training course was the first online education effort by the NWA Aviation Weather Committee and has received numerous military and civilian education awards.
We welcome your comments and suggestions on these courses and your thoughts on additional online training related to aviation weather. Please share the NWA homepage at www.nwas.org with your fellow pilots, fixed base operators, and even that instructor who told you over and over to hold the nose up and land on the mains. When it comes to weather, nobody can know too much!
National Weather Association