ice and rain affect normal operations-Emphasis on Icing
There is a clear inter-relation between safe and satisfactory travel by air and weather. Most of the accidents in airplanes occur due to adverse weather, and it is one among the different causes improving towards the occurrence of the accident. It can be blamed as the reason for most of the flight delays also. All flight operations are affected by unfavorable weather. This may prevent the handling of flight totally or sometimes partially. The expenses incurred due to delays and change in route due to such weather conditions is very high. Both the passenger and the aviation industry has to bear the brunt of these situations, due to the loss of time for the passenger, the hefty hotel charges, increase in fuel consumption, and additional spending on servicing, equipment and change in crew, as also they make flying more expensive beyond plans and budgets. Fogs, thunderstorms, freezing rain, snowstorms, crosswinds, poor visibility, icing, and en-route turbulence are the types of weather that make delays and changes in route unavoidable.
According to reports available the day the crash of 747-jetliner of Singapore Airlines occurred there was heavy rain in the city of Taipei. But the pilots are not prevented by such conditions. About the acceptable weather conditions for take-off and landing each airliner company has its own set guidelines, but to proceed or not is at the discretion of the pilot. Normally the aircrafts, especially the 747, fly even in heavy rain and winds. But as the jetliner sped up on the runway visibility of the pilot was, perhaps, affected by the incessant rain. The pilot had mentioned about the jetliner hitting an object on the runway during the process of the take off. It is the vision of the pilot that enable him keep the nose of the aircraft aimed straight while on ground. And when there is heavy rain outside, striking the window of the airplane it limits the visibility of the pilot. It is also possible that the engine fire of the jetliner was blown-off by the heavy rain entering its turbo engine. The fire can go off if the speed of the rain striking the engine exceeds two inches per hour. There is an intake of about 200 pounds of air in the case of a turbo jet engine, and when more than 10% of it is water the fire in the combustion chamber may go off.
All aerofoils, including the propeller, get affected and weight of the aircrafts is increased by ice, snow and frost. Even a light layer of frost can affect the airlift of the craft very much, and can be hazardous when it sticks on to the airfoils like wings, control surfaces, propeller and rotor blades. When the surface temperature of the airplane goes below dewpoint (below 32° F) or less than freezing point, formation of frost takes place. Though it forms early in mornings, it gets melted with sunrise and with the increase of heat and sunlight during the day. But a pilot has to take extra precautions to clear the frost before take off if the surface temperature is below freezing point and sky is not clear and heat from sun cannot be expected. When some pilots say there is no much frost on the wings, or that there is no change in shape by a simple look, they ignore the fact that the air flow across the airfoil may be blocked to a great extent by the surface roughness formed by the crystal formation of frost over them. This affects the airlift and makes take off and landing at low speeds risky. (Bernstein; Omeron; McDonough and Politovich, 1997).
While frost affects lift by reducing or destroying it, ice add weight besides affecting the lift. These factors together makes the pilot unable to maintain the required height, or prevents him from keeping the height below which he is not expected to fly for long. When thunderstorm also is present along with icing it becomes a nightmare for the pilot. Every year a number of accidents take place due to icing. 40% of the accidents are due to structural icing, which is also known as airframe icing. (Lankford, 2000). Carburetor and induction system icing, or engine instrument icing is the reason for the remaining 60%. The aircrafts may have to very often fly in such atmospheric conditions with high presence of aircraft structural icing. Most aircrafts, including even the Aerosonde, do not have equipments for proper de-icing fitted on them, to fly in such extreme conditions. And even if these are fitted, all aircrafts should avoid all icing conditions where its presence is high or very high-where ice formation can reach greater than 2 cm in a minute. (Lankford, 2000).
Ice additions even if bit by bit, can cause problems like reduced lift and speed and increased drag. Many a time icing is very risky. The sudden increase in drag due to ice accumulation on the main rotor and tail rotor blades affecting the lift necessitates additional power to drive the blades. Ice on the fuselage also demands more power as it increases the overall weight. The glassy formation of ice on the windscreens reduces or blocks visibility, making vision possible only through side or slide open windows. Vibration imbalances occur due to untimely fall of ice from blades. The engine air intake system gets blocked with ice formation over there. Ice falling from the fuselage comes like slabs or chunks and can damage rotors or enter into the air intake system blocking normal airflow. Freezing water comes in where they are normally not expected. Normal operation and control movements become difficult when ice is there on the flight controls like bell cranks, rod ends and pitch horn. Different temperature ranges are attributed to icing, in different texts. It may take place at temperatures of 0°C or below, and may vary based on a number of other factors.
Resistance to the forward motion of the aircraft through the air, known as the drag, is greatly influenced by the dirty formation of ice on the aircraft wing. There are two types of drags viz. The parasite drag and the induced drag. Induced drag is the one formed on lift, and builds up with the increase in angle of attack. Hence the induced drag will be higher in the case of aircrafts with contaminated wings than that with the normal ones as the ones with contaminated wings have to fly at higher angle of attack for the required lift at a particular air speed. More over there is a higher induced drag value at any angle of attack as the air flow is separated earlier from the upper surface of the wings due to the dirty form of ice on them. The increase in induced drag due to ice contamination is higher when compared to increase in parasite drag. The normal stall progression of a swept wing gets varied on a contaminated wing, which is influenced by surface roughness.
With a contaminated wing, the normal nose-down pitching moment in the direction of stall recovery, which accompanies a stall, is reduced. The effects of the degraded pitching moment quality vary with variation in angle of attack from an out-of- trim condition which can have abnormal responses to control column inputs. The sensitivity to ice contamination is high at the leading edge portion of the wing. The more the forward most extent of contamination is further away from the leading edge the less is the effects of this contamination. At temperatures just below the freezing point the slow accumulation of ice takes place and this calls for severe loss in aircraft movements. It is said that the major cause for take off accidents, specially in the case of jet transport aircrafts is the damaging effects on lift and drag due to ice accumulation and the related surface roughness of the wings.
To save aviation from risky icing conditions, proper and timely icing predictions are necessary. (Rasmussen et al., 1992). But it is not easy to give such a prediction of the icing conditions. Hence it becomes difficult to give flight plans leaving out areas prone to be of icing hazards. And the few available predictions give an exaggerated view of icing factors and on its severity. They are also limited to mesoscale icing conditions. These predictions can be of great use aircrafts, which are large in size. They are also of use in determining microphysical parameters acting upon the icing conditions. They would also be of great use to UAV and smaller aircraft operations along with of being benefit for Aerosonde operations. On noticing any adverse icing conditions while in flight the controller of the Aerosonde is to change the flight plan accordingly to save it from any possible loss or damage.
Predictions on icing may be verified with the pilot's report on icing including its type and severity (Bernstein; Omeron; McDonough and Politovich, 1997). The…