Icing Standing Waves
An Islander air ambulance flight was routing from Glasgow to Stornaway using the airway designated A1 D...
An Islander air ambulance flight was routing from Glasgow to Stornaway using the airway designated A1 D. Since moderate icing conditions were forecast, the pilot checked for correct operation of the pneumatic deicing boots whilst conducting the pretakeoff checks. The aircraft was equipped with an autopilot but did not have a weather radar.
The aircraft was cleared for departure from Glasgow at 0850. At 0856, whilst climbing through 4,000 feet, the flight was handed over to the Scottish Area Control Centre (ScACC) at Prestwick who cleared the aircraft to FL85 under a Radar Advisory Service. As the aircraft approached FL85 the commander requested FL95 but he was advised that this was an incorrect quadrantal level and was offered clearance to FL105, which he accepted.
At 0911 the commander reported that he was unable to reach FL105, possibly because of mountain wave activity, and he requested a descent back to FL85. At 0935 he requested an operating band from FL65 to FL85 because of more pronounced mountain wave activity. He was now in icing conditions and utilised both the propeller and airframe deicing systems, which operated satisfactorily.
At 1012 the aircraft was IMC at FL65 with an indicated airspeed of 110 knots when the pilot noticed a sudden buildup of ice on the wheels, struts and tyres. He was unable to maintain altitude and requested radar vectors to a clear area.
The ScACC controller suggested that he turn west, towards the coast, which was an estimated eight miles away. The pilot then declared an emergency and was asked to set the emergency transponder code of 7700. The pilot allowed the speed to reduce to seventy knots and then entered a descent at that speed; the engines remained at full power.
About three nm prior to reaching the coast, at an altitude of 4,200 feet, the aircraft entered clear air and the ice melted rapidly. The pilot then chose to continue towards Stornaway since the route appeared to be clear of cloud. He climbed to FL75 and the remainder of the flight was uneventful.
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The aircraft was cleared for flight into known or forecast icing conditions not more severe than 'light'. It was equipped with the following deicing systems, all of which were serviceable:*pitot head and stallwarning vane heaters*aircraft heater and windscreen demisting*propeller deicing*airframe deicing*heated panel in windscreen
There were warnings of moderate icing and moderate turbulence in cloud with isolated severe turbulence below 7,000 feet and severe icing in nimbostratus clouds. There was also a warning of mountain waves with a maximum vertical speed of 550 fpm near 9,000 feet. This wind profile is indicative of standing waves. The uplifted air associated with these waves can produce an upward motion to the clouds which further increases the water density and hence the risk of icing.
Clearly the upward motion of air is strongest in convective clouds such as cumulus and cumulonimbus but orographic motions can also produce severe ice accretion at times.
The operator had recently provided guidance for pilots when flying in forecast icing conditions. The Notice states, 'If you suspect that significant airframe icing is likely: Do not plan to route over the highest ground, Go round it.' The selected route passed over the Grampian Mountains and close to Ben Nevis (4,410 feet) whereas an alternative route was available over lower terrain.