A single engine pilot flew of the cascade mountain range at 10;000 ft IMC at night and lost 400 ft a...

Narrative:

The entire event took place in IMC. During cruise in IMC and rain at 10;000 ft up until the event I was not accumulating any ice. As I crossed the cascades I suddenly lost altitude. As I began to recover the controller called to verify my altitude (9;600 ft MSL) and advised me that the minimum IFR altitude in the area was 9;800 ft MSL. Even though I quickly established vy in recovering; that was only good enough to hold altitude for the first minute. It took nearly two minutes to recover the 400 ft lost. I attributed the sudden loss of altitude to mountain wave turbulence since there was about a 40 KT tailwind coming over the mountains. I emerged into VMC only a couple minutes later; and from the ambient light I could see that I had accumulated about 1/4 inch of rime ice on the leading edges of the wings. The ice built up very quickly; I had checked my wings with a flashlight only a minute or two before the loss of altitude and the water was still liquid. I think both the downdraft from the mountain wave and a sudden accumulation of ice (possibly also due to the mountain wave bringing colder air down) contributed to the loss of altitude and difficulty climbing back to my assigned altitude. To prevent a recurrence of this situation I will fly with an even wider gap between my altitude and the freezing level and also between my altitude and the MEA when mountain wave turbulence is likely.

Original NASA ASRS Text

Title: A single engine pilot flew of the Cascade Mountain Range at 10;000 FT IMC at night and lost 400 FT after some turbulence accompanied by a 1/4' rime ice build up.

Narrative: The entire event took place in IMC. During cruise in IMC and rain at 10;000 FT up until the event I was not accumulating any ice. As I crossed the Cascades I suddenly lost altitude. As I began to recover the Controller called to verify my altitude (9;600 FT MSL) and advised me that the minimum IFR altitude in the area was 9;800 FT MSL. Even though I quickly established Vy in recovering; that was only good enough to hold altitude for the first minute. It took nearly two minutes to recover the 400 FT lost. I attributed the sudden loss of altitude to mountain wave turbulence since there was about a 40 KT tailwind coming over the mountains. I emerged into VMC only a couple minutes later; and from the ambient light I could see that I had accumulated about 1/4 inch of rime ice on the leading edges of the wings. The ice built up very quickly; I had checked my wings with a flashlight only a minute or two before the loss of altitude and the water was still liquid. I think both the downdraft from the mountain wave and a sudden accumulation of ice (possibly also due to the mountain wave bringing colder air down) contributed to the loss of altitude and difficulty climbing back to my assigned altitude. To prevent a recurrence of this situation I will fly with an even wider gap between my altitude and the freezing level and also between my altitude and the MEA when mountain wave turbulence is likely.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	977757
Time
Date	201110
Local Time Of Day	1801-2400
Place
Locale Reference	ZSE.ARTCC
State Reference	WA
Environment
Flight Conditions	IMC
Light	Dusk
Aircraft 1
Make Model Name	Small Aircraft High Wing 1 Eng Fixed Gear
Operating Under FAR Part	Part 91
Flight Phase	Cruise
Route In Use	Vectors
Flight Plan	IFR
Person 1
Function	Pilot Flying Single Pilot
Qualification	Flight Crew Commercial Flight Crew Instrument
Experience	Flight Crew Last 90 Days 7 Flight Crew Total 322.3 Flight Crew Type 66
Events
Anomaly	Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural FAR Inflight Event / Encounter Weather / Turbulence

A single engine pilot flew of the cascade mountain range at 10;000 ft IMC at night and lost 400 ft after some turbulence accompanied by a 1/4' rime ice build up.