Air carrier medium large transport altitude deviation excursion from clearance altitude in severe turbulence caused by proximity of thunderstorm activity.

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Attributes
ACN	86955
Time
Date	198805
Day	Sat
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : fod
State Reference	IA
Altitude	msl bound lower : 36500 msl bound upper : 37000
Environment
Flight Conditions	IMC
Light	Dusk
Aircraft 1
Controlling Facilities	artcc : zmp artcc : zob
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : cfi pilot : atp
Experience	flight time last 90 days : 220 flight time total : 12000 flight time type : 2200
ASRS Report	86955
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp pilot : flight engineer
Experience	flight time last 90 days : 90 flight time total : 3630 flight time type : 510
ASRS Report	86955
Events
Anomaly	altitude deviation : excursion from assigned altitude inflight encounter : weather
Independent Detector	other flight crewa
Resolutory Action	none taken : unable
Consequence	Other
Supplementary
Primary Problem	Weather
Air Traffic Incident	other

Narrative:

In cruise at FL370, observed line of echoes north to south, tops appeared to be below our level. Requested and got ATC clearance for deviations. With echoes a considerable distance to south, we deviated northeast, encountered moderate turbulence, then a brief episode (1-2 min) of severe turbulence. No hail or rain, but severe up and downdrafts, wide speed fluctuations: +20 KTS (with thrust reduced), -30 KTS (with thrust advanced to mct). Unable to maintain altitude, lost about 500 ft, requested and got descent to FL240. Communications blanked by static for several mins. Conditions smoothed out in descent below FL330. No injuries to passenger or flight crew. While in turbulence, I experienced nystagmus (unable to focus eyes on the instruments) for the first time in my 30 yr career. We were apparently in the downwind, downwash turbulence from the cells, because we painted no echoes ahead of us at all. Supplemental information from acn 86959: although radar echoes were at least 20 mi off of our right wing tip, I will advocate that more distance be maintained.

Original NASA ASRS Text

Title: ACR MLG ALT DEVIATION EXCURSION FROM CLRNC ALT IN SEVERE TURBULENCE CAUSED BY PROX OF TSTM ACTIVITY.

Narrative: IN CRUISE AT FL370, OBSERVED LINE OF ECHOES N TO S, TOPS APPEARED TO BE BELOW OUR LEVEL. REQUESTED AND GOT ATC CLRNC FOR DEVS. WITH ECHOES A CONSIDERABLE DISTANCE TO S, WE DEVIATED NE, ENCOUNTERED MODERATE TURB, THEN A BRIEF EPISODE (1-2 MIN) OF SEVERE TURB. NO HAIL OR RAIN, BUT SEVERE UP AND DOWNDRAFTS, WIDE SPD FLUCTUATIONS: +20 KTS (WITH THRUST REDUCED), -30 KTS (WITH THRUST ADVANCED TO MCT). UNABLE TO MAINTAIN ALT, LOST ABOUT 500 FT, REQUESTED AND GOT DSCNT TO FL240. COMS BLANKED BY STATIC FOR SEVERAL MINS. CONDITIONS SMOOTHED OUT IN DSCNT BELOW FL330. NO INJURIES TO PAX OR FLC. WHILE IN TURB, I EXPERIENCED NYSTAGMUS (UNABLE TO FOCUS EYES ON THE INSTRUMENTS) FOR THE FIRST TIME IN MY 30 YR CAREER. WE WERE APPARENTLY IN THE DOWNWIND, DOWNWASH TURB FROM THE CELLS, BECAUSE WE PAINTED NO ECHOES AHEAD OF US AT ALL. SUPPLEMENTAL INFO FROM ACN 86959: ALTHOUGH RADAR ECHOES WERE AT LEAST 20 MI OFF OF OUR RIGHT WING TIP, I WILL ADVOCATE THAT MORE DISTANCE BE MAINTAINED.

Data retrieved from NASA's ASRS site as of August 2007 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.