A B747 cargo flight encounters a tall, dry, and small diameter cumulo nimbus while in cruise flight ...

Attributes
ACN	509119
Time
Date	200104
Day	Tue
Local Time Of Day	0001 To 0600
Place
Locale Reference	intersection : acmee
State Reference	FL
Altitude	msl bound lower : 35000 msl bound upper : 36000
Environment
Flight Conditions	IMC
Weather Elements	Thunderstorm
Light	Night
Aircraft 1
Controlling Facilities	artcc : zma.artcc tracon : d21.tracon
Operator	common carrier : air carrier
Make Model Name	B747 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	cruise : level
Route In Use	enroute airway : a315.airway
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 100 flight time total : 22000 flight time type : 4000
ASRS Report	509119
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Events
Anomaly	altitude deviation : excursion from assigned altitude inflight encounter : weather inflight encounter : turbulence non adherence : clearance other anomaly
Independent Detector	other flight crewa
Resolutory Action	flight crew : returned to assigned altitude flight crew : regained aircraft control flight crew : overrode automation flight crew : exited adverse environment
Consequence	faa : reviewed incident with flight crew other
Supplementary
Problem Areas	Flight Crew Human Performance Weather Environmental Factor Airspace Structure
Primary Problem	Weather
Air Traffic Incident	Pilot Deviation

Narrative:

We were in straight and level cruise flight at FL350. It was a pitch black night (no moon) and the autoplt was engaged. We encountered a very small cumulo nimbus -- small in diameter, but obviously very high. This cumulo nimbus didn't show up on radar. Both our scopes were set to 80 mi scale, 1 degree down tilt, and gain in automatic position. Even though this cumulo nimbus didn't show on our radar, it had enough vertical windshear in it to cause about 10 seconds of severe turbulence. I immediately disengaged the autoplt. Leaving the autoplt on in the 'turbulence' mode is the preferred procedure, but the panel was shaking so badly I couldn't quickly locate or move my hand to the selector switch. I could easily push the autoplt disconnect button on the control yoke. Hand flying is the second most approved method in turbulence. I attempted to maintain level attitude with elevator and mach speed with power. Attempting to maintain altitude, besides usually being impossible, could cause structural failure of the aircraft. When the turbulence subsided, we had gained almost 1000 ft. We descended quickly back to FL350. ZMA questioned us about what had happened. There was no rain or hail in the cumulo nimbus we hit. It probably didn't show up on center radar either. These small diameter cumulo nimbus occur often over the water, usually over islands -- even tiny ones -- due to heat content. They're usually undetectable with old geriatric monochromatic radars like the ones we had that night. Also, when set even to the lower scale like 30 mi, some of the circuit breakers are the size of a pinhead!

Original NASA ASRS Text

Title: A B747 CARGO FLT ENCOUNTERS A TALL, DRY, AND SMALL DIAMETER CUMULO NIMBUS WHILE IN CRUISE FLT AT FL350, AND WAS LIFTED 1000 FT ON AIRWAY A-315 ON FREQ WITH ZMA, FL.

Narrative: WE WERE IN STRAIGHT AND LEVEL CRUISE FLT AT FL350. IT WAS A PITCH BLACK NIGHT (NO MOON) AND THE AUTOPLT WAS ENGAGED. WE ENCOUNTERED A VERY SMALL CUMULO NIMBUS -- SMALL IN DIAMETER, BUT OBVIOUSLY VERY HIGH. THIS CUMULO NIMBUS DIDN'T SHOW UP ON RADAR. BOTH OUR SCOPES WERE SET TO 80 MI SCALE, 1 DEG DOWN TILT, AND GAIN IN AUTO POS. EVEN THOUGH THIS CUMULO NIMBUS DIDN'T SHOW ON OUR RADAR, IT HAD ENOUGH VERT WINDSHEAR IN IT TO CAUSE ABOUT 10 SECONDS OF SEVERE TURB. I IMMEDIATELY DISENGAGED THE AUTOPLT. LEAVING THE AUTOPLT ON IN THE 'TURB' MODE IS THE PREFERRED PROC, BUT THE PANEL WAS SHAKING SO BADLY I COULDN'T QUICKLY LOCATE OR MOVE MY HAND TO THE SELECTOR SWITCH. I COULD EASILY PUSH THE AUTOPLT DISCONNECT BUTTON ON THE CTL YOKE. HAND FLYING IS THE SECOND MOST APPROVED METHOD IN TURB. I ATTEMPTED TO MAINTAIN LEVEL ATTITUDE WITH ELEVATOR AND MACH SPD WITH PWR. ATTEMPTING TO MAINTAIN ALT, BESIDES USUALLY BEING IMPOSSIBLE, COULD CAUSE STRUCTURAL FAILURE OF THE ACFT. WHEN THE TURB SUBSIDED, WE HAD GAINED ALMOST 1000 FT. WE DSNDED QUICKLY BACK TO FL350. ZMA QUESTIONED US ABOUT WHAT HAD HAPPENED. THERE WAS NO RAIN OR HAIL IN THE CUMULO NIMBUS WE HIT. IT PROBABLY DIDN'T SHOW UP ON CTR RADAR EITHER. THESE SMALL DIAMETER CUMULO NIMBUS OCCUR OFTEN OVER THE WATER, USUALLY OVER ISLANDS -- EVEN TINY ONES -- DUE TO HEAT CONTENT. THEY'RE USUALLY UNDETECTABLE WITH OLD GERIATRIC MONOCHROMATIC RADARS LIKE THE ONES WE HAD THAT NIGHT. ALSO, WHEN SET EVEN TO THE LOWER SCALE LIKE 30 MI, SOME OF THE CIRCUIT BREAKERS ARE THE SIZE OF A PINHEAD!

Data retrieved from NASA's ASRS site as of July 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.

A B747 cargo flight encounters a tall, dry, and small diameter cumulo nimbus while in cruise flight at FL350, and was lifted 1000 ft on airway A-315 on frequency with ZMA, fl.