An air carrier flight crew flying a B737-300 aircraft deviated from their assigned altitude.

Attributes
ACN	429940
Time
Date	199903
Day	Wed
Local Time Of Day	1201 To 1800
Place
Locale Reference	navaid : als.vor
State Reference	CO
Altitude	msl bound lower : 30300 msl bound upper : 31000
Environment
Flight Conditions	VMC
Weather Elements	Turbulence other
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zdv.artcc
Operator	common carrier : air carrier
Make Model Name	B737-300
Operating Under FAR Part	Part 121
Navigation In Use	other
Flight Phase	cruise : level
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : commercial pilot : flight engineer pilot : multi engine pilot : cfi pilot : atp
Experience	flight time last 90 days : 164 flight time total : 18000 flight time type : 6000
ASRS Report	429940
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : multi engine pilot : instrument pilot : commercial
Events
Anomaly	altitude deviation : excursion from assigned altitude inflight encounter : turbulence non adherence : clearance
Independent Detector	aircraft equipment : altitude alert other flight crewa
Resolutory Action	flight crew : returned to original clearance
Supplementary
Problem Areas	Flight Crew Human Performance
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Flight conditions: cruise, FL310, autoplt engaged, seat belt sign 'on.' approximately 30 NM east of ALS VOR we encountered a sudden onset of light, almost moderate turbulence. As I disengaged the VNAV to select a slower airspeed I heard the altitude alert horn. Checking the altimeter, I noticed that we were 400 ft below FL310 and descending at least 4000 FPM! I disengaged the autoplt and stopped the descent then flew the airplane back to FL310 before re-engaging the autoplt. The maximum altitude deviation was 700 ft below FL310. I had the first officer report the event to ZDV and we were advised that there had been no conflict. ARTCC thanked us for the PIREP. There were no injuries in the cabin. I believe that we encountered a 'rotor,' as the reaction of the airplane was not consistent with mountain wave activity. There is not much that can be done to prevent this type of altitude deviation. There was a strong westerly flow over the rockies that day. Mountain wave activity I did expect but a 'downdraft' of this magnitude I did not expect and have never experienced anything like it at flight level altitudes in 22+ yrs of airline flying.

Original NASA ASRS Text

Title: AN ACR FLC FLYING A B737-300 ACFT DEVIATED FROM THEIR ASSIGNED ALT.

Narrative: FLT CONDITIONS: CRUISE, FL310, AUTOPLT ENGAGED, SEAT BELT SIGN 'ON.' APPROX 30 NM E OF ALS VOR WE ENCOUNTERED A SUDDEN ONSET OF LIGHT, ALMOST MODERATE TURB. AS I DISENGAGED THE VNAV TO SELECT A SLOWER AIRSPD I HEARD THE ALT ALERT HORN. CHKING THE ALTIMETER, I NOTICED THAT WE WERE 400 FT BELOW FL310 AND DSNDING AT LEAST 4000 FPM! I DISENGAGED THE AUTOPLT AND STOPPED THE DSCNT THEN FLEW THE AIRPLANE BACK TO FL310 BEFORE RE-ENGAGING THE AUTOPLT. THE MAX ALTDEV WAS 700 FT BELOW FL310. I HAD THE FO RPT THE EVENT TO ZDV AND WE WERE ADVISED THAT THERE HAD BEEN NO CONFLICT. ARTCC THANKED US FOR THE PIREP. THERE WERE NO INJURIES IN THE CABIN. I BELIEVE THAT WE ENCOUNTERED A 'ROTOR,' AS THE REACTION OF THE AIRPLANE WAS NOT CONSISTENT WITH MOUNTAIN WAVE ACTIVITY. THERE IS NOT MUCH THAT CAN BE DONE TO PREVENT THIS TYPE OF ALTDEV. THERE WAS A STRONG WESTERLY FLOW OVER THE ROCKIES THAT DAY. MOUNTAIN WAVE ACTIVITY I DID EXPECT BUT A 'DOWNDRAFT' OF THIS MAGNITUDE I DID NOT EXPECT AND HAVE NEVER EXPERIENCED ANYTHING LIKE IT AT FLT LEVEL ALTS IN 22+ YRS OF AIRLINE FLYING.

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.