Narrative:

This wake turbulence encounter happened more than 6 months ago, but I am sending it anyway because it happened when all the indications dictated that it shouldn't have. We were approaching den from the west and were on a downwind for the west runways. We were still at least 20 mi southwest of the airport, level at 13000 ft and told to follow a B727 '12 O'clock, 5 mi and 11000 ft.' considerations: 1) the heavier aircraft was 5 mi ahead. 2) it was 2000 ft lower. 3) we were in a tailwind. 4) the known katabatic 'rules' say that the tailwind and vortices should have had a downward flow as we came over the rockies. The ride was average for the time of yr (spring) and we had the B727 in sight. Suddenly the aircraft performed an uncommanded roll to the right. At the time it felt like about 45 degrees but in fact, was probably only about 30 degrees. The captain pulled the power levers to idle and I used nearly full aileron deflection to level the aircraft. About 5 seconds later the aircraft did exactly the same thing, only this time to the left. There was no uncommanded pitch activity. The captain and I immediately recognized this as wake turbulence and advised ATC. We landed with no further incidents and no one was hurt. Callback conversation with reporter revealed the following information: this BE1900D first officer said that he has studied about the wake that can be generated behind large aircraft, but he has not read about an encounter such as this. He said that he does not remember the exact wind, but it was going to be no factor on the approach. They were in light to moderate turbulence about 8000 ft above the surrounding terrain and they had the preceding B727 in sight. The B727 was going to runway 26L at den and the reporter was being vectored for runway 26R. Additionally the B727 was descending and was about 2000 ft lower than the 1900D when they hit the wake. The BE1900D rolled to the right very quickly with the first officer putting in full left aileron to counter. The aircraft then started to correct, but as it approached a normal level attitude it suddenly rolled to the left. The reporter said that again he applied full opposite aileron. However, the aircraft still rolled to 30 degrees of bank. The captain's response to the initial roll was to reduce power to slow the aircraft because he thought that this was simply a stronger version of the normal turbulence that they had been in during the flight according to the reporter. He said that the captain advanced power as they were correcting from the first roll. The first officer said that the passenger did not comment on the rolls, but they all clapped upon landing. He did mention this event to his chief pilot, but he was not interested, the reporter said. He said that he has had some aerobatic training in the past.

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Original NASA ASRS Text

Title: A BE1900D ACR FLC HAS A WAKE TURB ENCOUNTER BEHIND A B727 WHILE BEING VECTORED FOR A PARALLEL APCH AND STILL SOME DISTANCE FROM THE ARPT. THE FLC INITIALLY MISTOOK THE ROLL AS BEING AN ORDINARY TURB EVENT BECAUSE THEY HAD BEEN IN LIGHT TO MODERATE TURB THROUGHOUT THE FLT.

Narrative: THIS WAKE TURB ENCOUNTER HAPPENED MORE THAN 6 MONTHS AGO, BUT I AM SENDING IT ANYWAY BECAUSE IT HAPPENED WHEN ALL THE INDICATIONS DICTATED THAT IT SHOULDN'T HAVE. WE WERE APCHING DEN FROM THE W AND WERE ON A DOWNWIND FOR THE W RWYS. WE WERE STILL AT LEAST 20 MI SW OF THE ARPT, LEVEL AT 13000 FT AND TOLD TO FOLLOW A B727 '12 O'CLOCK, 5 MI AND 11000 FT.' CONSIDERATIONS: 1) THE HEAVIER ACFT WAS 5 MI AHEAD. 2) IT WAS 2000 FT LOWER. 3) WE WERE IN A TAILWIND. 4) THE KNOWN KATABATIC 'RULES' SAY THAT THE TAILWIND AND VORTICES SHOULD HAVE HAD A DOWNWARD FLOW AS WE CAME OVER THE ROCKIES. THE RIDE WAS AVERAGE FOR THE TIME OF YR (SPRING) AND WE HAD THE B727 IN SIGHT. SUDDENLY THE ACFT PERFORMED AN UNCOMMANDED ROLL TO THE R. AT THE TIME IT FELT LIKE ABOUT 45 DEGS BUT IN FACT, WAS PROBABLY ONLY ABOUT 30 DEGS. THE CAPT PULLED THE PWR LEVERS TO IDLE AND I USED NEARLY FULL AILERON DEFLECTION TO LEVEL THE ACFT. ABOUT 5 SECONDS LATER THE ACFT DID EXACTLY THE SAME THING, ONLY THIS TIME TO THE L. THERE WAS NO UNCOMMANDED PITCH ACTIVITY. THE CAPT AND I IMMEDIATELY RECOGNIZED THIS AS WAKE TURB AND ADVISED ATC. WE LANDED WITH NO FURTHER INCIDENTS AND NO ONE WAS HURT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THIS BE1900D FO SAID THAT HE HAS STUDIED ABOUT THE WAKE THAT CAN BE GENERATED BEHIND LARGE ACFT, BUT HE HAS NOT READ ABOUT AN ENCOUNTER SUCH AS THIS. HE SAID THAT HE DOES NOT REMEMBER THE EXACT WIND, BUT IT WAS GOING TO BE NO FACTOR ON THE APCH. THEY WERE IN LIGHT TO MODERATE TURB ABOUT 8000 FT ABOVE THE SURROUNDING TERRAIN AND THEY HAD THE PRECEDING B727 IN SIGHT. THE B727 WAS GOING TO RWY 26L AT DEN AND THE RPTR WAS BEING VECTORED FOR RWY 26R. ADDITIONALLY THE B727 WAS DSNDING AND WAS ABOUT 2000 FT LOWER THAN THE 1900D WHEN THEY HIT THE WAKE. THE BE1900D ROLLED TO THE R VERY QUICKLY WITH THE FO PUTTING IN FULL L AILERON TO COUNTER. THE ACFT THEN STARTED TO CORRECT, BUT AS IT APCHED A NORMAL LEVEL ATTITUDE IT SUDDENLY ROLLED TO THE L. THE RPTR SAID THAT AGAIN HE APPLIED FULL OPPOSITE AILERON. HOWEVER, THE ACFT STILL ROLLED TO 30 DEGS OF BANK. THE CAPT'S RESPONSE TO THE INITIAL ROLL WAS TO REDUCE PWR TO SLOW THE ACFT BECAUSE HE THOUGHT THAT THIS WAS SIMPLY A STRONGER VERSION OF THE NORMAL TURB THAT THEY HAD BEEN IN DURING THE FLT ACCORDING TO THE RPTR. HE SAID THAT THE CAPT ADVANCED PWR AS THEY WERE CORRECTING FROM THE FIRST ROLL. THE FO SAID THAT THE PAX DID NOT COMMENT ON THE ROLLS, BUT THEY ALL CLAPPED UPON LNDG. HE DID MENTION THIS EVENT TO HIS CHIEF PLT, BUT HE WAS NOT INTERESTED, THE RPTR SAID. HE SAID THAT HE HAS HAD SOME AEROBATIC TRAINING IN THE PAST.

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.