A B727 on approach to dfw encounters wake turbulence from a preceding B757 4 mi ahead. Flight crew suffers a temporary loss of aircraft control as the aircraft was on autoplt, which reduced the PIC's ability to combat the roll to the left and the following roll to the right.

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Attributes
ACN	408629
Time
Date	199807
Day	Fri
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : dfw
State Reference	TX
Altitude	msl bound lower : 11000 msl bound upper : 11000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : dfw
Operator	common carrier : air carrier
Make Model Name	B727-200
Operating Under FAR Part	Part 121
Flight Phase	descent : approach
Route In Use	arrival other enroute : on vectors
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	B757-200
Operating Under FAR Part	Part 121
Navigation In Use	Other Other
Flight Phase	descent : approach
Route In Use	arrival other enroute : on vectors
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp pilot : commercial pilot : instrument
Experience	flight time last 90 days : 150 flight time total : 10000 flight time type : 2800
ASRS Report	408629
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument
Events
Anomaly	inflight encounter other non adherence : published procedure other anomaly other anomaly other other spatial deviation
Independent Detector	other flight crewa
Resolutory Action	flight crew : regained aircraft control
Consequence	Other
Supplementary
Primary Problem	ATC Human Performance
Air Traffic Incident	Pilot Deviation other
Situations
ATC Facility	procedure or policy : unspecified

Narrative:

Descending into dfw we encountered the wake turbulence of a B757 that provided a moderate roll rate with the autoplt connected. Disconnected the autoplt and recovered without incident. We were being vectored for an ILS to runway 35C at dfw. Rain showers that were building in intensity were lying east of the field on our downwind leg. Callback conversation with reporter revealed the following information: the captain reporter felt that the preceding B757 was higher than his aircraft initially and had descended down through his altitude while about 4 mi ahead. The controller never advised of any traffic or possible wake turbulence. The reporter stated that in the future he will monitor his TCASII more closely for traffic separation and if turbulence is expected, turn off his autoplt for better recovery roll rate. While on autoplt, the aircraft is restr in its recovery as the outboard ailerons and some spoilers are locked out for reduced control input with a lessened roll rate.

Original NASA ASRS Text

Title: A B727 ON APCH TO DFW ENCOUNTERS WAKE TURB FROM A PRECEDING B757 4 MI AHEAD. FLC SUFFERS A TEMPORARY LOSS OF ACFT CTL AS THE ACFT WAS ON AUTOPLT, WHICH REDUCED THE PIC'S ABILITY TO COMBAT THE ROLL TO THE L AND THE FOLLOWING ROLL TO THE R.

Narrative: DSNDING INTO DFW WE ENCOUNTERED THE WAKE TURB OF A B757 THAT PROVIDED A MODERATE ROLL RATE WITH THE AUTOPLT CONNECTED. DISCONNECTED THE AUTOPLT AND RECOVERED WITHOUT INCIDENT. WE WERE BEING VECTORED FOR AN ILS TO RWY 35C AT DFW. RAIN SHOWERS THAT WERE BUILDING IN INTENSITY WERE LYING E OF THE FIELD ON OUR DOWNWIND LEG. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE CAPT RPTR FELT THAT THE PRECEDING B757 WAS HIGHER THAN HIS ACFT INITIALLY AND HAD DSNDED DOWN THROUGH HIS ALT WHILE ABOUT 4 MI AHEAD. THE CTLR NEVER ADVISED OF ANY TFC OR POSSIBLE WAKE TURB. THE RPTR STATED THAT IN THE FUTURE HE WILL MONITOR HIS TCASII MORE CLOSELY FOR TFC SEPARATION AND IF TURB IS EXPECTED, TURN OFF HIS AUTOPLT FOR BETTER RECOVERY ROLL RATE. WHILE ON AUTOPLT, THE ACFT IS RESTR IN ITS RECOVERY AS THE OUTBOARD AILERONS AND SOME SPOILERS ARE LOCKED OUT FOR REDUCED CTL INPUT WITH A LESSENED ROLL RATE.

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.