Narrative:

My aircraft was a PA28-181. When I obtained visual contact with dca I called ATC with a report of the airport in sight. The controller did not clear me for the visual to either runway 33 or runway 36, but responded that I could expect the ILS 36. I elected to remain IFR, primarily for wake avoidance. I was placed on about a 3 mi right downwind for the ILS, and when southeast of the field, descended to 1800 ft. Air carrier traffic appeared to be approaching in a steady stream from the south, essentially tracking the localizer northbound and descending at what appeared to be a fairly steep angle (more than 3 degrees) to intercept the GS about 1 mi south of the OM (oxonn). After passing abeam oxonn and flying perhaps 3 mi more, I was turned base, and then final. As I was being turned for the intercept, ATC advised that I was intrail of a 757, that I was cleared for the ILS 36 and 'caution wake turbulence.' at that point, the 757 had steeply descended from well above 1800 ft through that altitude just prior to reaching the marker, i.e., my tracking of the localizer at 1800 ft to GS intercept would place me below and converging upon the 757's-flight path. I do not know if the 757 was flying the ILS or on a visual approach. I would estimate that when I intercepted the localizer the 757 was ahead of me by about 3-4 mi. As I was about 1 mi south of oxonn, the 757 was outside the MM and I encountered what I believe to have been a mild form of wake turbulence. In what had otherwise been smooth air, the turbulence contained a strong rolling motion that required about 3/4 aileron deflection to retain a near- level attitude. That turbulence lasted for only a few seconds and, once I established on the GS, I did not again experience it. Shortly thereafter I contacted dca tower, who reported the 757 at the MM and I was cleared to land. The landing was without incident. I was surprised to see that ATC was descending the airline traffic (and, particularly the 757 I followed) from well above the GS, and also vectoring light aircraft at 1800 ft to intercept the localizer several mi south of the marker. While there may have been 'legal' IFR separation between the 757 and my aircraft, the wake turbulence was unmistakable and I would suggest that the separation behind 757's be increased to take into account what I have read is their wakes. In addition, in the future, I will both not trust operations under IFR, and flying the GS, to keep me free from wake turbulence encounters, and realize that as PIC I should have informed ATC that the intrail separation, given the descending flight path of the 757, was unacceptable to me.

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Original NASA ASRS Text

Title: WAKE ENCOUNTER.

Narrative: MY ACFT WAS A PA28-181. WHEN I OBTAINED VISUAL CONTACT WITH DCA I CALLED ATC WITH A RPT OF THE ARPT IN SIGHT. THE CTLR DID NOT CLR ME FOR THE VISUAL TO EITHER RWY 33 OR RWY 36, BUT RESPONDED THAT I COULD EXPECT THE ILS 36. I ELECTED TO REMAIN IFR, PRIMARILY FOR WAKE AVOIDANCE. I WAS PLACED ON ABOUT A 3 MI R DOWNWIND FOR THE ILS, AND WHEN SE OF THE FIELD, DSNDED TO 1800 FT. ACR TFC APPEARED TO BE APCHING IN A STEADY STREAM FROM THE S, ESSENTIALLY TRACKING THE LOC NBOUND AND DSNDING AT WHAT APPEARED TO BE A FAIRLY STEEP ANGLE (MORE THAN 3 DEGS) TO INTERCEPT THE GS ABOUT 1 MI S OF THE OM (OXONN). AFTER PASSING ABEAM OXONN AND FLYING PERHAPS 3 MI MORE, I WAS TURNED BASE, AND THEN FINAL. AS I WAS BEING TURNED FOR THE INTERCEPT, ATC ADVISED THAT I WAS INTRAIL OF A 757, THAT I WAS CLRED FOR THE ILS 36 AND 'CAUTION WAKE TURB.' AT THAT POINT, THE 757 HAD STEEPLY DSNDED FROM WELL ABOVE 1800 FT THROUGH THAT ALT JUST PRIOR TO REACHING THE MARKER, I.E., MY TRACKING OF THE LOC AT 1800 FT TO GS INTERCEPT WOULD PLACE ME BELOW AND CONVERGING UPON THE 757'S-FLT PATH. I DO NOT KNOW IF THE 757 WAS FLYING THE ILS OR ON A VISUAL APCH. I WOULD ESTIMATE THAT WHEN I INTERCEPTED THE LOC THE 757 WAS AHEAD OF ME BY ABOUT 3-4 MI. AS I WAS ABOUT 1 MI S OF OXONN, THE 757 WAS OUTSIDE THE MM AND I ENCOUNTERED WHAT I BELIEVE TO HAVE BEEN A MILD FORM OF WAKE TURB. IN WHAT HAD OTHERWISE BEEN SMOOTH AIR, THE TURB CONTAINED A STRONG ROLLING MOTION THAT REQUIRED ABOUT 3/4 AILERON DEFLECTION TO RETAIN A NEAR- LEVEL ATTITUDE. THAT TURB LASTED FOR ONLY A FEW SECONDS AND, ONCE I ESTABLISHED ON THE GS, I DID NOT AGAIN EXPERIENCE IT. SHORTLY THEREAFTER I CONTACTED DCA TWR, WHO RPTED THE 757 AT THE MM AND I WAS CLRED TO LAND. THE LNDG WAS WITHOUT INCIDENT. I WAS SURPRISED TO SEE THAT ATC WAS DSNDING THE AIRLINE TFC (AND, PARTICULARLY THE 757 I FOLLOWED) FROM WELL ABOVE THE GS, AND ALSO VECTORING LIGHT ACFT AT 1800 FT TO INTERCEPT THE LOC SEVERAL MI S OF THE MARKER. WHILE THERE MAY HAVE BEEN 'LEGAL' IFR SEPARATION BTWN THE 757 AND MY ACFT, THE WAKE TURB WAS UNMISTAKABLE AND I WOULD SUGGEST THAT THE SEPARATION BEHIND 757'S BE INCREASED TO TAKE INTO ACCOUNT WHAT I HAVE READ IS THEIR WAKES. IN ADDITION, IN THE FUTURE, I WILL BOTH NOT TRUST OPS UNDER IFR, AND FLYING THE GS, TO KEEP ME FREE FROM WAKE TURB ENCOUNTERS, AND REALIZE THAT AS PIC I SHOULD HAVE INFORMED ATC THAT THE INTRAIL SEPARATION, GIVEN THE DSNDING FLT PATH OF THE 757, WAS UNACCEPTABLE TO ME.

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.