SEA96LA173

The tower controller instructed the Cessna 182 pilot to expedite his visual flight rules (VFR) approach and follow an MD-80 airliner on straight-in final. The Cessna pilot reported the MD-80 in sight (at 2.3 to 3.6 nautical miles) and complied with the instructions. The Cessna encountered the MD-80's wake turbulence about 200 feet above ground level on final; the pilot lost control and the aircraft crashed. Radar data showed that the Cessna closed to approximately 0.9 miles behind the MD-80, and descended 200 feet below the MD-80's glide path (contrary to wake turbulence avoidance guidance in the FAA's Aeronautical Information Manual [AIM]). A 4-mile minimum separation distance would have been required for radar separation, but no minimum distance was required for visual separation. The AIM states that controllers are required to apply minimum landing separation distances for wake turbulence, without distinguishing between visual and radar separation environments. In February 1994, the NTSB issued recommendations to the FAA indicating a belief that VFR aircraft should observe instrument flight rules (IFR) wake turbulence separation distances.

On July 27, 1996, approximately 1927 Pacific daylight time, a Cessna 182G, N3135S, was substantially damaged when it collided with terrain following a loss of control on final approach at Portland International Airport, Portland, Oregon. The private pilot and a pilot-rated passenger received minor injuries. The 14 CFR 91 flight originated at Redmond, Oregon and was not on a flight plan. Visual meteorological conditions prevailed at the time of the accident.

N3135S was a visual flight rules (VFR) arrival to Portland (a Class C airspace area) and was inbound to the airport from the southeast. According to a transcript of the Portland Tower local control position communications for the accident time period, Portland Tower originally instructed the pilot of N3135S to enter a straight-in for runway 3, upon N3135S's initial check-in on frequency at 1923:34, then one minute later amended N3135S's clearance, instructing the pilot to enter a left base for runway 28R and telling him to "keep your speed up." At 1925:34, the controller told N3135S to "proceed direct towards runway two eight right you're following a reno air [sic] m d eighty on four mile final." The pilot acknowledged the instruction and stated that he had the MD-80, which was on a straight-in final for runway 28R, in sight. Portland radar data indicated that at the time the controller issued the instruction to N3135S to proceed direct toward runway 28R to follow the MD-80, the separation between the Cessna and the MD-80 was between 2.3 and 3.6 nautical miles.

At 1925:47, the controller told the pilot of N3135S: "...keep your speed up follow that reno air [sic] m d eighty caution wake turbulence runway two eight right cleared to land wind three two zero at eight." The controller cleared the pilot of N3135S to resume normal speed on final at 1926:26; the pilot replied "okay slowing down to final." According to the controller, he then observed that the MD-80 might need the full length of the runway for its landing rollout, and therefore instructed N3135S to square his turn to final; and when the pilot of N3135S did not acknowledge this instruction, he canceled N3135S's landing clearance and instructed him to overfly the runway.

The transcript showed that the instruction to N3135S to square its turn to final was issued at 1927:11 and was acknowledged, but an FAA air traffic control (ATC) report of the accident, submitted with the transcript, indicated that the pilot of N3135S transmitted the acknowledgment on Portland Approach frequency. According to the transcript, the controller canceled N3135S's landing clearance at 1927:44. Thirteen seconds later, according to the FAA ATC accident report, N3135S was observed to bank sharply left and crash in the grass on airport property, southeast of the approach end of runway 28R.

According to a Port of Portland Police report of the accident, the pilot told a responding officer that his aircraft encountered the MD-80's wake turbulence at "a couple hundred feet" above the ground. The pilot indicated on his NTSB accident report that no mechanical malfunction or failure was involved, and recommended "further separation by control towers" as a way that the accident could have been prevented. The passenger also stated that the aircraft encountered the MD-80's wake turbulence and expressed an opinion that "ATC chose to put us in a poor position for [approach and] landing" while also remarking on his statement to the NTSB that "we should have pulled out of the pattern before the situation became critical."

Portland Approach radar data submitted by the FAA indicated that N3135S angled in directly toward the runway 28R threshold from the southeast, as initially instructed by the tower controller at 1925:34, for approximately the final two minutes of flight. At 1926:42 (the time of the last return from the MD-80), with the MD-80 on approximately 1/2 mile final for runway 28R at 300 feet, N3135S was approximately 0.9 miles from the MD-80 at the MD-80's seven o'clock position and was at 500 feet. At 1927:01, N3135S was 1 mile from the runway threshold, 1/4 mile left of centerline at 300 feet. At 1927:19 (8 seconds after the controller's instruction for N3135S to square its base turn), N3135S was recorded on 1/2 mile final, at approximately the MD-80's 1926:42 position, at 100 feet (200 feet below the MD-80's altitude at that same position). A final radar position was recorded on N3135S at 1927:42 (two seconds before the controller canceled N3135S's landing clearance), approximately 1/4 mile east of the runway threshold with no altitude reported.

Per FAA Order 7110.65J, Air Traffic Control Handbook, ATC controllers are required to separate a small aircraft landing behind a large aircraft on the same runway by 4 miles, to ensure wake turbulence separation, when radar separation is being provided under instrument flight rules (IFR). This separation distance (for small aircraft landing behind a large aircraft on the same runway) also applies to radar separation between VFR and IFR aircraft in class C airspace. When visual separation is being utilized, there are no minimum distance criteria specified in 7110.65J for landing aircraft wake turbulence separation. For visual separation, 7110.65J instructs controllers as follows with regard to wake turbulence separation:

Aircraft may be separated by visual means...when other separation is assured before and after the application of visual separation. To ensure that other separation will exist, consider aircraft performance, wake turbulence, closure rate, routes of flight and known weather conditions....Visual separation may be applied between aircraft under the control of the same facility within the terminal area, provided...[a] pilot sees another aircraft and is instructed to maintain visual separation from the aircraft as follows: (a) Tell the pilot about the other aircraft.... (b) Obtain acknowledgment from the pilot that the other aircraft is in sight. (c) Instruct the pilot to maintain visual separation from that aircraft....

7110.65J also instructs controllers, in all separation environments, to "Issue cautionary information to any aircraft if in your opinion wake turbulence may have an adverse effect on it...." although it states that the controller is not responsible for anticipating the existence or effect of wake turbulence.

The FAA's Aeronautical Information Manual (AIM) states that acceptance by a pilot of ATC instructions to follow an aircraft "is an acknowledgment that the pilot will ensure safe...landing intervals and accepts the responsibility for providing wake turbulence separation." However, AIM paragraph 7-3-9, "Air Traffic Wake Turbulence Separations," also states that "because of the possible effects of wake turbulence," controllers are required to apply no less than 4 miles separation, measured at the time the preceding aircraft is over the landing threshold, to small aircraft landing behind large aircraft. Paragraph 7-3-9 of the AIM does not distinguish between visual and radar separation environments, nor between VFR and IFR, in stating that controllers are required to apply this minimum separation; i.e. AIM paragraph 7-3-9 does not explicitly state that the minimum wake turbulence separation distances for landing given in that paragraph apply only to a radar separation environment. Subparagraph 7-3-6b.1 of the AIM states that for aircraft landing behind a larger aircraft on the same runway, the pilot should: "Stay at or above the larger aircraft's final approach flight path-note its touchdown point-land beyond it."

The Air Traffic Operations and Procedures Division (ATO-120.2) at FAA headquarters in Washington, D.C., which is the FAA office of primary interest (OPI) responsible for paragraph 7-3-9 of the AIM, was contacted and queried regarding the lack of distinction in the paragraph between radar and v