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 visual separation for wake turbulence. The ATO-120.2 representative stated to the NTSB investigator that no distinction had been made because FAA Order 7110.65J required wake turbulence separation distances to be applied for landing in all cases. The ATO-120.2 representative cited a note at the end of Order 7110.65J, paragraph 7-8-3 ("Separation", under "Class C Service-Terminal"), as the basis for this requirement. The note, which follows subparagraph 7-8-3c of Order 7110.65J, states: "Apply the provisions of para 5-5-3 [the radar separation distances specified for wake turbulence], when wake turbulence separation is required." However, subparagraph 7-8-3c pertains to separation using radar systems. By contrast, subparagraph 7-8-3a, which contains instructions for visual separation, has a note immediately following which states, "Issue wake turbulence cautionary advisories in accordance with para 2-1-20."

The editor of Order 7110.65J at FAA headquarters was subsequently queried regarding the intent and applicability of the note following subparagraph 7-8-3c. The editor stated that per FAA's editorial standards for its publications, which dictate that notes are inserted immediately following the applicable subparagraph, the note applied only to subparagraph 7-8-3c and did not broadly apply to the entire paragraph.

The NTSB issued a special investigation report in February 1994 concerning wake turbulence accidents ("Safety Issues Related to Wake Vortex Encounters During Visual Approach to Landing", NTSB/SIR-94/01, Notation 6264, Feb 94.) In the section of its report entitled "Air Traffic Control Procedures Related to Visual Approaches and VFR Operations Behind Heavier Airplanes", the Safety Board stated: "...controllers sometimes issue visual approach clearances when the separation distance and closure rate preclude the pilot from maintaining a safe separation distance without excessive maneuvering....the Safety Board believes that controllers should be provided annual refresher training related to wake turbulence separation and advisory criteria. The training should emphasize the need for controllers to avoid using phrases or terminology that would encourage pilots of VFR or IFR airplanes to reduce separation to less than that required for IFR operation, thereby increasing the chance for a wake turbulence encounter when operating behind a turbojet airplane." In a footnote to this passage, an example given of such terminology was "keep a tight pattern and follow the large airplane."

The same section of the NTSB report also stated: "...a pilot, if provided with a wake turbulence cautionary advisory and other information relevant to the avoidance of wake turbulence, such as separation distance and the existence of an overtaking situation, would be better able to maintain an adequate separation distance. Thus, the Safety Board believes that controllers should be required to provide this information, as a minimum, to pilots prior to allowing visual operations behind or in-trail of heavier, turbojet airplanes."

The findings of the NTSB report included: "Current air traffic control procedures and pilot reactions can result in airplanes following too closely behind larger airplanes while on a visual approach to landing", and "Pilots of arriving visual flight rules airplanes...often do not have sufficient information to maintain adequate separation distances or to determine relative flight paths." As a result of these findings, the Board issued, among others, the following recommendations to the FAA:

A-94-48: "Amend FAA Handbook 7110.65H, Air Traffic Control, to require that arriving visual flight rules airplanes that have been sequenced behind a heavier turbojet provided...information relevant to the avoidance of wake turbulence, such as separation distance and the existence of an overtaking situation. (Class II, Priority Action)"

A-94-50: "Develop annual refresher training for air traffic controllers regarding wake turbulence separation and advisory criteria. The training should emphasize the need for controllers to avoid using phrases or terminology that would encourage pilots of visual flight rules...airplanes to reduce separation to less than that required during IFR operation, thereby increasing the chance for a wake turbulence encounter when operating behind a turbojet airplane. (Class II, Priority Action)"

A-94-51: "Expand the current guidance in the Airman's Information Manual and develop other training material to help pilots to determine that their flight path remains above the flight path of the leading airplane and that their separation distance remains consistent with that required for instrument flight rules operations. (Class II, Priority Action)"

At the time this report was submitted, all three of the above recommendations were classified by the Safety Board as "Open-Acceptable Response."

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