I was at the tail end of three months of training for my CFI reinstatement check ride. My instructor Kevin and I were flying into a deepening Pacific Northwest twilight as we descended westbound over Lake Washington toward Boeing Field/King County International Airport (BFI). We began maneuvering to enter right traffic for Runway 31R in four-one Romeo, one of our flying club's Cessna 172RGs. The wind was almost directly straight down the runway at five knots, and the air was perfectly smooth.
I had slowed and performed a GUMP check before we crossed the shoreline descending to the traffic pattern altitude of 1,000 msl on a 45-degree entry onto downwind. Boeing Field's twinkling runway lights in the valley ahead slowly slid into view at our two o'clock as we began clearing the high Mount Baker bluff. From my position in the right seat, I could see a heavy air cargo Boeing 767 rotating about halfway down Boeing's 10,000-foot-long Runway 31L and pitching into a steep initial climb. I recall briefly thinking, "hmm ... ." The rotation point was adjacent to the approach end of the general aviation 3,710-foot-long Runway 31R on which we were cleared to land. That, plus the fact that the runways were very close together, made little alarm bells go off in my head. However, I quickly became absorbed in setting up our downwind leg, following landing traffic ahead and setting up for the 180-degree power-off accuracy landing Kevin suggested. No worries, though. Air Traffic Control would keep us out of trouble. With the mix of Boeing Company experimental and production flight tests, heavy jet commercial cargo, corporate jet transport movements and lots of general aviation flight training to deal with, Boeing Field controllers are justifiably fastidious about wake turbulence. Like all light-airplane pilots at BFI, Kevin and I had spent a lot of time waiting out wake separation delays and performing go-arounds at pattern altitude.
This was my first attempt at a power-off landing in the Cutlass, so I turned base with plenty of altitude. As I turned final, I still looked high, but I thought the RG's draggy full-idle descent rate with full 30-degree flaps would save my bacon. The sight line didn't get any prettier, though, even after I started holding full-forward slip controls. On short final, I was still way high, and disapproving noises began coming over the intercom from Kevin. I glanced across the cockpit to the airspeed indicator to see it quivering around 85 knots, a good 20 knots too fast. I was just about to say "this isn't working" when Kevin beat me to it and said, "Let's go around." We were about 200 feet above ground level. At that instant, something abruptly shoved the airplane to the left. I think Kevin and I glanced at each other with a thought something like, "What the heck was that?" Then, in a flash, the airplane snapped into about a 90-degree left bank and pitched down. We were suddenly headed for the ground in a shallow dive perpendicular to the parallel runways. This was even though Kevin and I were by now frantically holding full right aileron, full up elevator and had the right rudder buried.
What we were experiencing, of course, is dryly referred to as a "wake turbulence upset event sequence," but believe you me, that phrase doesn't begin to do justice to the experience. It is surreal to be flying an airplane that suddenly goes into an unusual attitude and then doesn't respond to control inputs. I couldn't believe what was happening and remember thinking incredulously that "this is it!" Then, at about 50 feet agl, the airplane seemed to squirt out of whatever had seized it and we finally were able to roll out and pull up. The whole episode may have lasted five seconds, but as the old saying goes, it seemed like an eternity.
Somewhere along the line, I had relinquished the controls to Kevin. As he turned back onto runway heading and began climbing to the accompaniment of a lot of verbal high-fiving at our miraculous escape, I radioed Boeing tower that four-one Romeo had experienced a wake turbulence event and was going around. Apparently, in the gathering darkness, no one in the tower cab had seen our extreme gyrations. The controller sounded surprised and asked for more information "when able." Kevin chimed in with a brief statement that we'd had a serious problem, and the controller then warned another aircraft on final to 31R.
As he flew the pattern, I told Kevin about the heavy jet that I had seen rotating on 31L as we approached Boeing Field. I hadn't mentioned it previously, and from his position in the left seat as we were on the 45-degree entry, he had not been able to see it. Given the option to land on either 31L or 31R, Kevin prudently elected to land on the left runway. We slowed, turned off the high-speed runway and taxied over to our flying club's ramp, still marveling at our survival. After we shut down, Lellius, one of the club's instructors, was there to greet us with an incredulous look. He had been performing a preflight check and had witnessed the whole episode up close as the approach path for 31R is almost directly above our ramp. He confirmed that we indeed instantaneously rolled left but that it looked like it was beyond a 90-degree bank as we headed down toward the left runway before we recovered.
The evidence certainly points to our having flown into the departing 767's wake vortex, and then having been fortunate enough to exit the vortex in time to recover. The literature says that higher-pressure air rolling up into lower pressure air along wingtips and outboard sections of flaps forms wake vortices. Wake vortices on the right side have a counterclockwise rotation, and an air mass rotating to the left is consistent with our uncommanded left bank. Why didn't established wake-avoidance procedures work? When Kevin telephoned the tower after we landed, they told him that it had been more than three minutes since the heavy jet had departed, and besides, the wake turbulence delays were for takeoffs, not landings.
Since they generally descend at about 500 feet per minute, any wake generated at rotation and initial climb by the heavy jet should have long since drifted to the ground and dissipated by the time we showed up. So, what happened to us? Two theories. One discussed in Advisory Circular AC90-23F is that persistent wake vortices may descend to the ground and then bounce up to twice their nominal steady-state height. The AC says the "bounce" height of an aircraft with a 200-foot wingspan could reach approximately 200 feet agl. (The Boeing 767 wingspan is 156 feet.) In our case, we were definitely high, thanks to my botched power-off landing attempt, and this theory sounds plausible. Another theory is that perhaps a vortex generated some distance ahead by the departing heavy 767 had drifted back and down in the gentle northwesterly breeze. Either scenario implies a vortex rotor that would have lost a lot of its rotational energy, though it may have started out with a peak tangential speed up to almost 300 feet per second. But then it would not have taken much rotational energy to exceed the roll rate of our RG. The wake encounter that banked and turned us 90 degrees induced rolling moments that initially exceeded our limited roll-control capability, but evidently dissipated as we were flung toward the outer perimeter of the decaying vortex. Otherwise, we could have gone inverted, and that would have been that. Needless to say, I have a greatly enhanced respect for the threat that wake vortices pose. You can bet that I will be more prone to give the stinky eye to any potential wake turbulence and happily will spend a few minutes waiting it out even on approach even if I have to do some 360-degree orbits.
Ironically, years ago as a newly minted private pilot, I experienced my first wake turbulence on the ground. I was standing under the approach end of Dallas Love Field's Runway 13L/31R on a still night when a Braniff BAC-1-11 twin jet roared overhead. As the berm at the end of the runway masked the sound of the landing jet, I first heard a shrill whistle and then a banshee wail from what I realized was settling vortices. Little did I know that, many years later, I would have a nearly deadly encounter with the same kinds of vortices at the approach end to another Runway 13/31.
Note: Bill "Bubba" Vaughn is a lifelong pilot who started flying at age 14. By profession, he's a risk manager with an extensive background in aviation insurance and aviation safety. Kevin is an aeronautical engineer with Boeing working on the 787 program. He had previously written a paper on wake turbulence.