Wake Turbulence Avoidance through Situational Awareness

Background

Before the advent of jet engines, pilots would sometimes experience turbulence or even an upset when flying in the vicinity of a larger airplane. This disruption of the air was attributed to the action of the propellers and was termed "prop wash." Then came the jets. Mysteriously the prop wash phenomena still existed even though there were no props to churn the air. Research quickly revealed that swirling vortices are produced by everything that flys by means of creating lift. That includes birds, gliders, helicopters, airplanes. The high pressure from under the lifting surface (wing or rotor blade) spills from under the tip and attempts to fill in the low pressure created on the top of the lifting surface.

Our purpose here is not to present the whole concept of wake turbulence and its avoidance. Much material has already been produced toward that goal. Anyone desiring a definitive and practical publication on the subject should click here to download the FAA's Advisory Circular AC 90-23F.

But, even with all that has been written about wake turbulence and its avoidance, including test questions on FAA examinations, pilots still encounter these vortices. The wake is invisible air traffic controllers are required in some circumstances, to maintain separation between airplanes. In other circumstances, controllers are simply required to provide a warning to pilots such as, "Caution, wake turbulence arriving seven-thirty-seven." Of course, when not using the ATC system and at operations at non-towered airports, pilots are mainly on their own. Sometimes the consequences of a wake turbulence encounter are catastrophic.

The Psychology of the Threat

As children, we were afraid of the things that we couldn't see like monsters under our beds. Somehow, as we transitioned into adults, we stopped fearing what we couldn't see or at least we stopped reacting to those fears. As pilots, we fear those big, dark cumulus clouds and avoid flying into them. But unseen threats like wingtip vortices somehow seem less ominous. As children, each time mom or dad checked under our bed and proclaimed the absence of monsters, we were a little more convinced that maybe those monsters weren't really there. Each time a controller issues a wake turbulence advisory and we avoid the wake without taking any evasive action we are reinforced that the threat wasn't really there. That makes it just that much easier to ignore the threat the next time. Unfortunately, unlike the imaginary monsters under our beds, the threat posed by wake turbulence is very real.

My Personal Experience

I acquired a healthy respect for wake turbulence as a young flight instructor in the early 1970s. flying westbound between the UCA and SYR VORs at 4,500 feet in a Cherokee 140, my student and I observed a B-52 descending into Griffiss AFB. Having operated in that area extensively, I was used to seeing the B-52s and KC-135s of the Strategic Air Command flying about. Normally, the "heavies" remained high until north of the New York State Barge Canal (now renamed the Erie Canal) and general aviation stayed to the south. On this day, the B-52 was descending through about 7,000 feet and was about 5 miles ahead of us. Note that this was before the days of flight following and that our Cherokee didn't even have a transponder. As a flight instructor I had been taught the concepts involved in wake turbulence and the necessity of its avoidance.

I had even taught the subject in ground school myself. Somehow, I was of the mind set that one only needed to be cautious of wake turbulence when taking off or landing. I was completely oblivious to the fact that it might cause problems at altitude. Was I ever in for a surprise! As my student and I watched the huge airplane make its graceful descent, we inadvertently flew into its wingtip vortices. Without warning and without turbulence, our Cherokee was rolled to the left about 400 degrees. Yes, we rolled more than once completely around. Just as suddenly as it happened, we were once again flying under control but with about a 40 degree bank to the left. Not waiting for the student to react, I grabbed the controls and brought the wings back to level. We never stalled and we didn't stress the airplane. The entire event was over in probably less than 10 seconds. We were back in smooth air as if nothing had happened. We had lost only about 300 feet of altitude. Fortunately, my first and only serious encounter with wake turbulence happened at altitude and at cruise airspeed. Had we stumbled into that wake turbulence at low airspeed while in the traffic pattern, on final approach, or on climb out, I'm sure we would not have been so fortunate.

Situational Awareness

The key to avoiding dangerous encounters with the invisible wake turbulence is to maintain situational awareness. Most pilots know, or at least knew when they took their most recent FAA Knowledge Examination, that the wake turbulence is found below, behind, and downwind of the generating aircraft. Given a testing situation where provided with a an airport diagram, showing the generating airplane and the prevailing wind, almost any pilot could identify the location of the wingtip vortices. The problem becomes more difficult when taken out of the classroom or testing environment. It is complicated by all of the other duties associated with executing a departure or arrival in the terminal area.

Good situational awareness includes many aspects, one of which is the relative position of other traffic. It is a natural tendency to ignore traffic that is clearly not a collision threat. But, that traffic might still be a threat with respect to wake turbulence. One way to improve situational awareness regarding this threat is to take a two-step approach to analyzing the traffic. First, consider if it is a collision hazard and second to consider if it might be wake turbulence threat. The added second step only takes a couple of seconds and might be a life saver.

Accident Analysis

The following example illustrates how a routine flight, conducted by an experienced and otherwise competent pilot can end in disaster by not making wake turbulence avoidance part of the overall situational awareness. On July 2, 2003, at about 10 AM local time, a Beech Baron 58 was flying from Tampa to Memphis. The weather at Memphis consisted of scattered clouds at 3200 feet and a broken layer at 3800 feet. The visibility was 10 miles and the wind was from 290 degrees at 5 kts. As the Baron entered the Memphis terminal area, an EMB ERJ 145 regional jet was also in the area and on the approach to Memphis. The regional jet was to land on Runway 36 Center and the Baron was to land on Runway 36 Right. The Baron pilot was given a vector to intercept the Runway 36R localizer and was told that he would be following an EMB ERJ 145 and that he would be four miles in trail. He was also given the warning, "Caution wake turbulence." The Baron pilot was cleared to land on runway 36R by the tower and again told, "Caution wake turbulence." The pilot apparently disregarded the two warnings of possible wake turbulence which were provided by approach control and the control tower. The deadly wake was blown by the light wind across the approach end of Runway 36R. The regional jet had landed and turned off the runway by the time the Baron pilot approached the runway. It is entirely possible that the Baron pilot never saw the regional jet. Simulated pilot's view from MS Flight Simulator® According to witnesses, the Baron's approach appeared to be completely normal until it descended to an altitude of about 15 feet. Simulated approach from MS Flight Simulator® Suddenly and apparently without warning, the airplane began a sharp roll to the left. Simulated roll from MS Flight Simulator® According to the NTSB accident report, the passenger seated in the second row right-side seat of the accident airplane stated the airplane approached the runway. The passenger looked down inside the airplane during the approach. He stated that when he looked back out the window, he could see the runway off to the right side of the airplane, and that the airplane had turned. He stated the next thing he remembered was hearing the airplane hit the ground. Simulated pilot's view from MS Flight Simulator® The airplane rolled inverted and impacted the grassy area to the left of Runway 36R. The pilot and front seat passenger received fatal injuries and the two rear seat passengers received serious injuries. Official NTSB photo Summary So how could an accident such as this one be prevented? First, pilots must believe that wake turbulence is a very real danger even though it can't be seen. Second, pilots must not assume that ATC will provide adequate separation from the dangerous wingtip vortices. Wake turbulence avoidance remains the responsibility of the pilot. Third, as stated earlier in this article, a keen sense of situational awareness must be maintained and it must include an awareness of possible wake turbulence. Official NTSB photo Click here for a short refresher course (5 minutes) on avoiding wake turbulence.