The student pilot was making a routine approach in light wind conditions. A Boeing 757 had just landed on the parallel runway when, according to witnesses, the Cessna 152 rolled inverted and was hurled to the ground. It was as if a giant invisible wave had rumbled through the approach corridor and singled out the hapless trainer. The suspicion is wake turbulence. It has been a deadly nemesis for big and small aircraft alike.
The phenomenon was discovered in the late 1960s and studied in some detail by both the FAA and NASA. Before that time it was frequently referred to as prop wash or jet wash, a by-product of the engines. Actually, it is produced by all aircraft as soon as the wing begins to generate lift. Higher-pressure air beneath the wing seeks to move to the lower-pressure area above the wing. It curls around the wing tips to form a vortex. The hazard comes from induced roll which, when encountered by a trailing aircraft, can exceed roll control authority even with maximum aileron deflection. Just another bit of irrelevant aerodynamics to memorize for the knowledge test and the oral? Hardly. Since 1982 there have been at least 56 accidents attributed to the deadly wake.
Wherever the big iron flies you can expect to find disturbed air that produces anything from a very hard jolt to an uncontrollable roll that may result in a dirt bath. While you may think that it's only the big guys messing up the air for light aircraft, it is a significant problem for them as well. Eight business jets and air carrier aircraft were listed in NTSB's accident database as having been victims of wake turbulence.
The AOPA Air Safety Foundation works closely with NASA's Aviation Safety Reporting System (ASRS) which has been tracking this issue for some time, along with the FAA. ASRS's latest study, published late last year, tallied 166 wake turbulence encounter reports since April 1995. It is probable that many more encounters went unreported. So, this invisible bogey man is real and not entirely predictable.
Some fascinating numbers come out of this. The Boeing 757 was identified as the wake-producing aircraft 22 percent of the time. The Boeing 727 and Boeing 767 were each identified in 12 percent of the cases. Surprisingly, monster machines such as the Boeing 747, DC-10, and Lockheed 1011 seem to generate relatively few encounters, possibly because everyone knows to fear the horizontal tornadoes that heavies produce. They are treated differently by air traffic control, and pilots tend to be more cautious around the jumbos. But there are exceptions. An MD-11, successor to the DC-10, encountered wake at 100 feet from a 747 landing on a parallel runway. With wing rock and pitch excursions, the pilot attempted a go around but not before the tail of the MD-11 struck the runway. The aircraft suffered substantial damage.
You've probably heard the lecture on wake turbulence but a quick review is in order since there have been several recent accidents involving light aircraft. The avoidance procedures, while simple to describe, are not always so easy to practice.
Consider some unsettling things that NASA found recently. Forty-three percent of the encounters occurred when wind speeds were estimated at 10 knots or less. Remember that vortices sink; so if you're following at the appropriate distance and are on or above the ILS glideslope, there should be no problem — right? Sixteen percent of the encounters occurred on the glideslope and twenty-two percent happened above the glideslope. Only six percent were in the supposed danger zone below the glideslope.
There are relatively few encounters under instrument meteorological conditions since ATC is always left holding the bag for aircraft separation on that. In NASA's study 94 percent occurred under VFR weather conditions. In 63 percent of these encounters, there was no ATC warning about wake. The Aeronautical Information Manual is clear as to who is responsible under VFR. Once the pilot has received traffic information and instructions to follow an aircraft and has accepted a visual approach clearance, it is the pilot who will ensure safe takeoff and landing intervals.
Watch yourself. Most times ATC does an adequate job of keeping aircraft separated for wake, but as soon as the warning "Caution wake turbulence" is issued, the monkey climbs onto the pilot's back. Not long ago I had business in Atlanta at Hartsfield International Airport. When departure time came, the Beech Bonanza that I was flying was wedged into the lineup of 757s and MD-80s. I was asked to taxi into position and hold. As soon as the MD-80 ahead of me was airborne, the tower advised, "Bonanza Three-Six-Whiskey cleared for takeoff, maintain runway heading, caution wake turbulence." I asked for a right turnout to avoid that critical area that we've just been discussing — below and behind a large aircraft. The clearance was "Negative, maintain runway heading." My response probably earned a few choice comments from the tower. "Negative, I need an immediate right turn for wake avoidance. We'll wait right here." There was a pregnant pause, but not for long, as the Bonanza was sitting on a prime piece of operational real estate. Grudgingly, the tower granted a right turn and we were on our way, well clear of the roiling air that lay straight out. It was the only way to go.
The Cessna 152 accident cited earlier occurred this summer and was well documented by witnesses. A senior flight instructor and a student were following closely behind the accident aircraft. There were five or six other aircraft in the pattern, so the GA pattern was packed and the tower was very busy. There was a right crosswind of about 10 knots, which carried the wake of the landing 757 on the nearby parallel runway into the 152's arrival path. The witness aircraft was deliberately high, and its occupants estimated the accident aircraft to be tracking the VASI. The tower had issued visual separation and wake warnings, which the accident pilot acknowledged. At about 150 feet agl the Cessna pitched up sharply and rolled inverted, slamming into the ground. The entire incident lasted less than three seconds.
There are two other recent mishaps involving light aircraft. A Beech Baron 58 inbound for Port Columbus International in Columbus, Ohio, on a Part 135 cargo flight was cleared to land behind a 757 on a night visual approach. The approach controller advised the pilot twice about wake from the Boeing and issued a speed restriction not to exceed 170 knots. The Baron was estimated to be 3.5 to 4 miles behind the Boeing, and the wind was calm. The tower issued landing clearance and also issued a wake warning. A witness said, "As the first airplane landed, the second one was still high. It seemed to more or less maintain this altitude until shortly before the runway, when it descended down to the runway to land...the airplane rolled 90 degrees perpendicular to the runway...." The Baron impacted 152 feet beyond the approach end of the overrun and slid another 1,000 feet down the runway. It was the pilot's very first flight under Part 135; he had 1,250 hours total time and 30 hours in twins, but only three as PIC in a twin. How much better the outcome would have been if the pilot had aimed for the end of the touchdown zone rather than for the numbers.
Another accident involved a Cessna 182 at Portland, Oregon, that was following an MD-80 under VFR conditions. According to the transcript, the 182 pilot was first told to keep his speed up, cautioned about wake turbulence, and cleared to land. About 30 seconds later the tower controller asked for normal speed from the 182 and then, upon observing that the MD-80 was still on the runway, requested the flight to square its turn to final. Shortly after that, landing clearance was canceled but the pilot failed to acknowledge. The 182, according to the pilot, encountered the MD-80's wake "a couple of hundred feet" above the ground. When asked by the NTSB how the accident could have been prevented he suggested "further separation by control towers." The pilot-qualified passenger stated, "ATC chose to put us in a poor position for approach and landing" and "We should have pulled out of the pattern before the situation became critical."
Obviously, from a light aircraft perspective, all large aircraft command respect, with special mention to heavies and the 757. A NASA study suggests that the 757's vortices are particularly well formed and thus may hang together longer than other aircraft of similar or heavier weight. There are many who believe that the 757 should be treated as a heavy, even though it doesn't qualify by weight.
Several years ago, the FAA reclassified the aircraft weight categories and succumbed to industry pressure to keep the 757 as a nonheavy. It wound up in a pseudo-heavy class that doesn't require quite the same spacing as a full heavy but probably should — particularly in light of the number of accidents that seem to follow the 757. Additionally, the definition of a large aircraft was changed from the traditional 12,500 pounds to 41,000 pounds to accommodate many commuter aircraft and some medium jets. That means that a Cessna 150 and a Falcon 50 are both considered small aircraft. Obviously, this becomes a capacity issue at busy locations, since the airport acceptance rate is much higher with less separation. Balancing safety versus economics is never easy. The scales seem to tip toward economics until there is a major mishap, when it shifts the other way — until the memory fades.
NTSB has been concerned about wake for some time. It has suggested that the IFR separation standards be applied to visual approaches, particularly when the lead aircraft is large or heavy and the following aircraft is small.
For departing aircraft there are some time or distance intervals used to separate other aircraft from the wake of heavy or 757 aircraft: Two minutes or four- to five-mile separation when the following aircraft will depart from the same threshold; from a crossing runway where projected flight paths will cross; or from a nearby parallel runway. Controllers may not reduce or waive these intervals.
But suppose you're number one to follow a "garden variety" 727 or an MD-80? Is there any time interval required? Sometimes. Departing from the end of the runway you may get an immediate departure clearance and a "Y'all be careful of the wake out there," as was my situation in Atlanta. For a small aircraft departing from the intersection behind a large aircraft, the controller must provide a three-minute delay, which may be waived by the pilot. Is it safe to do that? Sure, as long as you can be airborne well before the jet's rotation point and can make an immediate turn to stay clear of the departure path. If you're not sure that you can avoid the wake, enjoy the view for three minutes and then be on your way.
Finally, consider that wake from light aircraft may be a possible hazard. A departing Piper Cherokee Six managed to slam a trailing Cessna 150 back onto the runway with damage. Some days it just doesn't pay to get out of bed.
To test your knowledge on wake turbulence go to ASF's Web site.
The Aeronautical Information Manual recommends:
Landing behind a larger aircraft — same runway — stay above the flight path and land beyond its touch down point.
Landing behind a larger aircraft on a parallel runway that is closer than 2,500 feet — same as above.
Landing behind a larger aircraft — crossing runway — cross above the larger aircraft's flight path.
Landing behind a departing aircraft — same runway — land well before its rotation point.
Landing behind a departing aircraft — crossing runway — note the rotation point, and if past your intersection — continue the approach. If rotation is prior to the intersection either abandon the approach or land well before the intersection.
Departing behind a larger aircraft — same runway — note rotation point and become airborne prior to it. Adjust flight path — preferably upwind to avoid flight below the large aircraft's path.
If a large aircraft is conducting a low approach, missed approach, or a touch and go on or near your landing runway — consider abandoning your landing or takeoff.
En route VFR altitude — avoid flight below and behind a large aircraft and adjust flight path as necessary.
See also the index of "Safety Pilot" articles, organized by subject. Bruce Landsberg is executive director of the AOPA Air Safety Foundation.