Sight unseen

March 1, 1994

Wake turbulence, the invisible horizontal tornado that rolls off aircraft wings as a by-product of lift, has been a phenomenon as long as there have been aircraft. But in the early 1970s, it was a newly discovered phenomenon, and as widebody aircraft were brought into service, the threat became large enough to merit everyone's attention. In 1972, the wake from a Lockheed L-1011 rolled a trailing McDonnell Douglas DC-9, causing it to crash. Wake turbulence education was born, and after hearing about it ad nauseam ever since, it is apparent that some of us have become complacent.

In the past year, there have been at least four accidents/incidents involving aircraft flying behind Boeing 757s. Although the investigations are not all complete, there is considerable evidence that wake turbulence from the Boeings was the cause.

What's different about the 757? It's not a "heavy" — an aircraft with a gross takeoff weight of more than 300,000 pounds. Heavies are accorded more respect and space by air traffic control and most pilots because the wing-tip vortices they produce are known to be big and powerful. It now appears that something about the aerodynamics of the 757's very efficient wing allows it to produce "vortex tangential velocities greater than any ever recorded, including those of the Boeing 747 and Lockheed C-5A." This is according to a study done by the National Oceanic and Atmospheric Administration back in 1990.

One of the recent accidents involved a Cessna Citation that was bound for Billings, Montana. The Citation, with a crew of two and six passengers, was on an IFR flight plan. At 1626 Mountain Standard Time, the Citation (N6887Y) began descent into Billings, sequenced behind a United Parcel Service 757 (UPS 2956).

The weather at Billings was reported as 10,000 scattered, 25,000 scattered, visibility 40 miles, temperature 2 degrees Fahrenheit, and wind 350 degrees at 5 knots.

At 1636, the Citation was advised of the 757's position: 12 o'clock and 6 miles ahead. The Boeing was cleared out of 11,000, down to 5,700 about a minute later. Approach control requested the flight maintain 250 knots. At this point, the Citation was descending out of 16,000 feet.

At 1638, the 757 reported the airport in sight and was cleared for the visual approach to Runway 27R. The controller, recognizing that the Citation was gaining on the Boeing, again requested the 757 to maintain maximum forward speed. Approach then advised the Citation that the 757 was at 12 o'clock and 4 miles, descending out of 8,000. The Citation's first officer responded that they were looking for the traffic.

The subsequent conversation from the Citation's cockpit voice recorder and the ATC tapes tells the story. (Intra-cockpit communication in the Citation: CAM [cockpit area microphone] -1 = captain, CAM-2 = first officer; 87Y = radio transmission from the Citation; UPS = the 757; APP = approach control; TWR = Billings Tower.)

1639:40 — CAM-2: Tell 'em we have the airport in sight?
1639:42 — CAM-1: Huh?
1639:43 — CAM-2: Tell 'em we have it in sight?
1639:45 — CAM-1: I don't care.
1639:56 — CAM-2: Oh, there's the traffic.
1639:58 — CAM-1: Got 'em.
1639:59 — CAM-2: Da da da dum.
1640:04 — CAM-1: Did ya tell 'em we got the traffic?
1640:06 — UPS: UPS, ah, 2596 we need to start slowing down now.
1640:12 — APP: I'm sorry, I stepped on somebody. [Approach and UPS had transmitted simultaneously and blocked the frequency.]
1640:14 — UPS: Yeah, UPS 2596, we got to start slowin' up now.
1640:17 — APP: UPS 2596, roger. Just keep your speed up as much as you can sir. If you have to slow down that's all right.
1640:23 — UPS: Okay.
1640:25 — 87Y: Billings Approach, Citation 87Y has our traffic.
1640:28 — APP: Citation 87Y, roger. The 757 is on the visual approach, follow him. Cleared visual approach Runway 27 Right.
1640:35 — 87Y: 87Y is number two for the visual 27 Right.
1640:43 — APP: Citation 87Y, do you have the runway in sight also or not?
1640:47 — 87Y: That's affirmative, I have the airport in sight.
1640:48 — APP: Thank you.
1641:03 — APP: Citation 87Y, the 757 is slowing. Groundspeed indicating 220 knots now.
1641:10 — 87Y: All right, 220 for the 757.
1641:39 — APP: UPS 2596, resume normal speed, contact the tower 124.2. Citation traffic following you has you in sight.
1641:46 — UPS: Okay, normal speed, going to the tower, UPS 2596.
1641:55 — CAM-2: What do they land at?
1641:56 — CAM-1: Huh?
1641:57 — CAM-2: What do you think they land at...a 757?
1642:01 — CAM-2: Quite a bit faster?
1642:02 — CAM-1: Oh, they probably got a ref speed of, oh, ah, 135.
1642:49 — APP: Citation 87Y contact tower 124.2.
1642:53 — 87Y: 24.2 for 87Y.
1642:55 — HOR 400 [another aircraft, Horizon 400, a regional airliner, reported on downwind to the tower]: Tower, 400 is with you, left downwind. We got UPS and, ah, the airplane behind him in sight.
1643:01 — Horizon 400, roger, the second aircraft is a Citation. Follow the Citation, cleared to land. Caution wake turbulence, wind 350 at 4.
1643:07 — HOR 400: 400.
1643:10 — 87Y: Billings Tower, Citation 6887Y with you, cleared for the visual, number two behind the 757.
1643:15 — CAM-1: Flaps, approach.
1643:16 — TWR: Citation 87Y, roger, ah, follow the 757. He's, ah, slowing down.
1643:21 — 87Y: 87Y.
1643:22 — TWR: 87Y, if you need to turn for, ah, spacing, turn to your right.
1643:27 — 87Y: All right 87Y, if we need it for space, we'll go to the right.
1643:30 — TWR: Roger, there's a Metro on a left downwind. He'll follow you. He has you in sight.
1643:30 — CAM [sounds similar to that of landing gear being lowered].
1643:35 — 87Y: 87Y.
1644:24 — CAM-1: Almost ran over a 757.
1644:34 — CAM-1: We could've slipped in before this guy.
1644:56 — CAM-1: Damn.
1644:57 — CAM [sounds similar to background (cabin) voices screaming].
1645:03 — [End of tape.]

The ground controller at Billings Tower stated that he was looking right at the Citation when it rolled inverted. He watched it roll from a shallow left bank to a shallow right bank. Then, as the aircraft began to roll back to level, the nose pitched up about 20 to 30 degrees, and the left roll rate increased rapidly until the aircraft was inverted. As the nose dropped to near-vertical, the Citation disappeared from view below the rim of the plateau on which the airport is located.

Another witness described the aircraft as "snapping real fast," similar to an air-show maneuver.

The Citation impacted in the middle of a street about 1.3 miles from the end of the runway and slid into a warehouse. There were no survivors.

The investigators were able to reconstruct the flight paths of the 757 and the Citation from radar data. The Citation was consistently faster for most of the approach as evidenced by the controller's constant reference to speed. Prior to the Citation calling the airport in sight, the distance between the two aircraft had stabilized at around 4.2 nautical miles.

After being cleared for the visual approach, the Citation's speed increased from about 215 knots up to 250 knots, making it about 70 knots faster than the 757 at one point. The speed differential then decreased to about 30 knots. The horizontal separation gradually decreased and was less than 2.5 miles at the time of the wake encounter.

The descent path of the 757 was relatively constant at about 3 degrees until approaching the glideslope. The Citation had a greater descent rate and converged on the 757's flight path. It crossed the 757's descent path about 6 miles from the runway and then stabilized about 200 to 300 feet below the 757's path.

We generally make the assumption that the preceding aircraft is going to track the glideslope. From radar altitude profiles and the flight data recorder, the 757 intercepted the glideslope from below, at about 6 miles out, and then, presumably to reduce speed, leveled off. The 757 was above the glideslope by several hundred feet until reintercepting about 1.2 miles from the runway threshold.

The Citation intercepted the glideslope from above, about 3 miles out, and tracked it until the wake encounter.

The Citation was indulging, perhaps, in the aeronautical equivalent of tailgating as noted by the captain's impatience toward the end of the tape. There seemed to be no concern that he was closing rapidly on a much larger, heavier aircraft. The approach controller made several speed references that indirectly indicated the Citation was overtaking the 757. The tower controller, when he mentioned that the 757 was slowing and that the Citation could make a turn to the right as needed for spacing, was also aware of the decreasing separation.

The only mention of wake turbulence by air traffic personnel was the tower's admonition to Horizon 400 who was on downwind following the Citation. It should be noted that pilots are responsible for safe takeoff and landing intervals under VFR when they accept instructions from ATC. This includes traffic information, instructions to follow another aircraft, and the acceptance of a visual approach. In this case, it might have been helpful for the controller to issue a wake advisory to the Citation.

That the 757 was above the glideslope is unfortunate but also not unexpected. The Citation captain did have visual contact and was expected to adjust his flight path. The National Transportation Safety Board determined the probable cause as "the [Citation] pilot not following wake turbulence avoidance procedures." In this accident, the Citation captain, no doubt having heard about wake turbulence many times before, either figured that his jet was immune or was not consciously considering the potential threat.

The Airman's Information Manual has an entire section devoted to wake turbulence, and a few reminders might be in order:

  • Avoid the area below and behind the generating aircraft, especially at low altitude, where even a momentary wake encounter could be hazardous.
  • Vortices from Transport-category aircraft will sink at several hundred feet per minute. It is generally thought that 1,000 feet vertical separation below the generating aircraft is considered safe. If possible, being above the lead aircraft's flight path solves the problem.
  • Strong winds and atmospheric turbulence hasten the breakup of vortices. Light winds may cause a vortex to remain or drift onto a runway and remain there considerably longer than normal. In the Billings scenario, note that the surface winds were very light. Surface wind was from 350, approach heading was 270. So, the Citation captain may have thought quartering headwind would push wake to south of his approach path.
  • Vortices move with the wind, so altering the flight path to the upwind side is good practice.
  • ATC will provide minimum separation between heavy and other aircraft as follows: Small/large aircraft landing behind a heavy jet — 5 miles. This can increase to 6 miles for small aircraft at the runway threshold. Small aircraft landing behind a large aircraft — 4 miles. Although the Citation II, Model 550, may not have technically been a small aircraft (less than 12,500 pounds), adhering to the separation standards is good procedure. However, as previously mentioned, pilots assume responsibility under VFR conditions.
  • A seemingly small aircraft, such as a Citation, Beech King Air, or Learjet, can generate dangerous wake for a similar-sized or lighter aircraft. There are several such accidents involving two small aircraft that have occurred in the past five years. For example, a Piper PA-18 was upset following a Piper Cherokee Six on takeoff.
  • On departure, don't plan to outclimb any jet even if the rotation point is several thousand feet down the runway ahead of you. Alter your flight path to the upwind side, and avoid crossing beneath the wake until there are several miles lateral and at the least 1,000 feet vertical separation.
  • Helicopters of any size generate a significant wake and deserve a wide berth. Be cautious of helicopters hovering during takeoff, landing, or taxiing.

The FAA recently issued an alert on the particular wake hazards of the 757 in a letter from the FAA administrator, David Hinson, to all pilots. Pilots of light aircraft should consider wake from almost any aircraft as a possible hazard and from any transport-type aircraft as a guaranteed hazard. When it comes to wake turbulence, what you can't see most definitely can hurt you. However, unseen does not mean unavoidable.

See also the index of "Safety Pilot" articles, organized by subject. Bruce Landsberg is executive director of the AOPA Air Safety Foundation.