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What you think you know about wake vortex behavior may well be wrong


Recently I had my own almost accident, which reminded me of a problem that is only going to get worse.

It was Thanksgiving Day and the traffic going into Denver International was normal for any holiday. Our Boeing 727 was cleared for the visual approach and landing behind an Airbus 319. Since the 727 is heavier than the Airbus, and since we had sufficient separation distance, the thought of a wake vortex encounter didnt enter my mind.

Just as I started to flare, we hit the Airbus wake and suddenly my wing dropped. It only takes 12 degrees of wing dip to strike an outboard flap during a normal landing in the 727. For a few moments, that wake had more control over the aircraft than I did. Luckily the encounter stopped just as quickly as it started and I managed to touch down with the wings level.

Officially, it was a non-event. No metal was bent, so in classic safety statistics, this flight was as safe as any other. However, it was close to bending metal and it certainly had my heart rate elevated for a minute.

As airports get more congested, this problem is going to get worse before it gets better.

Before you shake off wake vortices as being old news, recent research indicates that the vortex behavior is more complicated than most people think and wake encounters can pose a hazard to aircraft in situations that defy conventional wisdom. It isnt as simple as just staying far enough behind another aircraft and staying above its flight path.

A Decade of Debate
When I started looking at wake vortex research in 1990, air traffic delays were causing tremendous problems for the U.S. aviation industry and the FAA was looking for ways to decrease the distance between aircraft so it could increase the traffic flow for the limited number of runways. Environmental, economic, and other constraints had not allowed airports to increase capacity to meet demand. New runways and new airports werent being built.

Coupled with growing traffic, these constraints contributed to flight delays and extra fuel consumption. One study in 1991 estimated the cost of traffic delays due to wake vortices at Chicagos OHare International Airport exceeded $20 million annually. The average delay in 1990 was more than 10 minutes per flight. That translates into 2,000 hours of jetliner time per day or grounding 250 commercial transport aircraft.

Not much has changed since 1990, however. If anything, the problem is getting worse. There is serious pressure within the industry to move as many aircraft as possible, and this includes tightening up the space between aircraft near the runways. The FAA, NOAA, NASA, the airline operators and academics have discovered some data that indicate that some atmospheric conditions cause wake vortices to dissipate faster than expected, thus allowing the spacing to be safely decreased.

However, there are also conditions on the other side of the curve that increase the hazard. Former FAA Administrator Adm. Don Engen said at a wake vortex symposium in 1991 that much more is to be learned about the meteorological effects and certain conditions that may lead to exceptionally long-lasting vortices.

Scientists have presented several studies that show vortex hazards can persist longer and migrate significantly farther, under certain atmospheric conditions, than found in previous studies.

After a series of wake vortex accidents behind Boeing 757s, the NTSB concluded that most pilots had insufficient knowledge and training related to avoiding wake vortices. The NTSB also found that pilots are not provided adequate training related to the movement and avoidance of wake vortices or for determining relative flight paths and separation distances. The NTSB strongly recommended that pilot training material be updated to reflect this new knowledge.

The wake vortex problem is a combination of complex science, historical data, industry pressures, human factors and some procedures that have subtle pitfalls for the pilot. This isnt an easy problem with so many aircraft competing for precious runway space.

Over the next several issues, Aviation Safety will present a series of articles examining the problem, the research into wake vortex behavior, and the best way to cope with an inadvertent wake vortex encounter.

The behavior of wake vortices is not easy to understand – certainly not as easy as the AIM might lead you to believe. Still, there are some techniques that will help you avoid them. And once you understand what happens to your aircraft during an inadvertent encounter, you may look with new eyes on the current debate about recovery methods.

The Size of the Problem
The NTSBs database from 1983 to 1993 shows that at least 51 accidents and incidents were caused by wake vortex encounters. In those 51 events, 27 aircraft occupants were killed, 8 were seriously injured, and 40 aircraft were substantially damaged or destroyed. An unknown number of pilots have had experiences like my recent flight, where no metal was bent only through happenstance.

Accident investigators and researchers in the United States and Europe conducted studies of wake vortex encounters in the early 1990s. Those studies made it clear that heavier aircraft produce stronger wake vortices.

In a British study conducted at Londons Heathrow Airport, the Boeing 747 and 757 produced significantly higher incident rates of wake vortex encounters, while the DC-9, BAC 1-11, and B-737 were the most affected followers.

Consistent Results
An earlier study of 140 reports in NASAs ASRS database looked at wake vortex reports between 1983 and 1990. The typical lead (generating) aircraft was a heavy in 40 percent of the cases and a large aircraft in 53 percent. One surprising finding was that the typical trailing aircraft involved in an incident was a large aircraft 52 percent of the time, and a small aircraft in 36 percent of the incidents.

In that study, 25 percent of all reported incidents involved a large aircraft following another large; 22 percent involved a large aircraft followed by a heavy, and 21 percent involved a large followed by a small aircraft. In other words, more than half of all reported wake vortex encounters in the United States occurred between air carrier type aircraft.

Of course, its important to recognize the attributes and limitations of the ASRS database. ASRS reports are voluntarily submitted, and some individuals are more prone to filing reports than others.

It doesnt surprise me that the NASA ASRS database contains a high proportion of air carrier reports. Most air carrier pilots carry a NASA ASRS form in their crew bag and are simply much more likely to file a NASA ASRS report because any kind of safety incident could have career consequences. This observation would partly explain the higher proportion of air carrier reports. Secondly, air carrier pilots tend to fly into more crowded airports on a frequent basis, and thus are more likely to be exposed to a wake vortex encounter.

At face value, that may make it sound like the ASRS data indicates wake vortices are mainly an air carrier problem. However, think a little more and its clear there is practical meaning for all pilots. If heavy metal is getting bounced around by wake vortices, thats definitely bad news for those who like to fly light aircraft, especially in the vicinity of larger aircraft.

This database represents only the tip of the iceberg. Many more wake vortex encounters occur but just arent reported, either because they are not recognized as wake turbulence or the pilot decides to report it only as hangar talk around the airport.

A review of ASRS data from 1990 to 1999 I just completed showed roughly one quarter of the reports were submitted by pilots of small general aviation aircraft, while one fifth of the reports involved pilots in small transports, such as corporate jets and turboprops.

There are a few interesting lessons to be learned from this data. First, note that most of the encounters occurred when the trailing aircraft was flying behind an aircraft of equal or larger size. Thats consistent with previous thinking to watch out for wake vortices from aircraft that are heavier than what you are flying. But there are also some surprises.

Pilots flying light planes sometimes report the aircraft behaving oddly as they near the runway. Often this is chalked up to a wind gust, when it may be just as likely to have been wake vortices from a preceding light plane. Nearly one-fifth of the NASA ASRS reports involve aircraft mixtures of the same size. Do we need to be cautious when flying a Cherokee and trailing a Cessna 172? The answer is obviously yes.

Note that the pilots of several heavies reported encounters when trailing smaller transport aircraft. The number is relatively small when compared with the number of day-to-day air carrier operations, but it points out that occasionally the heavies will encounter a wake vortex from a smaller aircraft in such a manner that it will upset the larger airplane.

More Exposure Equals More Risk
Not surprisingly, the type of aircraft that seemed to produce the greatest problem for general aviation aircraft was the medium-large transports. Seventeen small general aviation aircraft reported wake vortex encounters behind medium large transports such as the DC-9, 737 and MD-80 during the period I studied. Since the 737 is the most common air carrier aircraft, its not too surprising.

The next most common category was the small transport. Ten pilots reported encounters behind small transports such as corporate jets and turboprops. This is significant because wake vortex separation criteria allows a transport weighing nearly 41,000 pounds to be spaced just three miles ahead of a small general aviation aircraft since both aircraft are now classified as small aircraft for wake vortex separation purposes.

Pilots who are upgrading into a small transport such as a corporate jet or turboprop may be interested to know that nearly half of the pilot reports of wake vortex encounters in that size of aircraft were following a large aircraft. An almost equal number were trailing heavy aircraft, and the remaining one quarter occurred behind aircraft such as the 737 and MD-80.

Common Risks
The earlier study of NASA ASRS incidents found that nearly 50 percent of the wake vortex encounters occurred during the approach or landing phases. In my review of more recent data, I found that roughly 50 percent occurred when the trailing aircraft landed behind another aircraft that was landing.

This was by far the most common combination. Clearly the ASRS data from both studies indicates that landing is the most problematic phase for wake vortex encounters. This is where aircraft tend to be jammed closer together, and this problem is only getting worse with airport congestion.

British researchers have gathered data from flight data recorders, ATC, radar data and the Meteorology Office to examine all reported wake vortex incidents occurring at Londons Heathrow Airport. The British study found a peak in the data at 100 to 200 feet above the threshold, and another broad band at 2,000 to 4,000 feet – where aircraft will typically level off until reaching the final approach fix. The British findings outline the cause for concern, because the chances of recovering from a wake vortex encounter at 100 to 200 feet agl are slim at best.

In the next installment, Ill describe some of the findings about wake vortex behavior such as rebounding vortices that will cast further concern about this phase.

My findings from the recent ASRS data backed up the British study in that a significant number of incidents occurred between decision height and the runway. Correlating the findings makes it clear that a high percentage of wake vortex encounters occur when the trailing airplane is landing behind another aircraft that just landed and has descended to a point very close to the ground.

The results are predictable. Seventy of the pilot reports indicated a temporary loss of control due to the wake vortex encounter. Forty-nine of the reports indicated that the encounter was severe or violent. Forty-two said the encounter was moderate.

While it may seem intuitive that landing would be so problematic, the next most common phase of wake turbulence encounters is not, as you might expect, during takeoff and initial climb.

You may not normally associate wake vortex encounters with cruise, but this is the second most common phase of flight for encounters. These encounters have seriously injured airline passengers and created substantial aircraft damage in some instances.

Part of the reason is that the vertical clearance has been reduced on some routes, such as some routes from North America to Europe. Aircraft flying 20 nm behind and 1,000 feet below another aircraft have sometimes experienced severe turbulence. Under most conditions, a vertical separation of 1,000 feet is considered to be safe by the AIM.

General aviation aircraft are exposed to the wake vortex hazard during cruising flight when their flight path passes through the wake of a transport climbing out or descending into a terminal area. In addition, airplanes overflying you can also pose a risk. Remember that IFR aircraft tend to fly on hard altitudes such as 6,000 feet, 11,000 and so forth, while VFR aircraft tend to fly the 500 foot levels, such as 5,500 feet, 10,500 feet and so on.

Dont forget that the wake vortices from an aircraft operating on an IFR flight plan that is just 500 feet above you can drift down to your altitude. You are expected to make slight changes in altitude or lateral position to maintain a sufficient distance from wake vortices.

Crossing Traffic Hazard
Normally you should scan the flight path ahead of you to avoid the wakes of preceding aircraft, but dont forget to watch the sides, above you and behind you. There are many situations in which you could cross the wake of an aircraft flying on a perpendicular course. It might surprise you to learn that some of the general aviation cruise encounters occurred when an airliner descended over them and the wake drifted down to their altitude.

My review and the earlier study of ASRS data found that up to 20 percent of wake vortex encounters occurred during cruise flight. There is a good reason for this.

Aircraft are in the clean configuration in cruise, thus they generally produce vortices that are more coherent, though not as strong. In addition, there is a general lack of turbulence at the higher cruise altitudes, and turbulence is one thing that helps break down vortices. Thus, the absence of turbulence during high altitude cruise creates an environment which maintains the strength and prolongs the lives of vortices.

Additionally, due to higher speeds in cruise flight, the in-flight transit time for a given distance is smaller. Therefore, even though aircraft are separated by larger distances enroute than during terminal operations, the faster speeds and possible longer lifetimes of vortices in the high altitude environment enhances the possibility of a wake vortex encounter.

The takeoff and climb-out phases of flight were the next most common phases. The majority of these involved the classic scenario of taking off behind another aircraft that had just departed.

Sixteen of the ASRS reports indicated that the pilot refused a takeoff clearance due to concerns about wake vortex duration. There really are pressures to move metal at busier airports, but these pilots stood fast and refused to be rushed into taking off when they were concerned about wake vortices.

Nine of the reports involved actual aborted takeoffs due to concerns about wake vortices. Dont forget that aircraft produce strong vortices during low approaches, missed approaches and touch and goes. If you are departing or landing behind aircraft during these operations, you are expected to adjust your flight path as necessary to preclude a wake vortex encounter. Intersecting runways further complicate the issue.

Is Separation Requirement Enough?
Even though the British employ separation standards stricter than the United States, the British noted that many of the incidents occurred with required separations, and a strong peak occurred where the actual separation was equal to required separation.

The actual and required separations for these incidents show that in most cases the separation at the time of the incident was within a nautical mile either way of the required separations.

In the earlier ASRS study, 50 percent of the pilots reported having the required separations. This would seem to complement research findings that under certain atmospheric conditions, vortices have persisted longer than those who designed the separation criteria would have expected.

When it comes to separation distances, there is a significant point to consider. Changing wake separation distances may not change the severity of a wake encounter, only the likelihood of it happening. The severity of an encounter with a wake vortex is largely a function of the portion of the vortex penetrated. Fortunately, penetrations of the very core are statistically very rare, and for reasons to be discussed later, it is actually somewhat difficult to penetrate the very core of the vortex.

The French government has also analyzed its data and determined that most wake vortex encounters occurred in light winds, in VFR conditions, and the pilot had usually been informed about the presence of the preceding aircraft. In my study, only 27 of the 165 incidents occurred in IMC. The vast majority of the events occurred in VMC (138 out of 165), and 118 occurred during daylight conditions.

Unfortunately, the presentation of the NASA data made it impossible to discern what type of airport was involved in the incidents. It seems intuitive that busier air carrier airports were the scene of most of these reports, but once again, that doesnt mean that operations at smaller airports are immune from this problem. On the contrary, it renders considerable insight that we should heed.

In order to avoid wake vortex encounters, pilots not only need to be aware of other aircraft nearby, but also need to predict the location and strength of the vortex. Sometimes that isnt as easy as you would think, and some very smart scientists are trying to design a detection system that will help protect against such encounters, but only at busier airports.

In subsequent installments well also look at some of the factors that affect vortex strength, persistence and motion. Well look at the physics of wake vortex encounters and how those affect aircraft handling and structural limits. In addition, well consider the best procedures for avoiding a wake vortex encounter.

Also With This Article
Click here to view “Altitude of Wake Vortex Encounters.”
Click here to view “Phases of Flight for Wake Vortex Encounters.”
Click here to view “Aircraft Involved in Wake Vortex Encounters.”

-by Pat Veillette

When not smoothing out runways with his wingtips, Pat Veillette flies a B-727 and mines the aviation world for safety data.


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