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Beware of jet blast

 

Many years ago a friend and I flew into Ohio’s Dayton International Airport for a jaunt over to the USAF Museum (a place you have to see at least once in your lifetime!).

As with any reasonably sized airport, there were lots of large airliners moving about and being in a Cessna 172 made us feel rather insignificant. Taxiing amongst the larger aircraft was quite awe inspiring but at the same time made us just a tad anxious. We were aware of the possibility of jet blast, so we watched the other aircraft carefully so as to not get too close.

On departure from Cox we were part of quite a long line up of impressive aircraft (with the exception of ours!) that was inching towards the only open runway, as runway construction had necessitated closures that day. It was my friend's leg to fly, and he was somewhat of a novice, so I was watching him carefully.

As we taxied along, the flight controls were positioned for the wind but at one point I cautioned him against following that rule. At that point during taxi we were at a run up pad area and ATC asked us to pull over behind a Boeing 757 that was already parked in the same area.

My friend offered to get nice and close to the "tail pipe" of the 757, but I politely refused his offer and instead recommended we keep a couple of hundred feet clearance. Another airliner was parked behind us close enough that the view out our rear window was essentially her nose-gear.

I suppose that I figured a couple hundred feet would be enough distance from an idling 757. What I didn’t figure on was the 757 getting the clearance to proceed prior to us! When the clearance came across, I recall using a few choice words like "oh s#$*."

What might be a safe distance behind an idling plane is not safe for one spooling up to get rolling. I was now considering requesting a takeoff clearance from the run-up pad, since that’s effectively what would happen if that 757 would have pushed up the throttles.

However, what I saw next was the most considerate act by a professional pilot I’ve ever seen. The 757 very gently added some power; just enough to get it barely rolling. They then turned with what little forward motion they had and once their tail was pointed away from us they added a judicious amount of power to continue taxiing.

I promptly keyed up with a "thanks for not blasting us" after which he promptly responded that he didn’t want to blow us into the airliner behind us. And I thought he was just being nice to the "little guy," oh well!

The moral of the story extends beyond having the courtesy to direct your jet blast away from small aircraft, but that small aircraft need to keep in mind jet blast dangers at all times.

The Aeronautical Information Manual (TP 14371E) devotes a single page to the topic and it clearly shows the danger areas for jet blast. However, a thorough description of what constitutes that danger is lacking.

The magnitude of the blast to be expected, and updated blast distance figures are needed to fully understand the nature of the danger from today’s jet aircraft. For instance, for a large aircraft like a Boeing 777 taking off, the blast velocity out at about 2,200 feet can still reach 35 MPH.

I know that no one is likely to be sitting behind one of these during its takeoff, but many airports have taxiways that cross within that distance off the end of a runway. This is where a small plane might get into trouble if the pilot and/or controller is not paying attention.

How bad can it be you ask? Well several years ago a KingAir was flipped, killing both pilots, as it taxied behind a Boeing 747 doing a static run up on a runway at O’Hare airport in Chicago. The controller was apparently unaware of the status of that 747. Some of you may also have seen a video clip of the staged demonstration of a 747 jet blast that proceeds to blow a pickup truck off the ramp like a rag doll.

Scare tactics? You bet they are. I’m scared and you should be too! Our little aircraft just can’t handle the breeze from those bigger aircraft. Even jets like the Boeing 737 produce 35 MPH of blast up to 140 feet from the rear just to get rolling.

If you’re closer, say your prayers; the blast created to get one rolling can reach 70 MPH up to about 70 feet from the tail. These are figures for the older lower thrust 737s and the newer ones are even worse.

State-of-the-art in airliner engines means jet blast velocities exceeding 200 MPH out to a couple hundred feet at takeoff. Even idle thrust can produce velocities of up to 50 MPH out to 100 feet for some aircraft.

Business jets pose a potential jet blast hazard as well. First, it’s a bad assumption to assume you can get closer to a smaller jet than a larger airliner. Most business jets have tail mounted engines which potentially puts you quite a bit closer to the higher velocity blast than with larger wing-mounted engine jets.

Even smaller commuter jets like the Embraer 135 can produce a dangerous jet blast within 50 feet of the tail at idle or break-away thrust.

Now for a word about the general dimensions of expected jet blasts. The distance of the blast zone behind an aircraft is dependent on the thrust being developed and the environmental conditions such as wind.

For instance, crosswind conditions will dramatically impact the blast zone shape. The width of the blast zone, also affected by wind conditions, is a function of the engine placement and number of engines, but most recommendations suggest using the wing-span as a guide for staying clear of the blast.

More engines do not necessarily equate worse jet blast at a given distance. However, the jet blast area for a four engine aircraft will be wider than for most twin-engine designs.

The Aeronautical Information Manual goes on to caution pilots in regard to supersonic transports and military aircraft by saying that data is not available. In response, I’m not surprised about the lack of data for supersonic transports, since I’m not aware we currently have such a thing. Maybe this was written when the Concorde was still in use.

As for the military aircraft, I can understand the lack of readily available data for these types. Despite the fact that military aircraft may or may not have afterburner capability, they can easily produce localized blast velocities that meet or exceed small jet aircraft blasts and even that of some airliners. In general, the best place to be when a military fighter aircraft is taking off is on the other side of the airport fence with a camera. Ok, that may be a little bit of an exaggeration but I’ll assume you get my point.

It’s easy to say that the Information Manual lacks good resolution on the jet blast problem, but in all fairness, there are few publications that do any better. The single best source for technical data on the expected velocities and size of jet blasts is the aircraft manufacturer.

Most manufacturers post what are referred to as an airport planning document for each aircraft they produce. These documents include an operating conditions section that typically covers jet blast characteristics as well as noise profiles in some cases. So if you want a more fine grain analysis of individual aircraft jet blast characteristics, the airport planning manual is the place to look.

When operating at airports that are served by jets of any kind or size, always be cautious of the potential of jet blast. Give all jet aircraft a wide berth regardless of where they are and never assume that their pilots are aware of your presence or that they have the courtesy to consider their own jet blast impact.

While it’s probably true that most pilots are conscious of there surroundings, considering the hazard of jet blasts, this is one time to be a little more pessimistic about what everyone else is doing around you.

This month’s Pilot Primer is written by Donald Anders Talleur, an Assistant Chief Flight Instructor at the University of Illinois, Institute of Aviation. He holds a joint appointment with the Professional Pilot Division and Human Factors Division. He has been flying since 1984 and in addition to flight instructing since 1990, has worked on numerous research contracts for the FAA, Air Force, Navy, NASA, and Army. He has authored or co-authored over 160 aviation related papers and articles and has an M.S. degree in Engineering Psychology, specializing in Aviation Human Factors.