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How Far Should You Taxi Behind A Jet?

JetPhotos: HA-KLS

Whether you're following a Boeing 747 or Citation Jet, turbine engines pose a unique threat to your aircraft. How far away you should stay from exhaust wake?

Exhaust Velocities Stronger Than A Category 5 Hurricane, Even On The Ground

Depending on the aircraft, turbine engines can produce anywhere from 2,000 to over 100,000 lbs of thrust. Hundreds of vehicles and aircraft have been damaged over the years due to exhaust wake (jet blast), and it's something you need to be aware of when operating around turbine aircraft.

The FAA briefly mentions jet blast in AIM 4-3-18, but doesn't get into much detail, so we looked to Boeing to find out more.

At full thrust, modern engines can produce exhaust wake velocities exceeding 400 mph at the exhaust nozzle. Keep in mind, a Category 5 hurricane is rated with sustained winds above 155 mph.

You're probably thinking to yourself, "well, that's only exhaust speed right at the engine," and you're right. But data from Boeing tells a different story about just how widespread and significant exhaust wake can be. The figure below shows a jet producing a maximum exhaust velocity of 375 mph. Exhaust speed exceeds 230 mph, even 100 feet behind the jet.

Following the same ratio, a jet that's producing a much lower 150 mph of exhaust thrust at the engine nozzle can still create gusts above 90 mph, 100 feet behind the engine. That's not something you want to get caught in.

Report: Jet Blast Bends C172

Exhaust wake doesn't just apply to large aircraft, like a Boeing. Take the following report from General Aviation News:

A Cessna 172 pilot reported that while on a taxiway at the airport in Morristown, N.J., under air traffic control (ATC) instruction, the Cessna 172 encountered jet blast, originating from a larger turbine-powered airplane being marshaled by ground personnel (Falcon 7X).

The 172's empennage lifted and the propeller and left-wing struck the ground, which resulted in substantial damage to the firewall.

Probable cause: The pilot's failure to maintain a safe taxi distance from a large turbine-powered airplane, resulting in an encounter of the turbine-powered airplane's jet blast while taxiing.

NTSB

Breakaway Thrust

Getting a large, turbine-powered airplane to move on the ground obviously takes a lot more muscle than lighter, piston airplanes. Pilots flying jets often have to momentarily use "breakaway thrust" to get the wheels moving.

This thrust is a lot higher than normal taxi thrust, and usually sits somewhere between 25-40% of total engine power, depending on aircraft type, surface conditions, and weight. Pilots initially apply breakaway thrust to get the wheels moving, and once rolling, bring the power much further back to somewhere between idle and 20% of total engine power. Breakaway thrust is one of the things you should be most concerned about if you're taxiing behind a turbine aircraft.

According to Boeing data, the exhaust hazard area for breakaway thrust extends to 400 feet behind large aircraft. For takeoff thrust, the hazard area extends up to 1,900 feet behind the aircraft.

The following demonstration shows just how powerful exhaust wake can be at high power settings. Ground vehicles and light aircraft simply don't stand a chance.

So, How Far Back Should You Taxi?

Let's say you're flying a Cessna or Cirrus out of a busy airport with a long line for departure. You're in line right behind a Boeing 737. According to the data above, you should give yourself at least 400 feet of room behind the preceding aircraft...let's call that 500 feet to be extra cautious.

If you're having trouble visualizing this, imagine how far it is from the threshold to the 1,000-foot markers when you're ready for takeoff. Half that distance is the most conservative limit for how far you should trail a large jet in front of you. The smaller the jet, the less room you'll need. But that doesn't mean the hazards disappear. Use good judgment and never stack up right behind any turbine-powered aircraft.

Wikimedia

Are You A Turbine Pilot?

If you fly a jet, it's partially your job to know what's behind your engines. It's easy to forget just how powerful your turbine engines are when you're sitting in a quiet cockpit. In a busy ramp area, keep your thrust to a minimum. If you need to do a cross-bleed start or runup, ask ATC where you can perform the procedure. Use the minimum breakaway thrust to prevent damage to aircraft directly behind you.

Also, don't forget that exhaust wake can affect you too. There are dozens of reports from large, turbine aircraft which sustained damage to flight controls and forward-facing windscreens resulting from taxiing too close to other jets.

Video: CRJ Engine Runup Destroys Hangar And Aircraft

Even if you're not flying, exhaust wake can be hazardous. Take this video posted just a few weeks ago, for instance. A CRJ in San Luis Obispo, California, completely destroyed a hangar (and many aircraft inside) during what appears to be an engine runup for maintenance checks.

This video has some harsh language.

Long story short, be careful where you park your airplane and always have it tied down, with the gust lock installed, even if you don't think it'll be windy outside. If you're parking at an airport with lots of jet traffic, have the FBO park your airplane safely away from the main loading area.

If you're taxiing around an airport with jets nearby, give them a wide berth! When you're in a long line for departure, remember how breakaway thrust works and don't park too close to the jet right in front of you.

And if you're ever traveling to St. Maarten... PLEASE don't try this yourself...


Have you gotten caught in jet blast? Tell us about your experience in the comments below.

Swayne Martin

Swayne is an editor at Boldmethod, certified flight instructor, and an Embraer 145 First Officer for a regional airline. He graduated as an aviation major from the University of North Dakota in 2018, holds a PIC Type Rating for Cessna Citation Jets (CE-525), and is a former pilot for Mokulele Airlines. He's the author of articles, quizzes and lists on Boldmethod every week. You can reach Swayne at swayne@boldmethod.com, and follow his flying adventures on his YouTube Channel.

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