By Scott Dyer

You’ve seen the videos and pictures of tourists hanging onto the fence at the St. Maarten airport as transport jets depart just a few feet away, and that wonderful practice (NOT!) has resulted in deaths and serious injuries.  Jet blast is awesome power. 

The NASA Aviation Safety Reporting System (ASRS) files are replete with reports of light aircraft being caught in the jet blast of taxiing or departing turbine aircraft and it’s been this way for years.

A recent report had a homebuilt aircraft taxi behind a Challenger, which was waiting to cross a runway, with ATC then authorizing the Challenger to cross that runway:

I was cleared to taxi from via Taxiway A to Runway XXR in my [Homebuilt Experimental] tailwheel aircraft. A Challenger 300 had been cleared from the West Ramp to hold at Taxiway A1, while waiting to cross XXR to get to XXL for departure. Just as I was taxiing directly behind [the Challenger 300], Ground cleared him to cross XXR. [The Challenger 300] applied immediate power to taxi. I was directly behind him; I tried to accelerate but was unable to get past his jet blast before we were blown in a violent 330 - 360 degree right ground loop. My aircraft suffered damage to the left wing tip, both wheel fairings, and the left gear fairing. There was no prop strike and no contact with any flight controls. Neither I nor my passenger was injured.

The cause of the problem was the Ground Controller not noting my position directly behind [the Challenger 300] when he cleared him to cross; I also believe [the Challenger 300] used excess power to begin his taxi roll.  NASA ASRS ACN: 1100774

There are situations of light aircraft taxiing behind a jet doing a maintenance runup, being cleared either onto or across a runway, or being cleared for takeoff with the jet blast pushing landing traffic on another runway well off the runway heading while in a flare.  We’ve seen some jet blast incidents over the years at HPN, and a memorable one in recent years occurred at Boston Logan as a C172 was flipped on its back while in line for takeoff behind an air carrier jet.

Anticipation of these incidents by the light aircraft PIC, the turbine crew and ATC, can help avoid them almost entirely.  Good airmanship requires situational awareness in all phases of flight, including taxiing before departure or after landing.  There is very little FAA flight training guidance on jet blast avoidance while taxiing, a situation that needs to be remedied.  How we can anticipate these situations is the purpose of this short article.

Terminology

We’ll start by defining some terms and the extent of the problem.

Turbine powered aircraft use a range of power, from idle thrust while stationary or in a rolling taxi, to what’s called “breakaway thrust” to start moving, to takeoff power when starting the takeoff roll. 

Idle thrust is the lowest setting as you would expect it to be, with the engines still generating thrust (and wind!) behind the aircraft.  Behind corporate jet aircraft at idle, you can easily experience 30kt or higher exhaust winds even 100 feet behind the aircraft.  That number can grow to 200 feet behind or more idling large and widebody transports (e.g., 757, 767, 777, A330, etc.).

Breakaway thrust is what crews use to start an aircraft moving.  How much is enough thrust to start moving depends on the weight of the aircraft, the slope and/or contamination with snow or ice of the taxi surface, and temperature of the surface (think sticky asphalt), among other things.  The amount of thrust can and does vary, but the important thing to remember is that it is much HIGHER than idle thrust.  That means, the danger area behind an aircraft using breakaway thrust is much LARGER than one stationary at idle. 

Here’s a Boeing chart (annotated by the Australian Transportation Safety Board) for a 737 variant that makes the point:

You see from the chart that 75 mph winds extend back at least 100 feet from the tail, and 35 mph winds extend more than 200 feet back.  This is with minimal breakaway thrust, not a higher value that may be operationally necessary or used by crews who want to hurry onto the runway to save some time. Note to jet crews:  use the lowest amount of thrust to start motion that you can, as you won’t know what is behind you (and how close it is) on the ramp or taxiway.

Higher thrust levels are used for takeoff, but also for maintenance runs or burning off a few hundred pounds of excess fuel on hot/high/heavy departures.  A Boeing chart for takeoff thrust shows just how large the jet blast danger zone can be for a large transport jet:

Avoidance Techniques

Avoidance means knowing what’s going on all around your aircraft, and what can be expected.

Keep a good distance, perhaps several hundred feet, as you taxi behind a turbojet to the runway.  Don’t get closer than that when the aircraft ahead stops: the jet will need (at least) breakaway thrust to get rolling again, oftentimes more with the uphill slant of HPN Taxiway L heading to Runway 16, but even a level breakaway causes significantly greater jet blast than mere idle thrust. As a distance guide, the taxiway lights on straight sections of Taxiway L at HPN are spaced roughly 190’ apart.  The lights are spaced closer in turning sections.

Switch to the tower frequency as you get closer to the runway, even before you enter the runup area.  Listen carefully, and get a picture in your mind on whether and when the aircraft ahead may start moving as you wind your way to the runup area.  You can hear if a jet is cleared onto the runway, which will cause jets behind it to move up in sequence with (at least) breakaway thrust.  You’ll want to know that before you taxi perpendicular to a stopped jet as you head from Taxiway L into the runup area for either Runway 16 or 34:  you do not want to be passing behind a jet that is powering up for further taxi, or to enter the runway, without at least several hundred feet between you and the tail of that aircraft.

There may be some advantage to being aligned with the jet blast if you are being caught in it while taxiing rather than being cocked at an angle to the wind.  But, remember that even aligned aircraft can be flipped on their back by jet blast.  Use proper aileron/elevator position if you feel the jet blast wind just as you would for crosswind taxi, it may be all you can do at that point. The better technique is to leave enough room behind, and stay off-axis a preceding jet (and therefore out of the jet blast), rather than hoping for the best.

We’ll treat the ends of each of our HPN major runways separately, as they are a bit different in the terms of jet blast avoidance. 

Here’s a sample HPN taxi diagram to help orient you to the airport:

  

HPN taxi diagram 

Runway 34

As you can see in the diagram above, as you  taxi down L to Runway 34, immediately past the ILS Hold Short Line, the taxiway turns left by 90 degrees.  If there is a jet in line holding short of Runway 34 with its tail around the corner, you are at risk of a jet blast upset as you taxi into the runup area. 

Listen to Tower as you proceed down L to hear if the line of traffic holding sort of 34 is going to be moving up, in which case something more than idle jet blast from the jet around the corner is likely.  You don’t want to be perpendicular to the jet blast as that aircraft applies power, better wait to enter the runup area until you know the aircraft is stopped or there is sufficient distance.

Runway 34 runup area, be careful of potential jet blast as you taxi behind. 

While using the runup area is always a good option, you may be in a line of 5 or more aircraft on a summer Friday afternoon heading down L.  There is no rule against doing your runup while in line, with good practice being to cock the aircraft 30 degrees or so to not subject an aircraft following you to your prop blast.  This way you don’t need to enter or exit the runup area at busy times with lots of taxiway traffic.  Only do this if you are comfortable, because you can enter the runup area to take more time with preparations for takeoff with the jet blast avoidance technique above, and the strategy another paragraph below.

Assume you don’t need to enter the runup area.  If you can turn the corner toward 34 just past the ILS hold short lines, be wary of jet aircraft in line holding short of the runway, there can be several jets lined up.  If you are on the centerline of the taxiway as you make that turn, you are likely in a prime jet blast location as you will be lined up squarely behind the jet with little distance from it.  

If your aircraft is small enough (as is the case with most piston singles and many twins) consider staying to the left side of Taxiway L, well off the centerline, with the left main gear remaining well on the pavement. The benefit of this strategy is that it will keep your aircraft on the inside of the jet blast radius described by the turning (or stationary) jet.  We have the luxury of broad taxiways at HPN, and ½ of Taxiway L is plenty enough for most light piston aircraft…so use it!

Staying on the left side of Taxiway L as it turns the corner to Runway 34 will usually keep light aircraft out of the jet blast danger zone for jets holding short of the runway.  Stay off-axis from the engines.  

Now let’s assume that you use the runup area and are ready for departure.  There’s a jet or two holding short of 34 with a string of arrivals to that runway, and aircraft taxiing down Taxiway L to get in line.  If you can maneuver at least several hundred feet behind the last stopped jet on L, and you are CERTAIN that it will not be applying any power while you get in position, you may be able to line up behind that jet while still leaving 200 feet or more between you.  More likely, that isn’t going to be possible.  Rather than moving into a potential jet blast situation, call Tower on the radio to advise that you are ready for departure but in the runup area.  Tower will get you going with minimum jet blast risk in an appropriate sequence.  You’ve avoided a potentially hazardous situation.

One last thought before we leave the Runway 34 scenario, if you pull up in the runup area at the far eastern end (i.e., closest to the runway), you may have minimal distance between you and a departing jet that is cleared onto the runway for departure.  The jet in front of you may, as it comes around to the runway heading, apply far more than breakaway thrust to get moving quickly.  That jet blast can sweep across the end of the runup area nearest the runway. It is advised to use the other end of the runup area.

Runway 16

We’re on Taxiway L again, but this time heading northwest to Runway 16.  Many of the same considerations for Runway 34 departures apply here, too, but there’s a lmore for light GA to worry about.

First, L leads uphill to Runway 16, so higher jet thrust will be needed to start (and keep) rolling than on a flat taxiway. If you are in the Runway 16 line, be aware of that.

Second, there is less room on the stub end of Taxiway L where it intersects Runway 16, than at the other end of the airport.  There is room for one jet with little distance for another aircraft behind it.  Use the right hand half of the taxiway, well off the centerline, as you come up around the corner of that perpendicular turn to stay clear of the blast coming from the jet holding short of 16.  That way you can stay further away from the jet blast danger zone.

Just as with Runway 34, staying on the inside radius of the turning taxiway will usually keep light aircraft safer than staying on the taxiway centerline.  This is a picture of the turn just before Runway 16, and light aircraft can stay on the right side of the centerline.

Third, the runup area for Runway 16 on the Taxiway L side is small.  Once again, you don’t want to be in the jet blast area on the far eastern side of the runup area (closest to the runway). If you are going into the runup area, use the same strategies we covered earlier when talking about Runway 34 to (a) cross into the runup area behind a waiting jet, and (b) exit the runup area for departure.

Conclusion

We’ve covered main jet blast concerns when taxiing to Runways 16 and 34 at HPN.  These concerns will sensitize you to similar threats at other airports you frequent.  Be aware that maintenance/runup pads can also contain aircraft doing near- or full-power runups, and even aircraft parked on an FBO ramp can, with breakaway thrust, be felt by nearby taxiing light aircraft on established taxiways.  Those same jets doing high-powered maintenance runups can be dangerous for aircraft taxiing behind, with an area of concern on Taxiway K as it passes by Signature West with large aircraft sometimes parked undergoing maintenance with the tails toward Taxiway K.

While this article’s focus has been on operators of light aircraft, the message to jet crews is to use the minimum thrust necessary to start taxiing, and not use more than taxi power in motion until aligned on runway heading.  Air traffic control plays a role here, too, with appropriate jet blast warnings and being a good resource for light GA pilots in getting into/out of the runup areas.

Thank you to  Francisco Tejada and Andrew Bowser of HPN Operations for their help with photos from HPN to illustrate this article. 

About The Author:

Scott Dyer is a Westchester Aviation Association Board member, coordinator of our quarterly ATC Seminars and an active CFI/II at the airport. For more info on Scott check out his bio and meet the rest of the WAA Board at  - https://www.westchesteraviation.org/page-18130