Welcome to Gadling’s feature, Plane Answers, where our resident airline pilot, Kent Wien, answers your questions about everything from takeoff to touchdown and beyond. Have a question of your own? Ask away!
I hope this is the appropriate place to ask this question.
Why do some aircraft identify themselves as “heavy” on the radio? Does this refer to its size or load? Why is it important to so identify?
It’s the perfect place to ask, actually. The “heavy” designator is attached to aircraft that weigh over 255,000 pounds in the U.S. This term informs the controllers to add spacing between heavy aircraft and non-heavy types since the heavier aircraft create their own turbulence which can be rather dangerous to smaller airplanes spaced too closely.
A vortex is generated from the wingtips of high gross-weight airplanes and is known as ‘wake turbulence.’
It has become a bit more confusing for air traffic controllers lately, since some 757s have been modified to allow for a heavier gross weight takeoff. These 255,500 pound gross weight capable airplanes are now full fledged ‘heavy’ aircraft with spacing requirements that are the same as a 747..
I had no idea just how complicated the spacing criteria could be until I posed the question to Dayron Fernandez who works at the Miami tower.
Read on to hear Dayron explain just what is involved for an air traffic controller when dealing with the different types of aircraft:
Required radar separation minima is 3nm for all IFR [instrument flight rules] aircraft in a terminal environment. In other words, say, with Miami Approach.
In the center environment [en route] it is 5nm for all IFR aircraft. Since the Centers have everyone separated by 5nm, wake turbulence doesn’t apply and so they don’t use the “heavy” designator behind the call sign in the U.S.
In the terminal area, we can use a reduced separation because our radars sweep faster, and since the separation is down to 3nm, wake turbulence rules apply. So here they are:
- Any IFR (non-heavy) aircraft behind a Heavy : 5nm
- Heavy IFR behind Heavy IFR : 4nm
- Any large or heavy IFR aircraft behind a B757: 4nm
- Any Small IFR (PA28, SW4 etc) behind B757: 4nm
- Any IFR behind any other IFR aircraft: 3nm
The increased separation applies only to those aircraft following Heavy or B757 aircraft.
And just when it couldn’t get any more complex, because the effects of wake increase as airplanes fly slower and the AOA [angle of attack] increases:
- Small landing behind a Heavy: 6nm
- Small landing behind a B757: 5nm
- Small landing behind a Large: 4nm
As you are aware, for departures we use 2 minutes behind heavy or even the non-heavy 757s OR we can use radar separation. For the latter, what counts is that the wake separation exists at the time the trailing aircraft becomes airborne. (i.e. 5nm) For both landing and departing traffic we must issue cautionary advisories to aircraft departing/ arriving behind any heavy or any 757.
The FAA weight cutoff for a heavy is 255k. However, ICAO uses 300k. Thus the increased gross weight of some of the B757s brings it above 255,000 pounds but sits below ICAO’s standard.
So we now have to wonder which 757 we’re dealing with. And what’s more, since we now file flight plans in ICAO format, and they don’t recognize that weight class as heavy, pilots/dispatchers can’t file these larger 757s as heavies, so we manually have to make that change in our host computers for every H/B757 flight.