Plane Answers: A heavy question

by Kent Wien on

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!

Ken asks:

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?

Thanks Ken,

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.

Cause for Turkish Airlines flight known: Dutch Safety Board issues warning

by Jamie Rhein on

Last Wednesday, a Turkish Airlines flight crashed in a field just 1 kilometer short of the runway at Schiphol Airport in Amsterdam. The report as to what caused the accident has been released. After reading through it myself, and thanks to the paired down version of Gadling’s own Kent Wien, pilot and writer of Cockpit Chronicles, here are the details.

According to the report, there was a malfunction of one of the radio altimeters, the device that displays the distance of the airplane from the ground. The left altimeter, instead of reading the Boeing 737’s actual height at 1950 feet when the plane was descending, it read 8 feet.

At the point of the glitch, the auto-throttles went to idle because the reading said the plane was just above the runway thus about to land. This caused the plane to slow down more than it should have. The pilots didn’t have enough time to recover the speed needed to pull the plane out of a stall to a higher altitude in order to achieve a safe landing.

Along with determining the malfunction in the radio altimeter, the investigation also found out what happened to the plane upon impact. The tail of the plane hit first, then the undercarriage. When the plane hit the ground it was going at 150 kph. A normal landing speed is 260. Because the ground was soft, the plane had a “rapid halt” within 150 meters.

During landing, the tail broke off and the plane ruptured at the business class section which is where most of the fatalities and injuries occurred. Eighty passengers in all were injured and nine people died (4 crew, including the pilots and five passengers). The area of the plane around the wings was the most intact.

There are still investigations being made surrounding the altimeter’s malfunction and the Dutch Safety Board has issued a warning to Boeing.

For the report, click here. Prior to these findings, one theory about the cause of the crash was wake turbulence caused by a larger plane landing right before this plane’s attempt. (See article.)

Plane Answers: Wake turbulence and the TCAS traffic avoidance maneuver

by Kent Wien on

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!

Patrick asks:

Greetings from SE Texas!

I enjoy your Cockpit Chronicles and Plane Answers as much as I enjoy surfing the airliners.net site. I’ve been an aviation enthusiast my whole life, and will often listen to streaming ATC feeds on liveatc.net.

I notice that very frequently I’ll hear the “caution, wake turbulence” callout by ATC and while I understand its importance, does it’s frequent use “dilute” the message? I know that hearing something, over and over and over can make it become just more background clutter.

When you fly a “heavy,” I guess you don’t get that callout, as I’ve never heard a “heavy” receive it from ATC (not that they don’t, I’ve just never heard it). In your opinion, FAA requirements notwithstanding, do you think this “overuse” will have a diluting effect on the warning?

Thanks Patrick,

As you know, we’ll typically receive that warning when operating close to the airport, either before takeoff or on final approach. As you mentioned, it’s an FAA requirement for ATC to warn us when we’re following a “heavy” aircraft, which are jets with a gross weight capability of more than 250,000 pounds.

The wake that a wing produces on these heavy jets has the capability to create significant turbulence for lighter aircraft following a few miles behind. In practical terms, these warnings are noted, but we don’t generally change our course, altitude or speed as a result. It’s simply offered as a ‘heads up’ in case we do begin to experience the effects of a heavy jets wake, at which point we could then slightly offset to smoother air.

As for the warning becoming overused and diluted, I’d agree that it probably has become that way, but in the back of our minds, we’re always aware of the power of these wingtip-generated vortices, even if we rarely come into contact with them.
Calvin asks:

Hi,

I recently had a flight where we took off and then about 5,000ft up we backed off totally where the engines actually sounded like they stopped. Then we felt like we were falling, then the pilot increased the power for about 10 seconds and then sounded like the engines were out again, then we fell again, you could feel the drop.

Then the engines sounded like they fired up and we climbed again. Everyone in the plane thought we were crashing and was very scary. The pilot said afterwards that there was a computer on board that said we were too close to another aircraft and made us go down.

Is this true? I have flown at least 200 times and have never experienced such a thing. I am very scared to fly after this flight and don’t know if we just dodged the bullet or if this happens but the experience of the pilot came into play. Can’t wait for your response.

Hi Calvin,

What you probably experienced was a TCAS (Traffic Collision Avoidance System) resolution advisory. Since the mid-’90s we’ve had a device on board that can direct us away from other aircraft by climbing or descending.

There may have been an airplane above yours that caused the TCAS to alert the pilots to stop their climb and descend. This can happen when an airplane is above you and you’re climbing at a great rate. We’ve learned to slow the climb rate down when we know we’re in an area with a popular arrival corridor above us.

A good example of this is at DFW where we may be cleared to climb to 10,000 feet. At 11,000 feet, airplanes are approaching the Dallas Love Field airport. When we’re climbing at a great rate, the TCAS sees only the potential for a collision and the computer doesn’t know we’re leveling off soon.

It’s hard to know for sure, but I’m convinced that TCAS is no doubt responsible for saving thousands of lives. It’s a great technology that has the potential to help when a pilot or controller makes a mistake.

We’ve learned that having multiple layers of safety in this industry is what prevents accidents. Not that this is what happened in your case. It’s entirely possible that the other airplane was never at the same level, but the rate of closure ‘tricked’ the TCAS into thinking a collision was imminent. Since this resulted in not just one, but two moderately evasive moves on the part of the pilots, it’s entirely possible that there was more than just two aircraft involved.

A display shows the proximate traffic even if it isn’t a hazard.

I’ve received less than a handful of TCAS resolution advisories. They always start out with a “Traffic, Traffic” proximity alert by the magic box. And then comes a command to “Climb. Climb now,” or “Descend, descend now” followed by a pointer on our vertical speed indicator directing us at what rate to climb or descend. When this happens, the other aircraft is being told to perform the opposite maneuver milliseconds after the TCAS systems decide what the best evasive action is.

If we’re directed to descend, there’s a good chance that we’ll pull the power back to idle which will feel very much like the engines were shut off. Virtually every descent you’ve probably experienced is accomplished at idle throttle, so you can imagine how startling it would be to go from a climb at a high power setting to a descent at idle so suddenly.

That said, our procedure is to turn off the autopilot and to smoothly but without delay, follow the directed commands of the TCAS while letting ATC know we’re responding to a TCAS resolution advisory.

Do you have a question about something related to the pointy end of an airplane? Ask Kent and maybe he’ll use it for next Monday’s Plane Answers. Check out his other blog, Cockpit Chronicles and travel along with him at work.