Click on a label to read posts from that part of the world.
Plane Answers: Wake turbulence and the TCAS traffic avoidance maneuver
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?
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.
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.
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.
Filed under: Plane Answers