Cockpit Chronicles: Nearly a near midair collision

“Traffic, Traffic!” Announced the computer voice from the speaker on the ceiling just above me.

This is something we hear frequently enough, perhaps once every three or four flights when an airplane in close proximity is climbing rapidly with a clearance to level off 1,000 feet below us. The TCAS (Traffic Collision and Avoidance System) is just giving us a warning that, should the airplane not level off, we may have to take action.

We were at FL390 (39,000 feet), an altitude where the traffic warning was far less likely. The captain and I looked down at the TCAS screen to get a quick idea where we should be looking for the other airplane. It was 800 feet lower than us and at our 2 o’clock position. It was easy to spot, with huge puffy contrails billowing out and slightly below it. A 737 for sure. We were both satisfied that it would pass behind us, since it was moving from left to right across the windscreen. A stationary position in the sky would mean it was coming right at us.

But before we could discuss this passing airplane, the computer voice came on once again.

“Climb, Climb now!”
Our procedures dictate that we should honor thy TCAS request, known as a Resolution Advisory or RA, by disconnecting the autopilot and following the rate of climb commands computed by the TCAS system.

Since it was my leg, I immediately disconnected the autopilot, while glancing down at the vertical speed indicator to find out just how many feet per minute of a climb would be needed. It wasn’t much, in fact. Just 200 feet per minute, hardly even noticeable to the passengers. It commanded a level off when we were at 39,100 feet and shortly after allowed us to settle back down to our original altitude.

All this was done in a matter of seconds, with no input or guidance from Air Traffic Control. In fifteen years using TCAS, this was only my second resolution advisory-the other one having occurred while on approach just east of Port-Au-Prince Haiti years ago.

“Center, confirm we were cleared from 380 to 400?” The other aircraft asked.

The controller said yes, which made us think this could have been an error on the part of the controller.

“Can you explain then what just happened?” The 737 pilot queried.

There was no answer from the controller.

We let the controller know that we had also just responded to a resolution advisory. The other pilot asked for a phone number of the Air Traffic Control center that he could call. We copied this number down as well.

There was some discussion between the captain and I whether we needed to report this as a near midair collision (NMAC). I pulled my manuals out, now conveniently located on an EFB equipped iPad (Electronic Flight Bag) and searched for the NTSB criteria for a near midair collision. Nothing came up.

But I did find an interesting recent change to our procedures. The NTSB (National Transportation Safety Board) requires that any flight responding to a TCAS resolution advisory above 18,000 feet must pull the voice recorder circuit breaker after completing the parking checklist. This would allow the NTSB to analyze the tapes from ATC and the aircraft involved in the loss of separation incident.

Just knowing that the NTSB would be listening to our conversation for the next two hours tends to make you aware of every word you’re saying. In fact, I debated with myself about getting into a discussion during our approach briefing about wind and gust additives that we would be applying for the approach.

I recognize that there’s value in allowing the NTSB access to the conversations that led up to an incident. They’ll hopefully study the procedures and policies that could prevent this kind of situation. There’s still a big brother feel to it.

I couldn’t help but feel bad for the controller on duty. While the captain and I were waiting for the employee bus, he phoned the air traffic control center. The controller explained that a clearance was given to the Trans-Siberiana 1701, but that Trans-Siberiana 1790, who had also asked for a climb, had accepted the clearance instead. All airline names have been changed to protect the innocent.

I looked up the FAA definition of a Near Midair Collision:

A near midair collision is defined as an incident associated with the operation of an aircraft in which a possibility of collision occurs as a result of proximity of less than 500 feet to another aircraft, or a report is received from a pilot or a flight crew member stating that a collision hazard existed between two or more aircraft.

It turned out we were just a 100 to 200 feet away from the NMAC definition. So I guess it was “nearly a near midair collision.”

We both filed a report detailing the events. I recently received the response. We did everything by the book and it obviously wasn’t our fault, which meant that the case was closed as far as our involvement.

Someday I hope we’ll have a third layer of safety in addition to the protection offered by ATC and TCAS in the form of a two-lane airway using a half mile offset to the right. Ever since GPS was invented, we have reduced the normally 8-mile wide airways down to just a few feet thanks to the precise nature of the technology. But with that came greater reliance on TCAS to keep us out of trouble. I wrote about an inexpensive offset airway proposal previously and I’d love for the FAA to take another look at it. Adding layers to our safety net is what has made air travel so much more safe than in the early years of flying.

Cockpit Chronicles takes you along on some of Kent’s trips as an international co-pilot on the Boeing 757 and 767 based in Boston. Have any questions for Kent? Check out Plane Answers or follow him on Twitter @veryjr.

Plane Answers: Zeroing in on a few airplane systems

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!

Jason asks:

On a recent round trip on board 757-200s, when we would descend there was a sound like air leaking out of a tire coming from the engine area. It would last from 1-3 minutes in some cases or a matter of seconds at other times. What causes this noise?

Air conditioning and pressurization on an airliner both originate from what’s called bleed-air that comes from the engines. On the 757, as the thrust is reduced to idle for descent, another valve opens allowing ‘high-stage’ bleed air to supplement the existing air.

This hot air is then run through air conditioning “packs” that heat or cool the cabin and provide pressurization.

You may have been hearing the rushing air associated with this valve.

Not being a mechanic, that’s the best explanation I can provide. Any mechanics out there want to tackle this one? Big Ed?

Michael asks:

While working, I flew Northwest from MSP to AMS monthly. When flying DC-10s, the cabin display showed the same flight level the captain reported – 35,000 feet = 35,000 feet. Now flying the A330, the captain reports 35,000 feet but the IFE reports something like 34,824 (a guess, not the actual number). Does the IFE not get its travel info from the plane’s instruments? Since FL350 is about 10,600 meters, does the IFE work in metric units (rounded to the nearest 100 meters) and convert to an exact English equivalent i.e. 34,777 feet?
On older aircraft such as the DC-10, I can’t recall where the inflight entertainment screen pulls the altitude information from, but the newer jets all use GPS data to display the altitude on the map back in the cabin. In addition to locating an airplane’s position on a map, the GPS can also compute an approximate altitude.

Since we refer to an altimeter in the cockpit that’s entirely based on air pressure, our version of 35,000 feet actually varies slightly depending on whether we’re flying through a low or high pressure area.

It’s not important that we’re exactly 35,000 feet above the ground, but it IS important that we’re using the same measurement as all the other aircraft. And this barometric altimeter is considered the most accurate way to establish our altitude relative to all other airplanes.

Down low (below 18,000 feet), we manually correct the altimeters for the variations in pressure with a small knob using a setting provided by air traffic control. This keeps us at a safe and known altitude above the terrain.

So you may notice that the altitude displayed in the cabin while in level flight below 18,000 feet is much closer to a given 1,000 foot measurement, such as 11,000 or 5,000 feet when temporarily leveling off during a climb or descent.

Michael goes on to ask:

Also, the system in the cockpit that warns of impending collision – does it work by getting transponder information from nearby aircraft or is the information derived from the ground? Over the mid-Atlantic, out of ground based radar coverage, is separation dependent on seperation leaving Canada and maintaining a constant Mach number or will the system warn if you are getting too close?

The TCAS system (Traffic Collision Avoidance System) works over the Atlantic, since it uses the transponder of each airplane to decide who, in the case of a controller or pilot mistake, needs to climb and who needs to descend to avoid a collision.

But you’ll almost never hear that warning over the Atlantic since the Gander (Newfoundland) and Shanwick (Shannon, Ireland and Prestwick, Scotland) controllers place aircraft on ‘tracks’ at a proper spacing and altitude that should stay relatively constant with an assigned speed for the crossing.

It’s far more common to receive a TCAS alert while flying domestically or while in Central or South America. And since the introduction of GPS, which has effectively reduced our airways from up to 8 miles wide to a much more narrow .1 of a mile wide due to vastly improved accuracy, TCAS has saved numerous lives already, usually without passengers even knowing it.

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

Plane Answers: Wake turbulence and the TCAS traffic avoidance maneuver

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 site. I’ve been an aviation enthusiast my whole life, and will often listen to streaming ATC feeds on

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:


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.

Plane Answers: Is “Free Flight” the answer to ATC delays?

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 know a lot has been written lately about airport delays, I have also read something about “Free Flying.” With TCAS is ATC obsolete? Should ATC be more focused on ground operations, to get planes in the air? I know from most recent articles the ATC system is operating on antiquated systems and in need of a massive overhaul. I am interested to hear your opinion, is “Free Flying” in our future?


Thanks Justin,

We’re not able to navigate or adjust our spacing using our Traffic Collision Avoidance System (TCAS). This device, which is almost like a radar screen showing the other traffic within 40 miles of our airplane, is solely to keep us from running into someone. Think of it as a backup to the Air Traffic Control system. And just like passengers aren’t generally interested in pilotless airplanes, pilots may not be interested in a world without controllers directing traffic and keeping us safe.

The ability for airplanes to fly directly to a destination is one thing that would shorten travel times, but it’s important to put the benefit in perspective. The FAA is hoping to develop a system that would allow for a direct routing versus today’s system of waypoints and VOR’s that define a more jagged path, but it will only save a few minutes of flight time.

The FAA is even more interested in the ability to space flights closer when near the airport using a new technology called NextGen. Why are they so excited about this?

Because it’s low hanging fruit.

Even at $20 billion, it just might offer the best answer to the capacity problem. Using computers and GPS, we can have more direct flights and airplanes can take care of their own spacing as they approach the airport. Take a look at this video on “NextGen” by David Pogue for CBS News that explains what the FAA is trying to do (after the jump):

While they make the small airplane owners look like the bad guys in this story, it’s hard to see any reason to force Cessna 182 pilots to pay $6,000 for a box that will help ATC control traffic around a hub like DFW or ORD, an area that most general aviation (Cessna) pilots avoid anyway.

Unless we start doing formation takeoff and landings on ultra-wide runways out of JFK and other saturated airports-a highly unlikely scenario-we’re not going to see delays improve without capacity reductions or huge investments in new infrastructure. So the next step is to improve the infrastructure at airports by adding gates, revising taxiways and adding more runways. The trouble is, people living near these airports equate that to more traffic and subsequently, noise. And it’s yet another cost.

At airports with most frequent delays, airlines need to ‘bump-up’ the size of airplanes. A 19-seat Beech 1900 takes up nearly the same airspace that a 747 does. It might take the government to step in and mandate a minimum size of aircraft at these ultra-saturated airports, but this could be an effective way to fix the problem. The smaller airplanes might begin flying more point-to-point trips from lesser used airports in the same way Southwest does now.

We’re going to get some short term relief from the airlines that are cutting back later this year for economic reasons. But that’s no reason to sit back and wait until we’re near gridlock once again to fix the problem.

Thanks for the great question, Justin.

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 Friday’s Plane Answers feature.