We're in Aspen, departing on an IFR flight plan. During startup, our transponder stops working.

It doesn't affect our ability to navigate, but it impacts ATC's ability to see us.

Two Types Of Radar

ATC uses two types of radar: primary, and secondary.

The naming is a little backwards. Secondary radar is officially known as the "Air Traffic Control Radar Beacon System".

The system started out as "Identification Friend or Foe" (IFF) in World War II. During the war, German aircraft would infiltrate British bomber formations on their way back to England, and try to "sneak" into British airspace under the radar cover of the bomber aircraft.

IFF helped identify returning friendly aircraft. The British called the system "The Parrot", and since it blinded early primary radar systems, radar operators would tell crews to "squawk your parrot" when they wanted transponders on, and "strangle your parrot" when they wanted transponders off.

That's where the term "squawk" came from.

Not a lot has changed in secondary radar since then. The system started with two numbers ranging from 0-7, and now it has four numbers, still ranging from 0-7.

And now Mode-C adds altitude information to the mix, so ATC can see your position and your altitude.

Primary Radar

Primary radar, the original radar system, bounces high energy radio waves off your aircraft, and displays the return on a radar scope. When most people think of radar, this is what they imagine. But primary radar only paints a 2-D picture.

Primary radar can plot your position, but not your altitude.

ATC can still tag a primary radar return with your flight information, including your callsign and your reported altitude.

But there's a problem on our flight out of Aspen...

No Primary Radar In Aspen

According to ATIS, Aspen's primary radar is out of service.

We don't have a transponder, and Aspen doesn't have primary radar. They can't see us depart, and we'll need to climb out under IFR, in the clouds, through the mountains.

The FARs

We can fly with inoperative equipment, but to do so, we need to follow the regs.

FAR 91.213 lays out the requirements.

We're dealing with a specific part of the reg, 91.213(d), because we don't have a Minimum Equipment List (MEL). MEL's are typically used by airlines, charter operators, and large flight schools, and we don't have an MEL for our plane.

FAR 91.213(d) is pretty straight forward even though it's a little wordy:

According to the reg, we're good to go, until we hit paragraph (d) (2) (iii): "Required by FAR 91.205 or any other rule of this part for the specific kind of flight operation being conducted".

A transponder isn't required for IFR flight under FAR 91.205, which lists out the required instruments and equipment for Day VFR, Night VFR, and IFR flight.

But it is required under another rule, FAR 91.215.

When You Need A Transponder

According to 91.215, on our flight, we'll need a transponder when we're:

• Above 10,000' MSL
• Inside Class B Airspace
• Within 30 NM of Denver International airport (Mode-C veil)

Because we need to file 17,000' MSL for our cruise altitude, that's pretty much our entire flight home. (There are other times when you need a transponder as well, but these are the times we'll need it on our particular flight.)

But if you read a little further on in FAR 91.215(d)(2), it says that:

We're trying to get back to Denver Centennial so we can get the transponder fixed. So that's our plan: ask each ATC facility along our route of flight, and see if we can get permission to fly IFR without a transponder.

Call ATC On The Phone, Finish Placarding

From the FBO, we called each ATC facility on the phone, explained our situation, and asked if we could transition through their airspace under IFR without a transponder.

We talked to Denver Center, Denver Approach, Centennial Tower, and Aspen Approach/Tower.

Each of them gave us the OK to make the flight. After that, we filed our flight plan, and headed out to the plane to finish placarding the transponder inoperative, which is a requirement of FAR 91.213.

Altitude And Position Reports

The flight wasn't much different than any other flight, except that Aspen couldn't see us depart.

Because of that, they needed to put all arriving aircraft in holding while we departed, essentially shutting down the airspace so we could get out.

On our departure, ATC requested position and altitude reports, so they could plot our position, and know when we were out of the way so arriving aircraft to Aspen could start approaches back into the airport.

Handover To Center

Once we were handed over to Denver Center, we were back in primary radar coverage. Center was able to radar identify us, and tag our return with our reported altitude and callsign.

However, we weren't always in their view. With 14,000' mountains in the area, we were in and out of radar contact periodically throughout the flight.

Denver Approach, Centennial Tower

The same was true with Denver Approach. They were able to radar identify and tag our primary return, and track us through the remainder of the flight.

With a vector toward the airport, approach cleared us for the visual, and handed us over to Centennial tower. From there, it was pretty much business as usual. Tower cleared us to land, and a few minutes later, we were on the ground.

Getting Home Safely

The transponder problem was more work for ATC than it was for us.

But other than some position and altitude reports, the flight wasn't a lot different than a normal IFR trip.

Working through the regulations to get out legally isn't that hard. In fact, it's a routine part of flying. And it's a lot better than saying your transponder broke in flight.

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