Wednesday, May 09, 2012

Something to Squawk About

Note: This is my first attempt authoring a post with blogsy on the iPad, sooo ....

Several years ago a class C tower controller chided a student I was instructing for mistakenly squawking altitude while taxing, remarking that doing so "cluttered up" his screen. Recently an astute reader pointed out a change to transponder operation procedure contained in the February 2012 Aeronautical Information Manual 4-1-20, to wit:

Civil and military transponders should be turned to the “on" or normal altitude reporting position prior to moving on the airport surface to ensure the aircraft is visible to ATC surveillance systems.
This is a significant change from the previous procedure of keeping the transponder in "stand by" mode and squawking "altitude" just before departure. If you're operating an aircraft with a transponder that provides a "gnd" or "ground" mode, my understanding is that mode meets the new requirement.

The motivation for this change would seem to be to prepare pilots and controllers for the deployment of ground surveillance systems, current and future (NextGen/ADS-B). But there might be some problems and here's why.

ATC Not up to Speed

After learning this change had taken effect, I queried two different ground controller (one class C in northern CA and on class D in southern CA). I was startled to hear that neither controller had heard anything about the change nor had they been briefed on it. This leaves dilligent pilots and instructors in an odd position: Should they adopt and teach the new procedure or not? I'd be interested in hearing from other tower or ground controllers on their experiences.

Traffic System Conflicts

The other issue has to do with TIS and other traffic alerting systems (TAS) installed in aircraft. For instance, my initial experience in Cirrus aircraft several year ago revealed an annoying problem: All it took was a pilot holding short with their transponder turned on and the cockpit would be flooded with enough aural traffic alerts to drown out all other radio communications. Thank goodness newer aircraft with TAS have a mute function. TIS-equipped aircraft can suffer from the same issue, barking "TRAFFIC" at inoportune times. G1000 aircraft provide a means to turn off traffic, but this isn't the best use of time in single-pilot operations.

Unintended Consequences

Being a tech nerd who's had my share of close encounters during ground operations, I'm all for technological solutions. What should a pilot do if instructed to taxi before being assigned a squawk code? In these cases I'm planning to set the transponder code to 1200 set the mode to "on." It may take some time to work out the wrinkles and get the FAA, ATC, pilots, and aircraft equipment working toward the same goal.


Chad said...

I think the key word in the first paragraph of 4-1-20 (a) (3) is "should."

Civil and military transponders should be turned to the “on" or normal altitude reporting position prior to moving on the airport surface to ensure the aircraft is visible to ATC surveillance systems.

I interpret that statement to mean: unless instructed otherwise, the transponder should be emitting Mode C replies prior to any aircraft movement. This probably means an ATIS announcement of "squawk standby for taxi" would be prudent on the part of the controller.

What I don't understand is how transponders with a "ground" mode could comply with this. The whole point of "ground" mode is to avoid emitting a reply until airborne, which seems to defeat the whole purpose of that section.

John Ewing said...


I urge caution when developing one's own interpretations of the FAA's regulations and directive, even though ironically they are often written so as to require interpretation.

Historically the FAA practical test standards have used "shall" and "must" to indicate mandatory actions while the terms "should" and "may" to indicate actions that are desirable or permissive.

No changes have been made to 14 CFR 91.215 "ATC transponder and altitude reporting equipment and use." So it's unclear that we can infer the intent based on one word when the other words would seem to indicate a different intent.

As for GND mode, here's what the Garmin G1000 transponder documentation says:

"A green GND indication appears in the mode field of the Transponder Status bar. In Ground mode, the transponder does not allow Mode A and Mode C replies, but it does permit acquisition squitter and replies to discretely addressed Mode S interrogations."

toddgrx said...

Avidyne's TAS600 is supposed to have an "airspeed" switch that should prevent traffice alerts with TAS on while conducting ground ops. Once aircraft is indicating "airspeed" (around 40kts), the TAS unit is supposed to start aural alerts. When it was first installed, I was getting alerts while taxiing... installation facility claimed "faulty switch". Later it was discovered as "faulty installation". Still... I find ground ops alerts to be hit-miss... sometimes I get alerts and sometimes not.