Aircraft Maintenance: Do you really need a compass?
“For every complex problem there is an answer that is clear, simple, and wrong.” - H.L. Mencken.
That’s exactly where I found myself when pondering the question, “Do I need to put a compass in the Titan T-51D Mustang that I’m building?”
After all, putting a mechanical compass in a modern aircraft seems akin to putting a wind-up clock in a Tesla. Searching for guidance, I spoke with fellow builders, flight instructors, and avionics experts. The simple answer I received every time was “Yes.” Then I made one last call to my good friend and consummate federal aviation regulations expert, Ric Peri at the Aircraft Electronics Association. Peri’s answer: “Well, let’s go through the FARs and figure it out.” And down the rabbit hole we went…
It started simply enough. FAR 91.205 specifies various instruments required for flight under VFR, starting with these three:
- Airspeed indicator.
- Altimeter.
- Magnetic direction indicator.
Part 23.1303 stipulates the minimum required flight and navigational instruments, and it matches up exactly with FAR 91.205. Part 23.1327 covers the accuracy and installation of the magnetic direction indicator, but it doesn’t define what exactly that is. Can it be an electronic instrument, driven by a magnetometer, or does it have to be a mechanical compass? If we look at the amendments to Part 23 and the associated commentary, we can see that the FAA has gone back and forth on the topic over the years.
In 1993, Amendment 43 to Part 23 addressed the issue directly, and the commentary on the revision explained that a magnetic direction indicator has to be a non-stabilized magnetic compass based on the following explanation: “The non-stabilized magnetic direction indicator, which does not require power from the airplane's electrical systems, provides directional information to the pilot when all other directional navigation systems have failed due to loss of power.” The FAR language was changed to read, “A direction indicator non-stabilized magnetic compass.” No powered instruments need apply. End of story.
This wording was maintained through other amendments, until Amendment 62 in 2012, where the wording for minimum required flight and navigational instruments mysteriously changed back to “A magnetic direction indicator,” language that remains to this day. So, what happened? Does an electronic instrument now count? Sorting through volumes of commentary that went into the rationale for many of the changes in Amendment 62, they all point to “Yes.” Beginning in 2012, there was an industry push to accept that electronic instruments could deliver levels of accuracy and reliability that mechanical instruments simply could not. And so, the requirement for a “compass” has finally been relegated to the history books. A “magnetic direction indicator” is the current requirement for VFR flight, alongside the airspeed indicator and altimeter. (IFR flight requires additional instruments including artificial horizon, directional gyro, slip-skid, rate of turn, and others.)
Given the numerous electronic flight instruments available for the general aviation pilot, will this soon make the classic wet compass a thing of the past? The answer is…maybe. Some primary flight displays and electronic flight instrument systems on the market still list a “compass” as required equipment under their supplemental type certificates. It would certainly have been better if their STC language matched the FAA’s “magnetic direction indicator.” But the pieces are certainly in place to support a retrofit, all-electronic panel that meets the FAA requirements for certification, redundancy, and emergency power backup.
In 2015, the FAA released a policy statement (PS-ACE-23-08) acknowledging the superiority of electronically driven attitude indicators, making it possible for thousands of aircraft to easily upgrade to electronic attitude indicators and dump their old vacuum pumps. These days, many new flight instruments on the market utilize modern magnetometers, delivering reliable and highly accurate magnetic direction indication. I suspect it won’t be long before the old wet compass and all of its issues are a thing of the past. The FAA removed the “compass” specific requirement from the private pilot certification standards back in 2013. Now it’s time for the retrofit avionics industry to step up and complete the process, relegating those compass correction cards to the museums. Until next time, I hope you and your families remain safe and healthy, and I wish you blue skies.
