Keith Thomassen's IFR Equipment Write-Up

What is required for navigating in the IFR system in an experimental aircraft? This question stimulates lots of opinion and much disagreement, even if you’re asking FAA representatives. That’s because there is room for “interpretation” in the rules. What are those rules and how are they to be interpreted? Let’s reason together starting with what is written.

The rules for flying IFR are given in Part 91.205 for “powered civil aircraft with standard category US airworthiness certificates”. These are not just the rules, they are in fact permission to fly IFR if you satisfy them (and are rated and current). So they are both necessary and sufficient conditions for IFR flight in certified aircraft.

For experimental aircraft your Operating Limitations, issued after the FAA inspection and licensing of your aircraft, determine whether you can fly at night, do aerobatics, or use the aircraft in the IFR system. For IFR approval there will be a statement in those Limitations making
Part 91.205 applicable to you.

The requirements of Part 91.205 can be lumped into two groups; 1) a list of instruments that include those for day VFR, night, and additions for IFR, and 2) the statement that you must have “two-way radio communications system and navigational equipment appropriate to the ground facilities to be used”.

Most of the instruments are self-explanatory, like airspeed, compass, altimeter, clock, etc., but there is some confusion on gyro instruments (rate-of-turn, pitch and bank, directional indicator) now that solid state AHRS devices are being used. The Experimental Aircraft Association (EAA) has worked with the FAA Small Airplane Directorate to resolve that issue; if it performs the function it is acceptable.

A more confusing issue that is addressed here concerns GPS units for IFR flight. There are several FAA documents on using GPS, such as the navigation chapter in the AIM or the circular on GPS (AC20-138a). But advisory circulars are just that, and are not regulatory. So let’s explore the use of GPS for IFR flight.

First, as the EAA also concludes, the equipment does not have to be certified. But they concluded in their written summary that the required navigational equipment statement in 91.205 says that you can’t use GPS for your primary navigation system because it is not ground-based. They conclude that your primary system must be ground based (VOR’s), but that is also true if you installed a certified GPS under TSO 129c, so more discussion is needed there. Also, a GPS certified under TSO 146 does allow a GPS to be your primary navigation system. Let me clarify these points in some detail.

If you had a Garmin 1000 (or a King KLN94, or Garmin 430/530) it cannot be used for your primary navigation system. These are all certified
under TSO 129c, and as such they are certified for supplemental navigation. That’s because the FAA has integrity criteria for discontinuing their use in IFR flight, and the requirement for an integrity monitoring system that can warn you not to use the GPS for navigation. The Receiver Autonomous Integrity Measurement (RAIM) system must warn you if you don’t meet the RAIM requirements for your phase of flight – 2 nm while enroute, 1 nm in terminal areas (within 30 nm of your departing or arrival airport, where you might do a SID, STAR, or missed approach), and 0.3 nm in the approach phase (final to missed).

Clearly, the reasoning goes, if it might be rendered unusable it can only be certified for supplemental navigation. That’s why such a receiver cannot be used at an alternate airport where there is no other type of approach (than GPS). If there is one, of course, you can do the GPS approach there.

Supplemental use is not all that restrictive in that you don’t have to be navigating by VOR and DME if your GPS is within RAIM limits. You must, however, be able to make that flight with the GPS turned off. All ground navaids must be operational (don’t file a route over an inoperative VOR), and your VOR receivers must be operational. Of course, you wouldn’t file GPS-Direct over routes that require RNAV equipment (long distances in areas of sparse navaids), unless you have RNAV equipment. While this is commonly done, ask yourself what you’d do if you had a RAIM failure on that segment.

So, how about your non-certified GPS? If it has the functional requirements of the certified equipment, you should (I say) be able to use it. If questioned, the burden of proof is on you that you have met the “navigational equipment” requirement of 91.205. The FAA could use FAA policy or applicable court decisions to decide otherwise, but here things are grey. At least you should ensure that your unit does the integrity monitoring that is at the heart of the TSO 129c requirements and limitations. Then, you should use it as supplemental to your primary system in the sense I just described.

If you believe your unit meets these standards, is it ok to do GPS (and overlay) approaches with it? If it contains the latest Nav data base and it does the RAIM check internally before you execute it, I say yes. The only difference between enroute/terminal and approach phases is the more stringent RAIM requirement or the latter.

But the world of aviation has evolved, and now there are receivers certified under TSO 146, which requires using the WAAS system. If you never wanted to do NDB, VOR, or ILS approaches, you could fly with this GPS and a COM transceiver for all your flying, and save the money needed for a DME, ADF, VOR, and LOC/GS receivers. This TSO has tougher requirements on position accuracy that can only be met by adding WAAS error correction to your raw GPS position solutions.

Ground stations around the U.S. receive raw GPS position solutions and send their 3D position error (they know where they really are) to geosynchronous satellites in the east and west. Your WAAS enabled GPS receives the errors from those satellites and, by interpolation using errors at ground stations near you, adds a 3D correction. The WAAS-corrected solution is claimed to be accurate to a meter horizontal and two meters vertical (best case, I suspect).

The raw GPS solutions have an accuracy affected by the dilution of precision (DOP), which comes from poor geometry (all the satellites lumped close together gives a lousy solution). As in any triangulation scheme, horizontal position is best measured if stations are on both sides of you (on the horizon left and right). Since vertical position cannot be determined by satellites above and below you (the earth is in the way) your raw vertical position is not nearly as accurate as the horizontal. WAAS corrections largely fix that, so WAAS is critical to vertical operations, such as using GPS altitude for terrain avoidance (TAWS systems) or doing GPS vertical approaches. There are other large errors due to the slowing down of the GPS signal through the ionosphere and atmosphere (light slows down a factor of about 9 in water), and these too are corrected in real time since the ground stations experience the same extra delays.

The Chelton Flight Systems are certified under TSO 146, so they can be used for primary navigaton. So if you purchase the experimental Chelton system, can it be used for primary navigation? Since the FAA also imposes integrity monitoring on 146 units, it must give integrity warnings as specified by that TSO. These include measuring the horizontal and vertical protection limits (HPL, VPL), which you will find on the satellite page of the Garmin 480, the only other GPS certified under TSO 146. The certified Cheton gives the required integrity warnings through its Free Flight GPS engine and software, so if your experimental Chelton uses that GPS engine (this is an option on their Pro system) those warnings are given and (I believe) this satisfies the FAA intent.

But the Chelton is currently certified for LNAV operations only, so LNAV/VNAV and LPV approaches are verboten. Why? If you are going to track a vertical GPS course to LPV minimums, for example, an FAA requirement in TSO 146 is to determine your position 5 times per second, not once as in all TSO 129 receivers and the Chelton. But there is more.

The LNAV/VNAV and LPV approaches are called Approaches with Precision Vertical (APV). This means that, in software, the full scale CDI deflection is reduced as you go down the glideslope much as both ILS localizer and glideslope courses reduce the full scale deflection as you proceed to the runway. The increased sensitivity keeps you in a smaller and smaller box, and you must abort if you can’t keep the needles off the pegs.

So here is another set of requirements on refresh times and CDI sensitivity. By the way, there is also an increased sensitivity horizontally for LPV approaches, but not for LNAV/VNAV, hence the former have the lowest minimums and visibilities (generally). At the moment, the only GPS available, certified or not, that can meet these requirements is the Garmin 480, so at the moment it is a moot point and the real issue today is whether your WAAS GPS can be used for primary IFR navigation.

The requirements extracted here from TSO 129c or 146 are by no means a complete set, and it’s not clear whether you need to meet others not listed. There are requirements on environmental, software, data, and manuals for example, but I believe as for gyros, the main issues are
functionality (which includes fault detection). As the pilot and manufacturer of your aircraft however, the burden of proof is on you to determine if you meet the 91.205 requirements for IFR flight.

Finally, only you can decide what equipment is sufficient for your type of flying. Redundancy is important, and everyone will have a different comfort level with various backup options. But I hope this gives more insight into using one of the many new GPS systems available to the experimental

Keith Thomassen