Simtech Aviation

How do I determine which GPS procedures my airplane is legal to fly?

If you’re operating a GPS equipped aircraft under IFR, then you need to know what routes and area navigation (RNAV) procedures your airplane is legal to fly.  The airplane flight manual (AFM) and AFM supplements are the document(s) you need to be familiar with.  They will give you part of the answer, but where exactly in those documents is the question.  Also, there are multiple other supporting documents and pieces of information that must be brought together to successfully plan a flight within the capabilities of your avionics.  

Why do I need to know this?

The Instrument Practical test will most certainly have a few questions about determining your aircraft's RNAV capabilities.  Or maybe you’re in the market for a new or used aircraft and need to determine what the avionics are capable of.  We’ll use a long cross country flight to illustrate the key points.  

The example below uses a new Cirrus (with factory-installed avionics); check out part 2 to see some differences with Cessna G-1000 aircraft. In part 3, I’ll go over the process for all older airframes with new avionics.  

All 3 aircraft have the same capabilities so that the flight planning decisions will be the same.

The scenario is that we are conducting a multi-day ferry flight of a Cirrus SR-22 from Duluth, MN (KDLH) to Camarillo, CA (KCMA).  

Here’s how we would determine RNAV capabilities before planning this IFR flight.  

Assume that the GPS database is current for the duration of the flight.  This example is a situation that you might be presented during the oral exam of an Instrument checkride.

It’s the night prior to our last leg from KPGA (Page Arizona) to KCMA.  We have done all the previous legs on the trip under VFR. Still, this last leg needs to be conducted under IFR due to weather at the destination: forecast at 900’ ceilings and 5 miles visibility with little change expected all day tomorrow.  

Fortunately, Camarillo has an instrument approach, RNAV Z 26, with LPV (Localizer Performance with Vertical guidance) minima of 250 AGL and ¾ miles visibility and another non-LPV GPS approach; either will get us below the ceiling.  

We want to stay at or below 10,000’, we have filed the following route: PGA V208 TBC V210 BASAL V12 PMD V386 FIM SESPE, direct, Maintain 10,000.  


KWJF is our destination alternate since it’s on our route, less than 50 miles from the destination, and is forecasting clear skies all week.  Also, we’ll say for this exercise, that the Hector (HEC) and Palmdale (PMD) VOR’s are out of service.  

But how do we know we are legal to file and fly these procedures?  

Prior to filing, we checked out the AFM - Section 2 - Limitations and found out what procedures and routes this Cirrus is certified to fly.  See the excerpts from the AFM Section 2: Limitations, and don’t get overwhelmed; we’ll highlight the sections below.

Looking at planning the flight backward from the destination, we are legal to fly CMA RNAV Z 26, with LPV approach minimums, since on the second-page item 4 (i and l) say the perspective avionics package is approved for SBAS (SBAS = Satellite-based Augmentation System) in the US we call SBAS Wide Area Augmentation System (WAAS), at any rate, SBAS navigation and is also certified to perform LPV approach operations.  

Radius to fix (RF) is one approach procedure you won’t find on the G-1000 or any other general aviation GPS unit.  Because RF is not listed in the limitations section, we know that our avionics can’t fly an RF approach procedure such as RNAV (RNP) Z RWY 6. 

Next, our alternate airport (KWJF) has only a GPS based approach since PMD is out of service. Still, we are legal here too, as item 4 (h) specifies the equipment meets the requirements of Technical Standard Orders (TSO), TSO-145, and TSO-146; these TSOs refer to WAAS capable GPS units.  If we correlate this with the AIM 1-1-18 (c)(9), it allows us to plan a GPS based approach at both our destination and alternate.

En route, we are legal to file using both the out of service Palmdale and Hector VOR’s.  A TSO-146 (WAAS) GPS does not require the planned route to remain within the service volumes of conventional navaids (VOR) underlying our route.  WAAS capable GPS units are a stand-alone “suitable source of navigation for the route of flight,” assuming there is not a GPS or WAAS outage along the route.

Finally, departing Page, our GPS allows us to select 10,000’ as our initial cruise altitude instead of 16,000 as the MEA is 14,500’ on V210 between Tuba City and Grand Canyon. Still, the MOCA is only 9600’, which will provide us terrain clearance, and GPS will guarantee navigation reception filing based on a MOCA is allowable using a WAAS GPS for navigation as of 2017.

Now let’s look at this same scenario in a G-1000 equipped Cessna and an older aircraft with a new GPS unit.