An unspoken truth in general aviation is that FAR 91.171 is one of the most chronically "violated" regulations on the books. It is the regulation that requires a VOR equipment check before any flight under the instrument flight rules. The violations are more technical than substantive. In fact, pilots are constantly checking the accuracy of their VOR equipment as a matter of good operating procedure. What they fail to do, as regularly as required, is follow the technical procedures of the regulation. And they often fail to properly log the technical check.
These failures do not ordinarily come to light unless there is an investigation of an aircraft accident or incident. Then the FAA will routinely do a paperwork check of certificates, logbooks, and other aircraft/airman records, which could uncover the discrepancy. By then it's too late to cure the technical violation.
So, pilots might appreciate this reminder. Let's review what is required to stay in compliance with FAR 91.171. The basic requirement is fairly simple. The regulation requires that before an aircraft may be operated under IFR, using the VOR system of radio navigation, the VOR equipment must have been operationally checked within the preceding 30 days, the results must be within specified limits, and the check must be recorded.
Notice that the requirement does not apply to VFR operations. Notice, too, that it does not apply if some other method of navigation is to be used, such as IFR-certified GPS equipment.
The regulation goes on to spell out the allowable methods of making the check. This part of the regulation is a bit more complicated.
Probably the easiest method allowable under the regulation is to compare one VOR receiver against another VOR receiver installed in the same aircraft. Many, if not most, VOR-equipped aircraft have dual VOR receivers installed. The units are typically independent of each other except for the antenna. This independence is a condition for using this method of checking.
Using this method, both receivers are tuned to the same VOR ground facility. The needles on the omni bearing selectors are centered and the bearing readings on each unit are compared. The maximum permissible difference between the two indicated bearings is 4 degrees.
If this method of checking is not used, and it does not have to be, there are other methods allowable under the regulation.
If the airport from which you intend to depart has an FAA VOR test facility (VOT), that is probably the next easiest method of checking your VOR receiver. Tune in the VOT frequency and center the course deviation indicator (CDI). The OBS should read 0 degrees with the to/from indicator showing "from" and 180 degrees with the OBS showing "to." If not, the maximum permissible indicated bearing error is plus or minus 4 degrees. (Outside the United States a test signal operated or approved by an appropriate authority may be used.)
Another type of VOT signal may be used. It is a radiated VOT signal from an appropriately rated radio repair station. This requires advance arrangement with the repair station to have the test signal transmitted, since repair stations are not permitted to radiate the VOR test signal continuously. If this method is used, the repair station must make an entry in the aircraft records, certifying to the radial accuracy and the date of transmission.
Of equal regulatory priority to the VOT check is an FAA-designated ground checkpoint. If the airport of intended departure has a point on the airport surface that has been designated by the FAA as a VOR system checkpoint, that checkpoint may be used to make the required check. The aircraft is physically positioned on the checkpoint. The checkpoint will indicate the radial from the VOR from which the bearing error is to be calculated. When the receiver is tuned to the VOR and the radial, note the bearing error. The maximum permissible bearing error is plus or minus 4 degrees.
If neither a test signal nor a designated airport surface checkpoint is available, an airborne checkpoint may be used. If available, the airborne checkpoint must be one designated by the FAA (or, outside the United States, by an appropriate authority). An FAA-designated checkpoint indicates the radial that should be received over a specific landmark. The maximum permissible bearing error when flying directly over the checkpoint is plus or minus 6 degrees.
If an FAA-designated airborne checkpoint is not available, you can make your own airborne checkpoint. Select a VOR radial that lies along the centerline of an established VOR airway. Select a prominent ground point along the airway (preferably more than 20 nautical miles from the VOR ground facility). Maneuver the aircraft directly over the point at a reasonably low altitude. The VOR bearing indicated by the receiver when over the ground point must be within 6 degrees or less of the centerline radial.
The locations of airborne checkpoints, ground checkpoints, and VOTs are published by the FAA. They appear in the VOR check section of each Airport/Facility Directory (A/FD). VOT frequencies also appear in the voice communications panels on the government IFR area charts and IFR en route low-altitude charts. They appear on the airport charts of Jeppesen's approach chart service, and in Jepp's enhanced version of the Aeronautical Information Manual.
Here is a good memory device. In all of these checks, notice that, in general, ground checks must be within plus or minus 4 degrees and airborne checks must be within plus or minus 6 degrees. I say "in general" because a VOT check may be made airborne in certain areas and at certain altitudes specifically published by the FAA, and because a dual VOR check may be made while airborne. The allowable bearing error for both of these tests remains 4 degrees even if the test is conducted in the air.
This is the part of the regulation that needs special emphasis. Often a check is made but not properly recorded. The regulation states that each person making a required VOR operational check must make a record of the check, and the record must contain specific information. The entry must specify the date, place, and bearing error of the check, and the person making the check must sign the record entry.
The required entry may be placed in the aircraft logbook. But it doesn't have to be, so long as it is properly recorded somewhere. These entries, which are required so frequently, could eat up a lot of valuable space in aircraft maintenance logs. They could be made elsewhere. For example, aircraft status sheets or flight (as opposed to maintenance) logs are good places to record VOR checks. The record should be kept in the aircraft where it will be available to other pilots who may fly it IFR.
How long should the records be maintained? There is no minimum period specified in the regulation. I recommend that the record should be maintained at least until the uneventful conclusion of the flights to which the check applies.
It may be a burden to comply technically with this regulation, recognizing that all instrument pilots as a practical matter comply. Remember, technical compliance could be important if there is an accident or incident investigated by the FAA.