WAAS Up & Stalls At Altitude – Part II

June 1, 2011 by Bruce Landsberg

A friend called the other day to say that he knew of someone who had received a violation for not having a current VOR check as required by FAR 91.171. Incase you’re a little foggy, that’s the reg for VOR equipment check in IFR  operations. To quote:

“(a) No person may operate a civil aircraft under IFR using the VOR system of radio navigation unless the VOR equipment of that aircraft—

(1) Is maintained, checked, and inspected under an approved procedure; or

(2) Has been operationally checked within the preceding 30 days, and was found to be within the limits of the permissible indicated bearing error set forth in paragraph (b) or (c) of this section.”

Now suppose that the aircraft in question is equipped with a WAAS-approved GPS unit? Since WAAS units are approved for sole source IFR navigation, as long as you do not use the VOR function of the WAAS receiver or another VOR radio, a check shouldn’t technically be required. That said, we highly recommend that you make the check anyway and please log it somewhere.

If you are using the VOR function of the WAAS receiver or any non-WAAS GPS unit (which is not approved for sole navigation under IFR) the check is required.

The check doesn’t count unless it is logged but the reg doesn’t specify where it has to be logged or that it has to be carried board the aircraft. But, if asked, you will need to be able to send it to the FAA inspector if requested.

Might be time to update this rule. I wonder if the guidance to FAA inspectors is keeping pace with technology?

Stalls at Altitude – Part II

Comments regarding Air France 447 continue to dominate the news and blogosphere. Thanks to all who commented on this blog. I am reminded of a few observations by Captain Bob Buck, who wrote the classic book, Weather Flying. He noted that attempting to top a thunderstorm was not smart as the aircraft would likely be close to its operational ceiling. One good bump could precipitate a stall. There will be much discussion on what the pilots saw or didn’t. It is perplexing to me why one would climb in those circumstances. The investigation will tell us more.

Finally, those of us who fly glass cockpit aircraft should be thinking about how to handle such aberrations when the infallible magic fails. Obviously, it happens.

Bruce Landsberg
Senior Safety Advisor, Air Safety Institute

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  • Pranesh Dey

    Hi Bruce,
    I’d compare your blogs on Air France 447 as pieces of a jigsaw that’s helping me think clearly. Flying wisdom pilots should never lose sight of. Keenly waiting for the final investigation report to come out and, subsequently, its simplified interpretation by you and other experts for non-pilots like me. Did a strong gust of wind put the plane in a climb? Would like to know till what point the pilots of AF 447 had the chance to recover the stalled plane and at what point they lost that opportunity. It appears stalling a jet close to its operational ceiling is as dangerous as continuing with an unstabilised approach instead of going around. Both cannot be handled by the average human pilot and, therefore, are best avioded.

  • Jose Guido Fernandez

    What about the feared COFFING CORNER

  • Cliff Crabs

    The way I understand it, It is “coffin” – not “coffing”!
    (At least it was in the B-57)

  • Avi Weiss



    The law seems reasonable as written: if you haven’t checked the VORs, you cant use them.

    What seems unreasonable is being violated for not checking them simply because they are in the aircraft, irrespective of their use or not during IFR flight. In your friends case, he might have been able to be saved by pulling the ATC tapes and showing that he was either navigating via /g OR that he was cleared for GPS approach, which would have clearly demonstrated he wasn’t using the VOR equipment, and thus not in violation of the reg.

  • Pranesh Dey

    Hi Bruce, can GA aircraft ride through thunderstorms?

  • Bruce Landsberg

    Pranesh – It’s not recommended. If you’re lucky, you’ll escape with bruises from the seatbelt and perhaps minor damage to the airframe. Or the aircraft could come apart with discouraging results.

  • Pranesh Dey

    Thanks Bruce (sir) for the reply.

  • Gerard

    I dont mean to split hairs, but I think what Pranesh meant was can small single engine aircraft of the type common to “GA”, fly thorugh CBs. I believe there are larger “GA” aircraft that are capable. Still best to keep your distance if possible.

    Wikipedia says – “General aviation (GA) is one of the two categories of civil aviation. It refers to all flights other than military and scheduled airline and regular cargo flights, both private and commercial. General aviation flights range from gliders and powered parachutes to large, non-scheduled cargo jet flights. The majority of the world’s air traffic falls into this category, and most of the world’s airports serve general aviation exclusively.”

  • http://vastaviation.com Earl Kessler

    I believe it was General Yeager who said there is never a reason to fly an airplane into a thunderstorm during peacetime. I suspect this means ALL airplanes.

  • Jeffrey Parker

    I recently moved to an airport that does not have a VOT on the field. What is the best way to check my VORs? My aircraft is WAAS equipped. Thanks.

  • Jim McSherry

    Jeff –
    Most of us have never been at a field where there was a VOT. You should feel blessed.
    There are several other approved methods for performing the VOR check in the absence of a VOT. In order of my preference:
    – (GCP) ground check point : an identified position on the airfield at which the radial to/from a nearby VOR is published. We have such a spot at KPNE on taxiway F, for the terminal VOR which is on the field. You taxi to the marked position, tune to the VOR, and center the needle. It should agree with the identified radial within +/- 4 degrees

    – (dual VOR) either on the ground or in the air, if you have two VOR receivers, you can tune them both to the same VOR, center the needles (TO or FROM, but the same), and read the OBS ring on each one; they should agree within 4 degrees on the ground, 6 degrees in the air.

    or, in the air above a known position on an airway, tune the receiver to the proper VOR, and it should agree with the published radial for the airway within 6 degrees

    In any case, record the check in writing using the Date, Error, Position (where tested) , and your Signature and have that written material available for the flight. Many pilots or clubs keep a small notebook in a seat pocket and routinely make a test every few weeks, to keep within the regs.

    Good luck.

    Blue skies,

  • Jim McSherry

    Here’s some reference material, compliments of cfidarren.com:

    >>”from IFR Checkride Reviewer:

    It’s summertime, and people are flying more. Operations are up, more avgas is being bought, and you can be sure ramp checks are on the rise. Don’t be caught arriving from an IFR flight without a current VOR check onboard.

    You can update yourself in the AIM 1-1-6, and take a look at 91.171 which says:

    Each person performing the check shall record the date, place, bearing error, and signature in an appropriate written log.

    Permissible error:

    * VOT 360 FR 180 TO, on airport, each VOR receiver ±4°
    * VOR checkpoint, on airport, each VOR receiver ±4°
    * Dual VOR check, ±4°
    * Ground reference point (from A/FD) against VOR radial, ±6°

    The A/FD has information about VOT and VOR checkpoints available at airports. Airports make great ground reference points, see the A/FD for exact locations (distance & radial) to perform the check. ”

    I’m not sure about that last method, though.
    Here’s the AIM on the subject (look at 1-1-4):

    I hope that’s more than you asked for.