Wednesday, April 06, 2005

Nav Aid Power Sources

Recently, a pilot discovered before taxiing for departure that the ADF was unable to receive the NDB required for the assigned instrument departure. They could tune and identify a number of other, weaker radio navigation stations, but the required one remained elusive. ATC had no information on the status of the beacon.

The pilot had checked NOTAMs before departure and there was no mention of problems or scheduled maintenance on that station. It is usually identifiable on the ground at that aerodrome, and the aircraft equipment had been verified as functional. The tower controller made a couple of telephone calls. A terminal controller asked a few aircraft in flight to verify reception but none of them could receive it either. Eventually the tower controller called the pilot to relay the obvious: the station was non-functional. The pilot was a little taken aback at the lack of concern anyone seemed to have for the sudden unexplained demise of a navigational beacon, but accepted an alternate departure clearance and did the flight.

Later, the radio news reported an electrical substation failure denying power to a large area--including the location of the uncooperative beacon. That solved one mystery, but opened another. Public nav aids used for instrument approaches, as this one is, are supposed to be monitored and have back up power supplies.

An hour later, Flight Services still had no NOTAM on the outage of a facility that serves as an initial fix for the approach to one airport, and both the departure and missed approach guidance for another. The specialist commented that such problems are usually located in either the aircraft equipment or the pilot's procedures. Well yes, that's probably true, but no amount of equipment or procedure is going to allow anyone to fly to a non-functional beacon. I asked, "isn't there supposed to be a back up power supply?"

I was told, "Sometimes, sometimes not." Something to think about when a flag flickers on a VOR or the ADF identifier sinks into static and engine noise. "The back-up power might take a while to come on line." How long? The briefer thought it might be hours.

Trying to salvage a sense of hope and care for my welfare from this exchange I asked, "Is there some way to tell which ones do and which ones don't have back-ups?" I couldn't remember any notation on that in the CFS or the CAP. Some miserable night when storms threaten aircraft and power substations alike, it might be nice to know that my alternate will still have approach facilities.

"Oh maybe in some technical publication." He couldn't give me any more information.

True, we are supposed to, and do, monitor the condition of nav aids while using them, so that a loss of signal would be inconvenient, not be catastrophic, but you'd think someone would pretend to care.

2 comments:

Lord Hutton said...

Wait until something goes wrong;-(

David Megginson said...

I don't think anyone cares because so few pilots actually use ground-based navaids. Most private pilots who fly IFR use an IFR GPS; just about all companies with scheduled air service use an IFR GPS; for all of these people, the NDB was just a 2- or 3-letter ident they punched into the box, not a LF/MF transmitter they had to tune in and identify.

The only exceptions are cheap-bastard owner-pilots like me, who can't (or won't) pay $10-20K to install and certify an IFR GPS in a $65K airplane, and cheap-bastard freight/charter companies, who don't want to pay for the installation costs or the training and procedural changes required to add primary GPS nav to their ops manuals (or whatever they're called).

I once had a controller call me to warn that I was two miles off the airway. Huh? I was more than 40 miles from the nearest VOR, making the zone of ambiguity nearly a mile on each side even for a perfectly-calibrated VOR indicator; add in a +/- 4 degree tolerance, and two miles off the airway is on the airway as far as you can tell by VOR navigation. ATC is pretty used to GPS these days, I think (where two miles off would be far past full CDI deflection).