Thursday, May 22, 2008

Transponder checked and accurate

Every couple of years, I'm required to get my transponder checked for accuracy. For those of you that just said "your what??" allow me to briefly explain. The transponder (aka Xpdr) is an electronic box attached to the altimeter (more accurately an altitude encoder, but just think of it as a box that knows how high the airplane is at any given time) that receives radar transmissions from ATC radar. Without the Xpdr, the ATC radar would see me as an unidentified blip on the screen. Could be anything, really, and that's not sufficient data to keep hundreds of airplanes from running into each other all the time.

That would be horrible for ticket sales numbers, if nothing else, and the airlines would have a tough time filling seats. The Xpdr recognizes when it has been hit by ATC radar and replies back with an increased strength signal that also has two very important pieces of additional data encoded in it: my four digit ID number (which is always 1200 for me and every other VFR airplane out there stooging around) and my altitude. If the altitude that is being reported is incorrect, ATC is keeping airplanes separated with faulty data, which is darn near as bad as having no data at all. So, they make you get it checked and, if needed, calibrated every other year.

I called the outfit that does the inspection for me yesterday afternoon to make sure they could fit me into the schedule and was told that first thing in the morning would be the best time. That suited me just fine, so I got out to the airport at about 0745 to find clear skies and light winds. But oddly enough, the rotating beacon on the control tower was still on which indicates that either it's night, conditions are IFR, or the tower is closed. Two out of those three were easily seen to not be the case, and the tower should have been open at 0730. There were planes in the pattern doing touch & goes, but that doesn't mean that there's anyone in the tower. Figuring I would know one way or the other once I get Papa out of the hangar and ready to taxi, I went ahead and did just that.

Engine started and a call to the ground controller requesting taxi clearance resulted in a quick clearance to runway 22 "via bravo to alpha." Ok, we're still playing the formal comms game, but I'm getting used to it. It's probably for the better anyway. Take off was non-eventful, and I was soon settled in for the ride to the airport where the avionics shop lives. Gas is over $5 a gallon now, so I throttled back from my normal 2,550 RPM to a more (hopefully) economical 2,200 RPM. That still gave me a quite respectable cruise speed of 135 knots, and it made for a much quieter ride. I think I'm going to start using that power setting as a matter of routine. It was only a minute or so out of the way, so I detoured over to the lake to see how the water level is doing. Here is a picture of the boat ramp that last fall ended quite short of the actual edge of the water:

Looks like my worries of a drought may have been premature. As I approached the destination airport from the north, I dialed up the frequency of their automated weather reporting system and found the winds to be within a few degrees of directly down runway 23, and a mere handful of knots besides. Should be easy! Of course, this particular airport has a lot of tree-laden hills just of the end of 23, so there's always updrafts and the like to deal with.

I stayed a little higher than normal while on final to keep out of as much of that disruptive stuff as possible, which naturally led to one of those express elevator arrivals at the runway. Handled with aplomb, if I do say so myself, and a terrific greaser of a landing. With no witnesses. Oh well. It's not like I'm in it for the ego, right? Right?

Taxied up to the hangar and shut down, then headed inside to report my arrival to the avionics guy. "Good morning! I'm here for the VFR Xpdr check I called about yesterday."

[blank stare]

"Aren't you the guy that I talked to?"

"Uh, no," he replied, "I'm not expecting anyone until the guy coming in at 9:00 for some work. You must have talked to the other guy, and he didn't write you on the schedule, and he's not here yet."

He didn't seem too happy about it, and I wondered if I was to be sent packing. That would be unfortunate - it takes a day off of work combined with good flying weather (a rare combination) to get this thing done. He went ahead and got the hangar door open and started getting the test equipment ready, though, so it appeared that all was well. The other guy showed up about 15 minutes later, and I went outside to avoid the storm clouds I saw developing in the office. I don't know what went on in there for the next few minutes, but as we got started working on the plane there seemed to be a distinct chill in the air. Not my fault, of course, but uncomfortable nonetheless, and a perfect situation for turning the BanterAmp(tm) up to 11 and see if I could change the climate a little.

It probably would have been easier if the 9:00 appointment hadn't shown up early. There I was parked in front of the hangar, and there he was in his plane trying to figure out where he was supposed to park. The guy that had arrived a little late tried to make amends by going out on the ramp to try to direct the newly arrived plane to a parking spot, but the pilot didn't see him and went ahead and shut his engine down, leaving the presumptive parking director standing in the middle of the ramp with both hands up in the air.

"He seems to have accepted your surrender," I yelled over to him. Say, was that the glimmer of a smile? Yes, yes, I think it was! This battle could still be won!

The newly arrived pilot got out and was welcomed by the plane parking guy like a long lost acquaintance. As they were walking over to the hangar, I could hear them getting caught up on that status of another acquaintance of theirs who is "currently being hosted by the county" in some sort of facility. Penal, would be my guess. "Well, at least he's losing weight. I think I might go in for a spell myself and see if I can drop a few pounds myself. Be good for me," he said, patting his belly.

Now, there comes a time now and then where you have to take a risk on a single roll of the dice. The nuclear option, so to speak. This, to me, seemed to be one of those times. I went for it (and I apologize in advance for this, dear reader): "It'd do wonders for your love life too, at least in quantity if not quality."

That could have gone either way, naturally, but the odds were with me and the gamble paid off. Big laughs all around, and a distinct change in the atmosphere. Things went along nicely after that. Papa got himself all hooked up to the equipment, and I stood back and watched:

He looks like he's in for surgery with all of those tubes attached to him, but those are just pressure and vacuum hoses attached to the pipes and holes (pitot and static, for those that like the technical terms) that sense the atmosphere to provide airspeed and altitude readings. By adjusting the pressure sensed at each, the test box can fool the instruments into thinking that the airplane is in the air flying. The Xpdr is itself fooled in turn, and transmits the encoded altitude data. An antenna attached to a separate test box receives the transmitted signal and decodes the reported altitude for display. The altitude shown on the altimeter should match that shown on the test box. Mine was indicating 200' low, but a quick adjustment cleared that up. Job done.

I wandered around the airport a little bit. Some of you will remember my description of airport courtesy cars from a few weeks ago:

Courtesy cars are a breed all their own in the automotive world, and are particularly well known for their quirks.

Well, here's a perfect example:

The flight home went as well as the flight down there, but the arrival back at Bolton was a bit odd. I reported in "over Boutn, inbound full stop." Boutn is a defined waypoint about five miles directly off of the runway, and I've used it as my initial reporting point for years without an problems at all. So I was a little surprised to receive a gruff "say direction from airport" in response. It's not like Boutn has moved - it's right off the south west of the airport where it was always been, but whatever.

"Five miles southwest, inbound, full stop." This new attitude from the tower is perplexing, but it's their airport and I can play by their rules. It's not wrong, it's just different. Like I said, I'm getting used to it, and if it's intended to be a safety improvement, that's just fine with me. But then I get this directive:

"Report entering downwind for runway 22."

Well, that's a poser. Remember, I was directly in line with the runway, so I could enter downwind on either the left or the right side. It would be nice to know, from my point-of-view, which it was to be. It's not intended to be up to me, after all. He seemed to realize by my lack of response that something was amiss, and before I could ask for clarification called back with "Correction. Report entering right downwind, 22."

Ah, good!

The wind by this time was something near or above 10 knots out of the north west and was, in fact, a 90 degree crosswind to runway 22. Right downwind put me upwind of the runway, and I didn't compensate enough for the resultant wind drift towards the runway as I plodded along on the downwind leg. That, combined with the same effect applied as I turned right base, caused me to overshoot final by quite an appreciable degree. This required a tighter, steeper turn to final, and I have read enough flying safety articles to know that this how a lot of people die. With a higher than normal bank in the tighter than normal turn, I knew that I had to be especially cognizant of my airspeed to avoid spinning and/or stalling, either of which would be a fatal mistake. The upshot is that by the time I was established on final, I was high and fast. I sailed right on by my preferred exit taxiway (Alpha 3) and slowed down in time to at least make the turn at Alpha 4. It was still a pretty smooth landing, though.

They have been clearing me back to the hangar with "turn right on Alpha 3 (or 4, as it was today), taxi to park via Alpha to Charlie and across the ramp, monitor Ground on point eight." I was ready for that today, but apparently they're trying to keep us on our toes. Instead of what I was expecting, I got "Turn right on Alpha 4, taxi to park via Alpha to Bravo."

Ok, that's the same as saying "do it like we always have in the past," so it was easy enough to do. Except for one thing. He didn't tell me what frequency to monitor. Sometimes they tell you to monitor "point eight," which is the Ground frequency of 121.8, and sometimes they tell you to "remain this frequency," which is the tower frequency of 128.1. I'm not sure what it means if they don't tell you one way or the other. I stayed on Tower. Figured that would be the first place he'd look. I never heard from him either way, so it remains a mystery.


  1. Question: Is the altitude encoder actually connected in any way to the altimeter? Or is its own beast?

  2. Mine is what's known as a blind encoder. It's a stand-alone box that is plumbed into the static line just like the altimeter, and is wired into the transponder to sent the encoded altitude info to the transponder.

    I think you can also get an encoding altimeter where it's all built into one instrument, but I suspect that they cost more, and unless the adjustment was made at the front of the instrument somehow, harder to adjust as well. Then again, those use the aneroid of the altimeter which in theory should require less periodic calibration than the solid state guts of the blind encoder.

    I didn't crawl under the panel to look - I let the guy that was getting paid do that - but I suspect mine is the solid-state one sold by Vans.

  3. Another alternative is to use EFIS output. My Xponder is connected to the GRT EFIS. EFIS has a table for altitude correction.