altimeter setting

[Singlecoil]

If you have weather radar on board, you can easily determine altitude +- a few hundred feet which will be close enough until you get down within the operating window of the radar altimiter (if you have one), otherwise the weather radar alone will give you a pretty close estimate. (This is good to know with an altimiter failure, and not only when you're just unable to get an altimiter setting).

I'll bite. How do you do that?

 

[prpjt]

The Archie Altimeter

Adjust the tilt on your radar so the bottom of your beam is 4 degrees below the nose of the aircraft. Your radar display will be dark on bottom and paint ground return from some distance and outward.

Note the distance that the ground return begins painting. Take that # and multiply by 4, then add 2 zeroes. For example, if you are painting ground from 40 miles and outward, 40 x 4 = 160 + 00= 16,000 feet. Ground paint from 25 and out would be an approximate altitude of 10,000 agl.

 

[avbug]

would that brick be Mode C equipped?

It would, but as bricks are still continuing on a waiver to operate without electrical systems, the transponder would be no good.

Aside from that, how does one deal with a RA from a brick falling directly from above? "Dive, dive, no, I mean, really, dive!"

Another means of getting in when you don't have a way to get the current altimeter is find another airplane that's going to your destination (or any destination for that matter, everyone is going somewhere ), and follow them in. Form up just behind them and land at the same time. When you're having coffee with them later, you can ride them about their deviations from the glideslope.

 

[A Squared]

The Archie Altimeter.

I wouldn't put too much faith in using weather radar as an altimeter.

First, assuming that everything is perfect, you know *exactly* what the terrain elevation is 25 miles in front of you, the radar is tilted *exactly* 4 degrees below the horizon, the radar scale reads true distance *exactly* .... even if all of those measurements have zero error, the rule of thumb is going to put you off by about 600 feet if it's nautical miles, if it's calibrated in statute miles, it's going to be about 800 feet off in the opposite direction.

As far as the terrain height, even assuming that you have a sectional with you and you're pretty handy with it, your altitude estimate is only going to be a good as the height of the hills above the surrounding terrain, not a very comfortable thought.

Do you really think that you can read 25 miles off that tiny little screen with any precision? Even if you *think* you can, how accurate do you think that reading will be? I think that it would be optimistic to believe that you could determine the distance of a return to better than +/- 3-4 miles in 25, which is fine for it's intended use. You see a big ugly red blob at the 25 mile line, do you really care if it's 22 miles or 28? That's close enough to avoid flying through it. However, the difference between 22 miles and 28 miles is the difference between estimating your altitude at 9700 feet and 11,800 feet, assuming everything else is perfect.

Finally, the tilt angle. If you turn the tilt knob to about where you think 4 degrees would be, what actually is the tilt of the beam? How accurately do you think that the radar antenna has been installed and calibrated for tilt? To within a degree? Probably not. Even so, what about aircraft attitude, that's going to change with airspeed, gross weight, altitude, probably more than a few degrees. Ok, let's give Archie the benefit of the doubt and say that when you dial in 4 degrees tilt, it's +/- 1 degree (that's being extordinarily generous), that one degree of tilt is the difference between estimating your altitude at 7900 feet and 13,200 feet.

Ok, let's recap, the errors in the Archie Altimeter are +/- 1100 feet due to range error, +/- 2500 feet due to tilt error, +/- the height of the hills in front of you, oh yes and the 600 foot error inherent in the rule of thumb.

Certainly, you could use the weather radar as a means to warn you if you were flying toward terrain which was close to your altitude, somewhere ahead of you, but determining your altitude to within a few hundred feet is a bit unrealistic.

 

[Singlecoil]

Ok, let's recap, the errors in the Archie Altimeter are +/- 1100 feet due to range error, +/- 2500 feet due to tilt error, +/- the height of the hills in front of you, oh yes and the 600 foot error inherent in the rule of thumb.

Certainly, you could use the weather radar as a means to warn you if you were flying toward terrain which was close to your altitude, somewhere ahead of you, but determining your altitude to within a few hundred feet is a bit unrealistic.

I totally agree with that. Even worse, non-standard temperature seriously messes with an altimeter, the ol' true altitude to indicated altitude thing. The notion that you could use a weather radar and derive an altimeter setting that would be accurate to "+- a few hundred feet" is certainly a stretch.

 

[prpjt]

(I wouldn't put too much faith in using weather radar as an altimeter)

I agree 100%

(I'll bite. How do you do that?)

Did I answer your question? If you were wanting to know how to do it and get it to work within a couple of hundred feet, I apologize, I dont know how to do that.

In Archies defense, nowhere at any time did he ever advocate trying to shoot an approach with a radar as an altimeter. This is a ground mapping exercise and a way to see if your radar is working properly. It's been 10 years since he came to Dallas and did our company training, and yes, he is as boring in person as he is on the tapes.

Xjcaptain, I think A Squared explained it pretty well. A radar wouldn't help in this situation(getting an altimeter setting), because what you are utilizing is a Radar Altimeter, and not a very accurate one at that. Also, you are guesstimating hieght above terrain that is 20, 30 40 or 50 miles ahead of the aircraft.

(Doesn't that depend on the width of your radar beam, and antenna size?)

No. A degree is a degree. Doesn't matter if you have a 6,8, or 80 degree beam spread. If you are using the bottom four degrees of the cone the math works.

As to the original post, if you can't get something by listening in the blind, you in essence have an inop altimeter. Go to your alternate(assuming the WX was forcast to be better than destination) or another suitable ILS that doesn't require a vector to get established. Stay high. Do the full procedure and intercept the GS well before the marker. Follow the GS down and you should get a rough altimeter setting at GS/marker intersection. If you have a radar altimeter you use it. I think in your scenario you were fuel limited. If it's low everywhere your alternate should have the best WX , but since altimeter out is a bonafide emergency, you may have to consider taking the GS all the way to the ground. Or get low enough to get a cell phone signal and call FSS.

Another option is to ask the drunk guy in 22c. He knows everything when it comes to planes.

 

[cessna_driver2]

Just whip out the cell phone and call.

Of course you may have to deal with the FCC and an extremely large phone bill. But you get low enough you shouldn't have to worry about the phone bill

 

[ekuflyer]

This happened to me in a 182 on a long x-c in IMC about 6 years ago. The alternator quit and not long after that the battery gave up. So I was left with vaccum and compass, and of course my trusty handheld GPS (the best $1000 I ever spent). I just followed my route till I was directly over Louisville and then called 1800WXBrief and talked to a briefer and he told me the wx was improving in Lexington. So I continued and broke out near Frankfort and landed. The tower was calling when I walked into the FBO, to close my flight plan and make sure everything was alright.

 

[TXCAP4228]

If you have a GPS and thre sat's in view you can get an altitude.

 

[A Squared]

If you have a GPS and thre sat's in view you can get an altitude.

Not quite, you need 4 satellites to get a 3-d position soulution, if you have only 3 satellites, you need to supply your altitude (through an encoder or by punching it in depending on the equipment) in order to get a position fix.

Additionally, even if you have 4 satellites, the altitude you get from a C/A code GPS fix (the kind you get from the average civilian receiver) is not terribly accurate. It's better than nothing, but I'd be extrememly reluctant to fly an approach to minimums relying only on a GPS altitude