altimeter setting

[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

 

[TXCAP4228]

It was either three or four and I couldnt remember. A Squared, you may correct me if I am wrong but if you have enough satelites for RAIM (I know its a different issue but I think its 6, or 5 with a barro-assist which obviously doesn't apply here) then you have the same vertical precision as lateral precision. IE, less than the error that is allowable for an IFR approved altimeter.

That's not bad. In any case its a heck of a lot better than using weather radar.

Am I completely off base here?

 

[A Squared]

Texcap,

Having more satellites will improve the accuracy (not precision) of the soulution, including the vertical componnent. I don't know how much.

RAIM isn't so much to increase the accuracy, but to verify the accuracy. It does it with redundant measurements.

If you have 3 satelites, you can get a non-redundent position fix if you supply the altitude.

If you have 4 satellites, you can get a non-redundant 3-d position, or if you supply the altitude, you get a 2-d fix with one redundant measurement.

If you have 5 satellites, you can get a 3-d position with one level of redundency, or if you supply the altitude, you get a 2-D fix with 2 redundant measurements.

If you have 6 satellites, you can get a 3-d fix with 2 redundant measurements.

Now what the redundency does is allow error checking. If you have no redundency, and you have a bad measuremnt, your position will bad, but you can't tell. If you have one redundent measurement, and one measurement is bad, you can tell that one is bad, but you can't tell *which* one. If you have 2 redundent measurements, and one measurement is bad, then you can tell which measurement is bad and drop it out of the soulution.

That conceptually is what RAIM does, how exactly that is programmed, I couldn't tell you, I'm sure it is buried in a TSO someplace.

I *think* (but I'm not sure) that if you're using baro aiding, the altitude displayed on your GPS screen is just your barometric altitude from the encoder.

Anyway, back to the question at hand, all of the things suggested have some use, If I was in the situation, I'd use a non-baro-aided GPS altitude as a reality check, I'd use the Wx radar to make sure I wasn't flying toward terrain at or above my altitude, I'd fly an ILS and check the altitude against DME readouts and crossing the outer marker. All of these things are a tool, none is the answer by itself.