altimeter setting

[Be200pilot]

You are flying along at fl350 when you lose all com radios. you know that an approach is required at your destination/alternate and everywhere else within fuel range. Your going to have to descend sooner or later but how do you get the correct altimeter setting??? You obviously need it below 18 and 29.92 could be way off from actual setting. comments....

 

[ERJdca]

dont you have any paperwork onboard as far as weather???? THose things called metars

 

[ERJdca]

as long as your nav's are working i would also assume that you could listen to a asos/awos over a vor through your speaker?

 

[CitationLover]

do you have a radar altimeter?

 

[The FNG]

Go with forecast then cross-check your glide slope altitiude crossing the marker and correct as necessary.

 

[jergar999]

That's a good one, FNG, I never thought of that.

 

[spngbobsqrpilot]

ok, getting creative here....

Use the "below" mode on your TCAS while still in the FLs or even descending through 180, see how many hundred feet your altimeter is off based on the a/c below. If you are going into a busy airport, you can compare your variance to a/c ahead.

Looking at METARS?? Don't know how much I would want to trust hours old data. I've seen .75" increase in pressure in 30 minutes. So depending on where you are, I'd guess you could see a rapid decrease as well.

 

[xjcaptain]

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).

 

[avbug]

Open a window, toss out a brick, and count the seconds until it strikes something expensive. Then calculate the altitude, and set your altimeter accordingly. Don't any of you read the AIM?

 

[spngbobsqrpilot]

would that brick be Mode C equipped?

 

[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.