Estimating technique

skidoo

skidoo

Line Up and Wait
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skidoo
What is your technique for estimating the height of the cloud bottoms from your flight altitude? There has been times where I passed under some clouds that almost looked like I could reach out and touch it, but by the speed of which they passed overhead, I would think they were still perhaps 2000 ft higher. So, I am wondering what is the simplest technique to estimate the bottoms to ensure you are greater than the required VFR 500 ft below clearance?
 
A helpful method is to pick a spot on the windscreen (a bug works nicely) then watch the bottom of the cloud. Is it moving up in relation to the bug? You're under. Not moving? You're even. Moving down? You'll be in the cloud soon.

Same technique as for traffic. The ones that don't move in the windshield are the dangerous ones.
 
I guess, which I think is probably the only thing any of us can do other than calculate where they should be. I've held and exercised and instrument ticket for several hundred hours so it's an educated guess based upon all the time I've spent bumping in and out of them but it's still just a guess.
 
Ron Levy said:
I check the relevant portion of 91.155 and pick a number greater than the legal limit.
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Depending on the reason(s) for wanting such an estimate (such as when answering a related question during a flight test) that might well be the best choice. But if your actual concern is doing the best you can to maintain legal separation with the bases, it's often possible to use the bases report from nearby ASOSs added to the associated airport's elevation. Subtract your altitude from that (using the altimeter setting from the same source) and you'll be pretty close if the bases are fairly uniform as they often are in good VFR conditions.
 
Personally I'm pretty bad at estimating so everything looks like 501' to me.
 
How does this sound? Say if I am cruising level at 148K, then that would be equivalent to about 250 ft / second. So, if I pick a point in the cloud base that is about 45 deg vertical, then if that point passes directly overhead in 2 seconds, then I would be 500 feet below. If it passes in more than 2 seconds, I am greater than 500 ft, and if it passes in less than 2 seconds, then I would be closer than 500 ft. Anyone do this as a double check?
 
skidoo said:
How does this sound? Say if I am cruising level at 148K, then that would be equivalent to about 250 ft / second. So, if I pick a point in the cloud base that is about 45 deg vertical, then if that point passes directly overhead in 2 seconds, then I would be 500 feet below. If it passes in more than 2 seconds, I am greater than 500 ft, and if it passes in less than 2 seconds, then I would be closer than 500 ft. Anyone do this as a double check?
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A *lot* of margin for error in that. Just don't hit the clouds and give yourself room so that you might have a chance to see an avoid someone popping out of the clouds.
 
skidoo said:
What is your technique for estimating the height of the cloud bottoms from your flight altitude? There has been times where I passed under some clouds that almost looked like I could reach out and touch it, but by the speed of which they passed overhead, I would think they were still perhaps 2000 ft higher.
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You answered your own question there (emphasis mine). ;)

The easy way, if you're near an airport with weather reporting is to add the reported bases to the field elevation as Lance stated. Else, use the perceived speed to estimate it, and realize that the point of the reg is to keep you from hitting that IFR bird that may pop out of that cloud at any nanosecond. Even if you have the required separation, you'll only have a couple seconds to react to the cumuloluminum.
 
jesse said:
Just don't hit the clouds and give yourself room so that you might have a chance to see an avoid someone popping out of the clouds.
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That's really pretty much the best answer, "If someone came busting out of that cloud, would I be able to "see and avoid?"
 
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