Altimeter Sensitiviy

[west33freefall 0]

Was just wondering how sensitive our altimeters are in comparison to say a pilots altimeter. Being a pilot you have to understand the difference in how certain things affect your altimeter, heat, open windows, where the static pressure comes from and a few others. How are our skydiving altimeters affected by things like this? for instance from inside the plane to outside the plane? In freefall to under canopy? Temp from altitude to ground level? Belly flying vs sit-flying vs back-flying? And what happens to it if it is in your burble?

[hackish 8]

I think the wrist mounted altimeter needs to be less accurate than the one in the aircraft. I have a digital one and it's temp compensated. It also appears to be very accurate. My optima also has beeps for under canopy and it seems to be very accurate.

-Michael

[riggerpaul 1]

Our altimeters are not considered "sensitive altimeters" in the sense that aircraft are required to have "sensitive altimeters" in the panel. When I was a student pilot, I was told that a "sensitive altimeter" was one that had the 2 needles. In earlier days, there were aircraft altimeters that had only a single needle. These are not "sensitive altimeters" either, and are not legal for use in instrument flight, for example.

The effects you mention, like attitude, certainly do affect the readings on our altimeters. The most dramatic effects will be found when you are in freefall and may have to consider the effect of a burble above a jumper.

If you are falling back to earth your chest mount altimeter will read higher because of the low pressure burble that is above a jumper. Exactly how much higher depends on how fast you are falling and how big a burble you create. In the case of the altitude sensors in an AAD, for instance, the effect is sufficiently pronounced that the manuals often have detailed information regarding how the differences will affect the activation altitude of the device.

My experience says the difference is in the range of a few hundreds of feet, perhaps as much as 500 feet. By the way, the AADs usually talk about a few hundred feet when talking about the difference in activation altitudes, so I think that my observations are well supported. Of course, the difference between 800 and 1200 feet for an AAD is much more important than the difference between deploying at 3000 or 3500 since you are still quite a few seconds from impact in the first place.

A wrist mount will have less of a difference or no difference at all because it isn't usually in a burble. But if you are on your back and you put your wrist in front of your chest to read it, there will be an error similar to that of a chest mount.

Variations under open canopies will be small, as there is not much of a burble to deal with anywhere.

The effect of ram air pressure on the altimeter sensing port should be small. Most altimeters have their sensing ports in a protected location to minimize this effect.

You ask about the difference inside and outside of the jump aircraft. Why not check it yourself on your next jump. It might depend somewhat on the position and type of door you have. A door that seals well might isolate the interior better than the slide up clear plastic door that is often found on ships like the King Air and Twin Otter (etc, etc).

For many or most jumpers, the differences should not be terribly interesting. We should be opening high enough that the errors will not mean the difference between getting a canopy in time or not. Of course, hard decks relating to cutaways and such are more interesting since these are typically much lower than you want to be opening your main. But we are all taught that when you are low and fast we should focus on deploying, not looking at the altimeter.

Swoopers, by virtue of being under open canopies, will see negligible differences. But single needle altimeters will not really let you see the differences of a few tens of feet. These differences are certainly of interest to swoopers. Many or most of the swoopers I know have digital altimeters, some with the extra digit for tens of feet.

[west33freefall 0]

A sensitive altimeter is just one that can be calibrated for sea level pressure. I.e. the barometric pressure window found in all most all modern aircraft. Some home builts and classics I have flown have altimeters that are stuck to 29.92 pressure setting and you have to do the mental math in the air. It kinda sucks but its fun sometimes. I was just curious really how exact the littler skydiving altimeters are. I have noticed that they are rather acurate which really amazed me when I started jumping.

[riggerpaul 1]

Thanks!

I apologize that I relied on old anecdotal knowledge without going to the reference source.

You are correct that a "sensitive altimeter" is defined as having a pressure scale in a window so you can set it in flight. There's nothing about how many needles it has.

I've never flown an aircraft with the earlier sort of altimeter, so you trump me on that point. Are you saying that you couldn't even adjust them to the known MSL altitude when you were on the ground?

I've never seen a "sensitive altimeter" that had only 1 needle. Are there any?

Yes, our altimeters are sensitive in the sense of the physics involved, perhaps amazingly so. But physical sensitivity aside, you can't really read our single needle altimeters to the sort of precision needed for instrument flight, or even really well enough to meet the performance standards of a (USA) Private Pilot certificate. The needle may be in the right place on the dial, but can one tell what that position indicates to closer than about a couple of hundred feet? (I am presuming a mechanical altimeter with a single hand that indicates from 0 to 12000 feet. The new AltiTrack from LB has a hand, but it is driven electronically, so that's not what I am talking about.)

Despite their sensitivity or accuracy, they are fairly easily fooled, and I tried to address the circumstances under which we must be suspicious of the readings we get, and by about how much they might be in error.

[west33freefall 0]

I didn't mean to come off full of myself if I did. But yeah in the old old altimeters there is no adjustments. One I flew you could adjust to the known field elevation. Similar to what we do on our skydive altimeters with adjusting to ground level. But it was just a movement of the dials and could not adjust to changes in pressure levels. Of course these aircraft can't be flown in IFR weather but they are fun to fly because you realy have to rely on your judgement and experience. FYI, one was an old Piper cub, it had an adjustable alt. one was a Dragonfly or something like that home built and one was an experimental C150 that was stripped out with only the required VFR instruments and had a stick instead of a yoke.

[riggerpaul 1]

No problem.

You correctly pointed out an inaccuracy and I wanted to acknowledge that.

As you had knowledge which I didn't possess, I wanted to learn more from you.

Again, thanks!