How to Use and Set an Altimeter

The Altimeter & Types of Altitude – Video Transcript

Today, in this video what I want to talk about is the altimeter. I want to dive into a little bit of how it works, and then talk a little bit about altimetry and setting the altimeter correctly and the value that has.

So, to get us started I wanted to first take a look inside of an altimeter. Behind the face of the altimeter in the back of the instrument, it’s a sealed case and ultimately inside what we’re going to find is what they typically call an aneroid, or aneroid wafer, or aneroid diaphragm. And the intention is this particular aneroid device is just like what we would find in a barometer, it’s an empty container and this material that it’s made out of will ultimately always try to expand against the atmosphere.

Then we put this static pressure from our static source in the case around this aneroid wafer, and that means then that the atmosphere around us is trying to squeeze those androids closed. So as this android expands and contracts that’s ultimately what moves the dials on the face of the altimeter. So, if we climb to higher altitudes, for example, the air becomes less dense, when the air becomes less dense it allows for these androids to continue to expand and that would move the hands clockwise.

Conversely if we descended it to a lower altitude, we know that the pressure of the air will become greater. If the pressure becomes greater, or the density of the air becomes greater, that will squeeze these aneroid wafers even smaller and that means the hands would rotate counterclockwise showing us a lower altitude.

So, looking at the face of the altimeter we see usually that there’s three hands. For our purposes, especially in the flight training world we rarely climb above 10,000 feet, so we’ll really focus mostly on these two. So this big, thick, short hand and then the longer, skinny hands and it basically reads kind of like a clock, where instead of this being hours the shorthand is thousands of feet and instead of this being minutes the longer hand is hundreds of feet. So, in this example we see that we’d be in about 1,500 feet is our altitude.

Okay next I want to talk a little bit about different types of altitudes. The first one we call Indicated altitude, pretty straightforward, it’s whatever the instrument is telling us our altitude is. Ideally our Indicated altitude and our True altitude are the same – that’s our goal. Our true altitude is our altitude above sea level we call it Mean Sea Level, or most people will just say ‘MSL’, and so that’s our altitude above sea level. We use that when we plot a course to fly to someplace just because that allows us to ensure that we’re going to stay away from any terrain, or any other obstacles that might exist.

Another altitude is known as an Absolute altitude, this is our altitude above the ground right, so we commonly refer to that as AGL. This is not something we necessarily read on our altimeter, but obviously while we’re flying, we want to make sure that we’re staying safe distances above any obstacles. In some regard we use AGL to help identify where we’re supposed to be, and we can just sort of translate it to a true altitude and then hopefully read that as an indicated altitude.

Okay so next what I want to do is jump over and talk a little bit about setting the altimeter and sort of why we do it and what value it has for us.

What I’ve done is put together a scenario that we’re going to use to better understand how the altimeter setting works inside the altimeter. In this particular scenario we’re going to assume that we’re going to fly our airplane from Point A to Point B. Over Point A we will ensure that the altimeter is set correctly for the current pressure there, so a sea level pressure or the altimeter setting in Point A is 30.00. That’s what our altimeter is set to and we’re going to leave it set to 30.00 all the way across the entire flight.

So, in this example at the beginning our altimeter is set correctly which means then that our indicated altitude and our true altitude of 5,000 feet are both equal. The altimeter is showing us 5,000 feet and we’re actually 5,000 feet above mean sea level, great. As we continue along the flight the sea level pressure changes, so we notice that the appropriate altimeter setting by the time we get to Point B is 29.90. So now they’re different and that difference is going to result in our indicated altitude being different from our true altitude, so I want to figure out now how much different would it be in this case?

So, to understand that we will make the assumption that the altimeter is assuming that the pressure at sea level is 30.00 because that’s what we still have set in our altimeter. This means then that it’s not measuring our altitude above the actual sea level, which I’m kind of indicating here with this black line but is instead indicating that sea level is somewhere below that. And we can kind of calculate how far below that by using a lapse rate. All right so we know especially at these lower altitudes there tends to be a lapse rate, or change in pressure, that is pretty standard as we change in altitude.

At these lower altitudes that’s approximately 1” of mercury per 1,000 feet. So, we apply that here, where we have a difference of about 0.1”. If I multiply that 0.1” of mercury by this same equation, I’ll find that there’s about a 100-foot difference. So what does that translate to us in the airplane as we’re flying? What that means then is if we do a great job flying the airplane and fly consistently at 5,000 feet indicated to us on our altimeter, but what that actually means is our airplane doesn’t remain at that true altitude. So even though our indicated altitude is consistently 5,000 feet the entire way, by the time we get to Point B we now see that the altimeter still tells us that we’re at 5,000 feet but if we measured our actual altitude above mean sea level we’re about a hundred feet off and in this case we’re below and therefore we’re actually at 4,900 feet.

The opposite would be true, if the opposite occurred in this scenario. So for example if we had flown from a place that had an altimeter setting of 30.00 to a place that had an altimeter setting of 30.10 then we would have drawn this the other way and our altimeter would have set 5,000 feet but we would have actually been at 5,100 feet above sea level.

As an example, we can use an FAA knowledge test question to help us practice with understanding this idea of altimetry. So, the question I have as an example is:

What effect does changing the altimeter from 29.85 to 29.15

have on the indicated altitude?

A) 700-foot decrease in indicated altitude

B) 70-foot increase in indicated altitude

C) 700-foot increase in indicated altitude

So, I have a few things here on the board that I want to go over. The question was about changing our altimeter setting from 29.85 to 29.15. So, what I wanted to do is kind of put this to sort of an altitude example. If we assume our plane is here parked at 1,000 feet, let’s say that’s the field elevation of our airport or something to that effect. We’re parked here and we have our altimeter set to 29.85 and let’s say right now that is the correct altimeter setting.

Okay then our altimeter would read 1,000 feet and that’s because the pressure that we are at is approximately 1,000 feet above, so the pressure is going to be about 1” below right because that’s our lapse rate. So let’s do a little bit of review there, we know that we could calculate the difference after we reset the altimeter to 29.15, we know we can calculate what the altitude difference is here and therefore we would be able to know how much of a change it would be. We can do that because we have this standard lapse rate and the lapse rate tells us that for every 1,000 feet that we climb we will lose about 1” of mercury.

All right so in this case we can use that in reverse, and I can say the difference between 29.15 and 29.85, that difference in pressure then is 85 – 15, so it’s 0.7 inches of mercury. If I apply that to the lapse rate of 1” of mercury per 1,000 feet, we could do a little bit of math. I kind of have that worked out already over here – so 1” of mercury per 1,000 feet, I’m trying to figure out how that equates to 0.7” of mercury over what altitude? What altitude would that mean? To solve this we simply cross multiply and what we end up with then is X equals 0.7 x 1000, so we’ll know then that this difference is equivocal to 700 feet of altitude.

Alright, so now my question is if I changed my altimeter from 29.85 to 29.15, would this show on my altimeter as my altitude going down or my altitude going up? So, I want to better understand how it would impact. Well 29.15 is closer to my aircraft, which means I would be basically saying well if sea level was here, and I measured all the way up to here and that was 1,000 feet, great. But if I set sea level to be here and measure up that obviously is a lower altitude, or a lower range, so what that means then is this 700 feet it’s going to show us lower than where we would be at.

So, in this case if we go back to the possible answers, the answers were

A) 700-foot decrease in indicated altitude

B) 70-foot increase in indicated altitude

C) 700-foot increase in indicated altitude

As we see from the image here, we know that it would be a 700-foot decrease in the indicated altitude.