The Pressure Altimeter — Page 63, Lesson 75

Let’s pick this up right where the datum problem left off, because the excerpt you’re looking at is the worked solution to that very problem — the one with aircraft overhead positions 'A' and 'B', and the two different QNH values. Here’s the setup. You have two airfields, A and B. Overhead A, the QNH is 1025 hPa. Overhead B, the QNH is 995 hPa. Mean sea level, MSL, is the reference for true altitude. Now, the key thing to remember: the altimeter does not measure height above the ground, and it does not measure height above mean sea level directly. It measures height above the pressure datum that you set on the subscale. That is the single most important idea in this whole passage. So at position A, the QNH is 1025 hPa, which is the MSL pressure. That means the 1025 hPa level is exactly at sea level. The aircraft is actually 10,000 feet above MSL, and the altimeter reads 10,000 feet. Fine — no problem there. Now the aircraft flies to position B. Overhead B, the MSL pressure is 995 hPa. But the subscale is still set to 1025 hPa. Remember that pressure decreases as height increases. So if the pressure at sea level is only 995 hPa, then the 1025 hPa level — the level where the pressure equals what’s set on the subscale — must be below sea level. It’s lower than the surface. How much lower? The difference is 1025 minus 995, which is 30 hPa. And the standard conversion is 30 feet per hPa. So 30 hPa times 30 feet per hPa gives you 900 feet. That means the 1025 hPa datum level is 900 feet below MSL. Now here’s the trap. The altimeter is still indicating 10,000 feet — but it’s indicating 10,000 feet above that datum, which is 900 feet below sea level. So the true altitude, the actual height above mean sea level, is 10,000 minus 900, which is 9,100 feet. So the instrument reads 10,000 feet, but the true altitude is 9,100 feet. The altimeter is over-reading. The aircraft is closer to the surface than the instrument says. And that is a potentially dangerous situation. This is the classic rule you must remember: flying from HIGH to LOW pressure causes the altimeter to read HIGH. High to low, altimeter reads high. And that means you are lower than you think. This is exactly why a datum diagram — like the one in Figure 5.8 — helps you sort out this type of problem. You draw the datum level, you draw MSL, and you see where the aircraft really is. Let me give you the two memory anchors from this passage, because they are the foundation of everything. First: pressure always decreases as altitude increases. Second: the altimeter indicates height above the datum set on the subscale. If you hold those two together, you can solve any of these datum problems. Now, the excerpt moves on to a second topic: time lag. This is a different kind of error, and it’s about the instrument’s response, not about pressure settings. With many types of altimeter, the response to a change of height is not instantaneous. There is a lag. And the direction of the error depends on whether you’re climbing or descending. The altimeter under-reads in a climb, and it over-reads in a descent. Think about that — in a climb, the pressure is dropping as you go up, but the instrument is slow to catch up, so it reads lower than the true height. In a descent, the pressure is rising, but the instrument lags, so it reads higher than the true height. The lag is most noticeable when the change in altitude is rapid and prolonged. So a fast, sustained climb or descent is where you’ll see the biggest error. Now, there’s a calibration standard here. In the laboratory calibration of the sensitive altimeter, the lag between increasing readings and decreasing readings should not exceed 150 feet. That’s the tolerance — 150 feet is the maximum acceptable lag. But then there’s a contrast with the servo-assisted altimeter. With servo-assisted altimeters, there is said to be no appreciable lag — unless the rate of change of height exceeds 10,000 feet per minute. That’s a very high rate of climb or descent. The reason is that the servo altimeter does not suffer from the linkage friction that causes a much larger error in the sensitive altimeter. The sensitive altimeter has mechanical linkages that create friction, and that friction is what causes the lag. The servo altimeter uses a servomechanism to drive the display, so it doesn’t have that friction problem. So to tie it together: you have two distinct error sources here. One is the datum error — flying from high to low pressure makes the altimeter over-read, and that’s a potentially dangerous situation because you’re closer to the ground than you think. The other is time lag — a mechanical response delay that makes the altimeter under-read in a climb and over-read in a descent, with a 150-foot calibration limit for the sensitive altimeter, and essentially no lag for the servo altimeter except at extreme rates above 10,000 feet per minute. Both of these are things you will need to recall precisely, because they affect how you interpret what the instrument is telling you in real flight.