The Pressure Altimeter — Page 63, Lesson 75

Let’s pick this up right where the pressure altimeter’s behaviour gets genuinely dangerous — the datum problem, and then the time lag. First, the core rule you must hold onto: the altimeter indicates height above the pressure datum you have set on the subscale. It does not directly tell you height above mean sea level. It tells you height above whatever pressure level you dialled in. Now look at the scenario. An aircraft flies from position ‘A’ to position ‘B’. At ‘A’, the QNH is 1025 hPa. At ‘B’, the QNH is 995 hPa. QNH, remember, is the pressure setting that makes the altimeter read altitude above mean sea level when you are on the ground at that location. So at ‘A’, 1025 hPa is the MSL pressure — the aircraft is genuinely 10 000 feet above MSL. But here is the trap. The pilot has set 1025 hPa on the subscale and keeps it there. When the aircraft reaches ‘B’, the MSL pressure there is only 995 hPa. Since pressure decreases as height increases, the 1025 hPa level — the datum the altimeter is still referencing — is now below sea level at ‘B’. How far below? The difference is 1025 minus 995, which is 30 hPa. And the standard conversion is 30 hPa equals 900 feet. So the datum the altimeter is measuring from is 900 feet below MSL. The altimeter therefore reads 10 000 feet above a datum that is 900 feet below MSL. The true altitude — the actual height above mean sea level — is 10 000 minus 900, which is 9100 feet. So the instrument says 10 000, but the truth is 9100. The altimeter is over-reading, and the aircraft is closer to the surface than the instrument indicates. That is a potentially dangerous situation. And this is the classic rule you must memorise: flying from HIGH to LOW pressure causes the altimeter to read HIGH. High pressure to low pressure, the instrument over-reads, and you are lower than you think. A datum diagram — like the one in Figure 5.8 — is exactly the tool to sort out this kind of problem, because it lets you visualise where the datum level actually sits relative to MSL. Two things to remember from this: pressure always decreases as altitude increases, and the altimeter indicates height above the datum set on the subscale. Now the second topic: time lag. With many types of altimeter, the response to a change of height is not instantaneous. This lag causes the altimeter to under-read in a climb and over-read in a descent. Think about why: in a climb, the pressure around you is dropping, but the instrument mechanism takes time to catch up, so it reads lower than the true height — under-reading. In a descent, the pressure is rising, and the lag means it reads higher than the truth — over-reading. The lag is most noticeable when the change in altitude is rapid and prolonged. There are calibration limits here. In the laboratory calibration of the sensitive altimeter, the lag between increasing readings and decreasing readings should not exceed 150 feet. That is the tolerance you hold the instrument to. But the servo-assisted altimeter is different. 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 is the threshold. Why is the servo altimeter so much better? Because it does not suffer from the linkage friction that causes a much larger error in the sensitive altimeter. The servo system removes that mechanical friction, so it tracks height changes almost instantly. So to summarise the two big ideas: the datum problem — high to low pressure, the altimeter over-reads and you are closer to the ground than you think; and time lag — under-read in a climb, over-read in a descent, with the sensitive altimeter limited to 150 feet of lag in calibration, and the servo altimeter effectively lag-free until 10 000 feet per minute.