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If you have read my previous article on density altitude then you will already be aware of the dangers of taking off in hot temperatures, at high altitudes, with a few full-sized pax, However, I have a new scenario for you. Let’s say you take-off from a field that has a QNH of 1020 hPa and climb to an indicated altitude (IAS) of 7000ft. The outside air temperature (OAT) at 7000ft is 25°C,  and we fly for two hours, maintaining an IAS 7000ft. En-route, you fly overhead an airfield (at 7000ft) where ATC indicates the same QNH of 1020 hPa, but the OAT is only -10°C?

The first thing we look at, would be the pressure, but as they are coincidentally of equal value, there are no clues there. With the temperature, however, we have significant differences with the International Standard Atmosphere (ISA) temperature at that altitude. ISA temperature at 7000ft is 1°C. How does this affect our flight? If we are maintaining the same QNH, we do not need to change the subscale on our altimeter, and so we should be maintaining our altitude of 7000ft…right?

The way a standard altimeter works, is that it senses the pressure of the air (just like a barometer) and then translates that pressure into an altitude. In other words, because the altimeter is maintaining the pressure, we perceive that the altitude that it is giving us, is correct. The key to understanding this, is that temperature affects the pressure lapse rate. As the altimeter is calibrated to the ISA standard 15°C, any variation from this temperature will provide an error. The altitudes it reads are based on this temperature setting.

If we fly from an area with a high temperature, to an area with a cold temperature, whilst maintaining a constant pressure level, the indicated altitude reads correctly, but we are in fact descending, and no longer at the same true altitude. This might seem puzzling if the altimeter has continuously shown IAS 7000ft, but there is a worthy explanation:

At our initial point, where the temperature is 25°C, and we are at an altitude of 7000ft IAS, we will in fact be flying at a true altitude which is significantly higher,  because the warm air rises, and is therefore less dense for that altitude. However, as we approach our second point, where the temperature is colder, the opposite occurs. A temperature of -10°C is lower than ISA at 7000ft, and therefore, we will be flying at a true altitude which is lower than what is indicated on our altimeter – this can be very dangerous indeed if we are close to the ground!

So how do we keep ourselves safe, and correct for this error ‘on the fly’? Here is the easiest way:

For every 1°C that our temperature deviates from the ISA, we need to add 4ft per 1000ft altitude.

At the warm temperature area of 25°C, the we would work it out like this: 24°C difference x 4ft = 96ft per 1000ft. Therefore, at 7000ft, we will be 672ft HIGHER i.e. True altitude will be 7672ft. At our second point, there is an 11°C difference x 4ft = 44ft per 1000ft, so our indicated altitude is decreased by 308ft, giving us a true altitude of  6692ft.

Even though the temperatures are a bit extreme in this example, it will help you to understand that even though pressure variances are the most significant, temperature can also be deadly! It is probably safe to say that this is the reason that South African law demands a clearance of 1500ft (as opposed to European countries which use 1000ft), particularly as we are prone to these conditions. Hopefully, the phrase ‘HOT TO COLD – DON’T BE BOLD‘ makes a little more sense :).