Change of Subscale or Pressure MET

A relatively straightforward set of questions once you realise the rules!

If you change the subscale setting, you change the "reference point" that your altimeter is using. If you increase the subscale setting, you are comparing the aircraft position to somewhere with a higher pressure and this "somewhere" must be lower down as this is where the pressure is higher. Because this new reference point is lower down, this means there is a larger distance between aircraft and reference and so your altimeter reading increases.

The opposite is true for lower subscale settings. It is easily remembered as higher subscale setting, higher reading and lower subscale setting, lower reading.

For a pressure change, (ie where the subscale is the same but the surface pressure has changed) the situation is exactly the opposite. If pressure increases, that means the "reference" (the subscale setting level) has been pushed upwards, meaning the altimeter will read less. Conversely, if the pressure decreases, the "reference" (the subscale setting level) moves downwards and the altimeter will read higher.

This can be remembered as pressure up, lower reading and pressure down, higher reading.

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An aircraft is climbing away from an airfield with a QNHQNH —Static pressure at MSL calculated from QFE using ISA temperature lapse rates of 995 hPa set and reaches the transition altitude. What happens to the altimeter reading when the subscale setting is altered?

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