In aviation, vertical navigation based on barometric altimetry and vertical references on navigation charts traditionally rely on the use of local barometric pressure, i.e., QNH (or QFE). Hence, operating with an incorrect altimeter setting could lead to flying closer to terrain or obstacles than expected. It may also lead to a loss of separation with other aircraft. In the worst-case scenario, having an incorrect barometric altimeter setting could lead to a loss of adequate terrain clearance and in the worst case, a Controlled Flight into Terrain (CFIT).

When vertical navigation relies on barometric altimetry, a precise barometric altimeter setting is paramount. Otherwise, an incorrect vertical profile will be flown, i.e., either lower or higher than desired, depending on whether the incorrect QNH (or QFE) is, respectively, greater or lower than the actual QNH (or QFE).

The diagram (refer to the attached figure), helps to highlight what the situation might look like from a practical perspective. A 10 hPa error in altimeter setting translates into 280 ft altitude error. This means that the altitude displayed may differ significantly from the actual altitude.

When flying from a high pressure area to a low pressure area without adjusting the altimeter, a constant altitude will be displayed, but the actual height of the aircraft above the ground would be lower than the indicated altitude. “GOING FROM A HIGH TO A LOW, LOOK OUT BELOW”.

When flying from a low pressure area to a high pressure area without an adjustment of the altimeter, the actual altitude of the aircraft is higher than the indicated altitude.

When flying into a cooler air mass while maintaining a constant indicated altitude, true altitude is lower. “FROM HOT TO COLD, LOOK OUT BELOW”.

When the air is warmer than standard, the aircraft is higher than the altimeter indicates.

An altimeter subscale which is set to an incorrect QNH leads to an incorrect altimeter reading. If the subscale is set to a higher than actual pressure, the indication is too high. This may lead to much closer proximity to the ground than intended.