What happens if the altimeter is set to 29.82 instead of 28.82?

When the altimeter is set to a pressure of 29.82 inches of mercury (inHg) instead of 28.82 inHg, the readings displayed by the altimeter will not accurately reflect the aircraft's true altitude. Altimeters function by measuring atmospheric pressure; they are calibrated such that a specific pressure setting corresponds to a particular altitude above sea level.

If the setting on the altimeter is higher than the actual atmospheric pressure at the aircraft's altitude, the altimeter will indicate a lower altitude than what the aircraft is actually flying at. This is because the altimeter, using the higher pressure setting of 29.82 inHg, assumes the aircraft is at a higher altitude than it really is based on the lower pressure of 28.82 inHg that should have been set. As a result, the aircraft will end up being lower than what the altimeter indicates.

This situation may lead to potential flight safety concerns, such as flying too low relative to the terrain or obstacles, thus emphasizing the importance of correctly setting the altimeter prior to flight.