Quantitative Examination and Comparison of Altimetry Rules-of-Thumb for General Aviation

General aviation rules of thumb (ROTs) for density altitude and true altitude are examined and developed. Both ROTs originate from the same basic principle of hydrostatic balance, but differ significantly in the assumptions made regarding the atmospheric temperature profile. While the ROT for DA assumes a standard atmospheric vertical temperature lapse rate, the ROT for true altitude requires information regarding the observed layer-mean temperature of the atmosphere. Since the layer-mean temperature between the aircraft and the surface is typically unknown, it must be inferred from the temperature at a single level by again assuming a linear lapse rate. This method is shown to perform poorly in cases of strong lower-tropospheric temperature inversions. Direct comparisons of the two ROTs are made to highlight the gross errors that can occur if the two are misused for one another.