We implement two methods to derive Planetary Boundary Layer Heights (PBLH) from AMDAR aircraft data - a Theta-Increase (TI) method and a critical Bulk Richardson (BR) method - and compare hourly PBLH over a four-year period (2021-2024) at 58 airports, using both potential temperature and virtual potential temperature as inputs. The TI method produces 10-30% higher average convective daytime PBLH and 30-60% lower average stable nocturnal PBLH than the BR method, which is likely due to how they define the top of the PBL. PBLH derived from virtual potential temperature produces roughly 10-15% higher PBLH compared to using potential temperature. Seasonally-averaged PBLH from the HRRR model compares well (within 5%) against the AMDAR TI method, with two exceptions. First, the HRRR has delayed morning growth and evening collapse of the PBL during all four seasons, likely due to errors in the land surface model and surface layer scheme. Second, the HRRR convective peak PBLH is too high in summer at 1- and 12-h lead times, likely due to a positive model surface shortwave (SW) radiation bias.