PA(ft) = Indicated Altitude(ft) + (29.92 − Altimeter(inHg)) × 1000.   ISA Temp(°C) at PA: 15 − 1.98 × (PA/1000).   DA(ft) ≈ PA + 120 × (OAT(°C) − ISA(°C)).   TAS computed from CAS with full compressibility: we derive impact pressure at sea level, solve for Mach at current static pressure, then TAS = a·M (a from OAT). Density altitude is the exact ISA inversion.