Army Aviation Information: Density Altitude and a Confined-Area Go/No-Go
A UH-60 crew plans a training insertion into a mountain landing zone. Field elevation is 4,200 ft MSL, the altimeter setting is 29.92 inHg, and the forecast temperature at the LZ is 32 C. The LZ measures 60 ft by 60 ft and is surrounded by 40-ft trees; winds are forecast calm. Planned gross weight is near the aircraft maximum, and the crew is carrying fuel for 2.5 hours. For planning, the crew uses two standard approximations: ISA temperature at a field is 15 C minus 2 C per 1,000 ft of elevation, and density altitude rises about 120 ft for each degree Celsius the actual temperature is above ISA. A crew member suggests launching at 0600 instead, when the forecast temperature is 18 C.
1. Compute pressure altitude and density altitude for the planned launch and state what each number means.
Read the worked answer
Pressure altitude: the altimeter is set to 29.92 inHg, the standard datum, so pressure altitude equals field elevation: PA = 4,200 ft.
ISA temperature at 4,200 ft: 15 - (2 x 4.2) = 15 - 8.4 = 6.6 C.
Temperature excess: 32 - 6.6 = 25.4 C above ISA.
Density altitude: 4,200 + (120 x 25.4) = 4,200 + 3,048 ~ 7,250 ft (accept roughly 7,200-7,300 ft).
Meaning: the aircraft will perform as though it were sitting at about 7,250 ft on a standard day - roughly 3,000 ft worse than the field's true elevation. That is the number the performance charts must be entered with, not 4,200 ft.
2. Given near-maximum gross weight and a 60-by-60-ft LZ ringed by 40-ft trees, what does a density altitude of about 7,250 ft change about the plan? State a decision rule.
Read the worked answer
High density altitude attacks exactly the margins this LZ consumes:
- Power margin for a hovering or steep approach is reduced, and the aircraft needs a balked-landing/go-around reserve that a confined area does not let it build with airspeed.
- Translational lift arrives only after climbing above the tree line, so the aircraft must be able to hover out of ground effect at that weight and that DA before committing to the approach.
- Tail rotor authority in a calm-wind, high-DA hover is at its weakest - no wind to help, no airspeed to trade.
Decision rule: compute hover-out-of-ground-effect torque at 7,250 ft and near-max weight. If the result leaves no reserve to arrest a sink or break a go-around at the decision point, the sortie is a no-go at that weight - it is not a judgement call to be settled in the air.
Levers available: reduce gross weight (fuel or personnel), pick an approach path over the lowest obstacle, or move the launch time.
3. Recompute density altitude for the 0600 launch at 18 C and evaluate whether the time change is worth it relative to other options.
Read the worked answer
ISA at 4,200 ft is unchanged at 6.6 C.
- Excess: 18 - 6.6 = 11.4 C.
- Adjustment: 120 x 11.4 = 1,368 ft.
- Density altitude: 4,200 + 1,368 ~ 5,570 ft.
That is roughly 1,700 ft lower than the 32 C case - a meaningful restoration of power margin and a legitimate reason to shift the launch.
Weigh it fairly, though: a 1,700-ft DA improvement helps, but it does not by itself fix a near-maximum gross weight in a 60-by-60-ft hole. The two levers are independent and should be modelled together: if weight reduction alone brings the required torque inside limits at 7,250 ft, waiting for cool air is optional; if it does not, cooler air plus a weight cut is the combination that makes the sortie legal and safe.