Baro-VNAV Not Authorised Below Zero
Flight 88 is planned into Echo Valley (KEVL): elevation 5 800 ft, one runway 16/34 of 2 300 m, and one instrument approach, RNAV (GPS) RWY 34, with LNAV/VNAV minima 6 420 ft and LNAV minima 6 540 ft (2 400 m visibility). The chart carries the note 'Baro-VNAV not authorised below 0 °C', a 3.3° final approach angle and a missed approach climb to 9 800 ft. Terrain 1.5 nm left of the final approach course rises to 7 900 ft. Forecast at the ETA: wind 340/12G22, temperature −22 °C, QNH 30.42 in Hg, 3 000 ft broken, 12 km. The crew asks whether they can fly the LNAV/VNAV and, if not, which minima to brief. The operator's cold-temperature table covers KEVL.
1. Which approach may the crew fly, and what is the corrected LNAV minimum?
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Approach selection: not LNAV/VNAV. The chart prohibits baro-VNAV below 0 °C and the temperature is −22 °C. The crew must fly the LNAV procedure (lateral guidance only, no barometric vertical guidance) and apply a cold-temperature correction to the MDA and to the step-down fixes.
Correction:
- Height above the altimeter source: 6 540 − 5 800 = 740 ft.
- ISA temperature at 5 800 ft: 15 − 1.98 × 5.8 ≈ 3.5 °C; deviation = 3.5 − (−22) = 25.5 °C below ISA.
- Δh ≈ h × ΔT / (273 + T_ISA) = 740 × 25.5 / 276.5 ≈ 68 ft, rounded up to 70 ft.
- Corrected MDA ≈ 6 610 ft.
Sanity check with the rule of thumb (about 4 ft per 1 000 ft of height per 10 °C below ISA): 0.74 × 2.55 × 4 ≈ 75 ft - the same answer.
The physical reason matters for the radio call: in cold air the altimeter over-reads, so the aircraft is lower than the altimeter says, and the correction is always added to the published minimum. Visibility is not normally corrected.
2. What correction applies to the missed approach, and why does the terrain note change the way you brief it?
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Missed approach: 9 800 − 5 800 = 4 000 ft above the airport, so Δh ≈ 4 000 × 25.5 / 276.5 ≈ 370 ft → brief 10 170 ft, use 10 200 ft. The correction scales with height, so the highest fixes get the largest correction - the same calculation must be run on every step-down fix and on the MAPt altitude, not just on the MDA.
Why the terrain matters: the correction protects the vertical path only. The 7 900 ft terrain 1.5 nm left of course means any lateral excursion in a balked landing or during the missed approach is a genuine terrain threat in IMC at −22 °C. Brief the published missed approach track, the bank angle limit (normally 25° for a straight or turning MAP), and the fact that the go-around must be initiated early rather than flown low and slow over rising ground.
Dispatcher checks: confirm the operator's approved cold-temperature procedure (and the values in the table) is quoted on the flight plan so the crew is not doing this arithmetic head-down at 6 000 ft, and confirm no other NOTAM changes the missed approach.
3. The release names Ridge (KRID) as the destination alternate: elevation 4 100 ft, LNAV minima 1 100 ft AGL, forecast 900 ft broken, 6 km, temperature −18 °C. Is the release sound?
Read the worked answer
No. Run the same correction on the alternate, which is where most of these errors happen:
- ISA at 4 100 ft ≈ 15 − 8.1 = 6.9 °C; deviation = 6.9 + 18 = 24.9 °C.
- Δh ≈ 1 100 × 24.9 / 279.9 ≈ 98 ft → corrected MDA ≈ 1 200 ft AGL.
The forecast ceiling of 900 ft AGL is below the published 1 100 ft, and further below the corrected 1 200 ft. The visibility (6 km) is irrelevant - the ceiling fails the alternate on its own, and the cold temperature widens the gap.
Consequences: the flight cannot be released as planned. The options are:
- Select a different destination alternate whose forecast, after cold-temperature correction, is at or above its minima, and confirm it is inside the alternate fuel already carried - if it is not, add fuel.
- Delay until the KRID forecast improves, if it does so inside the operating window.
- As a last resort, add enough fuel to use a more distant, weather-clear alternate, which is a payload decision as much as a fuel one.
The general trap: the correction is applied diligently at the destination and forgotten at the alternate. Both minima are affected by the same temperature physics.