Baro-VNAV Temperature Limit and a Corrected LNAV MDA
Destination aerodrome elevation 1,380 ft. You plan the RNP APCH to RWY 09: LNAV/VNAV DA 1,590 ft, LNAV MDA 1,700 ft, both with a required RVR of 800 m. The chart's baro-VNAV note says: 'Not to be used when OAT is below -15 C.' The current OAT is -23 C, QNH 1030 hPa, wind calm, runway dry, RVR 1,000 m. The aerodrome is surrounded by high ground; the missed approach climb gradient is 4.0%, and the chart's minimum sector altitude is 9,800 ft. Your aircraft is RNP AR capable and the crew is qualified. The PF wants to use LNAV/VNAV because it gives a lower DA and a stabilised approach; the PM says the temperature is only a note, not a limit. Fuel is not a factor; the alternate is 20 minutes away with weather well above minima.
1. May the LNAV/VNAV approach be used at -23 C? Explain the technical reason.
Read the worked answer
No. The chart's note 'not to be used when OAT is below -15 C' is a procedure temperature limitation, not advice. Baro-VNAV builds the vertical path from the altimeter and the computed barometric altitude. In air colder than ISA the altimeter over-reads the altitude: at a given indicated altitude the aircraft is physically lower than the altimeter suggests. The published vertical path and its obstacle clearance were designed for a temperature band, and below -15 C the aircraft would be below the nominal path, reducing obstacle clearance. At -23 C you are 8 C outside the limit, so LNAV/VNAV is not available. The lower DA is not a reason to use it. Options: fly the LNAV procedure with a cold-temperature correction, or use another approved vertical guidance if the aircraft and procedure are approved and that procedure's temperature limit is met. Do not 'fly the glidepath visually' to compensate; that does not restore the designed obstacle clearance at the DA.
2. Calculate the cold-temperature correction to the LNAV MDA and state the minima you will use.
Read the worked answer
ISA temperature at 1,380 ft: 15 - 2 x 1.38 = 12.2 C. Temperature deviation: -23 - 12.2 = -35.2 C. Height of MDA above aerodrome: 1,700 - 1,380 = 320 ft. Correction: 320 x 35.2 / 288 = 39 ft. Corrected MDA: 1,700 + 39 = 1,739 ft, rounded up to 1,740 ft. Use the operator's cold-temperature table as the primary source and the computation as a cross-check; if they differ, use the higher correction. The correction applies not only to the MDA but also to step-down fixes on the approach and to published altitudes on the missed approach (including the 9,800 ft area altitude), because the same altimeter over-reading applies there. Do not apply a correction to a DA that already accounts for it; check the chart. Minima to use: LNAV, MDA 1,740 ft corrected, RVR 800 m (reported 1,000 m). Set the altitude bug at 1,740 ft and brief the go-around at that height, not at the published 1,700 ft.
3. What is your decision and what do you brief for the approach and missed approach?
Read the worked answer
Decision: proceed with the LNAV approach, not LNAV/VNAV, using corrected minima. The RVR of 1,000 m is above the 800 m requirement, the runway is dry and the wind is calm, so the approach is permissible. Fuel is not a factor and the alternate is good, so there is no pressure to accept a lower standard. Brief:
- LNAV only; baro-VNAV not to be used at -23 C.
- MDA 1,740 ft corrected. Approach ban: if RVR falls below 800 m before the final approach fix, do not continue.
- Cold-temperature corrections on step-down fixes; verify each crossing altitude against the corrected table.
- Missed approach: published altitudes corrected for cold temperature. The 4.0% gradient is a minimum, and with high ground and cold-air altimeter error, confirm obstacle clearance and consider a higher gradient if performance allows. Brief the first action and the alternate.
- Go-around criteria: unstable by 1,000 ft, RVR below minima, or any doubt. Because the alternate is close and fuel is not limiting, a go-around is a low-cost decision. If the corrected minima cannot be met, divert. Do not continue 'to have a look'.