High-Density-Altitude Missed Approach and a Vacuum Failure
You are the CFII in a Cessna 172 with a maximum gross weight of 2,550 pounds, shooting the RNAV (GPS) runway 13 approach into a mountain airport. Airport elevation is 4,200 MSL, temperature is 24 degrees Celsius, and the altimeter is 30.02. The LNAV minimums are 1,200 MSL with 1 SM visibility. The published missed approach requires a climb gradient of 350 feet per nautical mile to 8,000 MSL before proceeding to the holding fix. At 2,550 pounds and 24 degrees Celsius, the POH shows a climb rate of 380 feet per minute at 75 KIAS at 6,500 feet density altitude. The GPS is IFR-certified with RAIM but not WAAS, so LNAV is the only line of minima. On final at 1,800 MSL, the vacuum pump fails; the attitude indicator and heading indicator become inoperative. The turn coordinator, altimeter, airspeed indicator, and VSI remain. There is no autopilot. The student wants to continue to the MDA and then go missed.
1. Evaluate whether the aircraft can meet the missed approach climb gradient at the planned conditions. What should you do before continuing?
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The aircraft cannot be expected to meet the 350 feet per nautical mile gradient. The pressure altitude is about 4,200 feet, and with a temperature of 24 degrees Celsius the density altitude is roughly 6,300 to 6,500 feet. At that density altitude and gross weight, the POH gives 380 feet per minute at 75 KIAS. True airspeed at 6,500 feet is approximately 75 plus 2 percent per 1,000 feet, or about 85 knots. Climb gradient equals 380 feet per minute divided by 85 knots, times 60, which is about 268 feet per nautical mile. Even using 75 knots instead of true airspeed gives only about 304 feet per nautical mile, still below 350.
Before continuing, you must recognize that the missed approach procedure is part of the approach clearance. If the aircraft cannot comply with the published gradient, you should not accept the approach clearance or continue the approach. Options include waiting for a cooler temperature or lower weight, selecting a different approach with a lower gradient, or using another airport. Do not assume ATC can waive obstacle clearance; the gradient exists for terrain and obstacles. Brief the student on the performance calculation and the go/no-go decision before the final approach fix.
2. If you continue and the vacuum pump fails at 1,800 MSL, what instruments remain, and what is the legal and practical effect on continuing the approach?
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The attitude indicator and heading indicator are inoperative. The remaining gyroscopic instrument is the turn coordinator, which provides rate of turn and bank information but not pitch or direct bank attitude. You also have the altimeter, airspeed indicator, VSI, magnetic compass, and GPS ground track. This is a partial-panel situation.
Legally, 14 CFR 91.205 requires a gyroscopic pitch and bank indicator and a gyroscopic direction indicator for IFR flight. With both failed, the aircraft no longer meets the equipment requirements for IFR. You should not continue the approach under IFR as though the failure were only a training inconvenience. Advise ATC, and if the aircraft is in VMC, cancel IFR and maintain VFR while landing or returning. If in IMC, execute the missed approach using partial-panel techniques and request the safest routing to VMC or an airport with better conditions. Practically, a partial-panel approach may be a training maneuver, but it is not a substitute for required equipment in actual IFR conditions.
3. What does the LNAV minima authorize, and what should you do if RAIM is lost on final?
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LNAV authorizes lateral navigation only. The MDA is 1,200 MSL and the visibility is 1 SM. LNAV does not provide vertical guidance, so the pilot must use the published step-down fixes and not descend below the MDA until the runway environment is in sight and the requirements of 14 CFR 91.175 are met. Because the installation is not WAAS, LPV or LNAV/VNAV minima are not available.
If RAIM is lost on final, the required navigation capability for the GPS approach is no longer assured. You should execute the missed approach and advise ATC. Do not continue to the MDA hoping the signal returns. If the RAIM loss occurs before the final approach fix, request vectors or a different approach. The instructor should use this to teach the difference between lateral-only LNAV and vertical-guidance approaches, and why a loss of integrity is an approach-ending event rather than a nuisance.