Airframe icing in a mountain pass
You are on a day VFR cross-country in a single-engine aeroplane that is not approved for flight into known icing, cruising at 9,500 ft toward a mountain pass where the terrain rises to 8,200 ft; the grid MSA is 9,900 ft. The OAT is −5 °C and the freezing level is forecast at 5,000 ft. The sounding shows a moist layer from 6,000 to 12,000 ft, and the sky above the pass is not visible; visibility is 8 km in light snow grains. Ten minutes after entering the pass, rime ice about 6 mm thick has formed on the struts and the leading edges, and the airspeed has fallen 8 kt at constant power and altitude. Behind you the terrain falls away to a valley aerodrome at 1,200 ft elevation, 20 NM back, reporting +4 °C. Your planned destination is 60 NM ahead on the far side of the pass.
1. What is happening to the aeroplane, and what does the 8 kt airspeed loss tell you?
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You are in supercooled cloud: visible moisture (snow grains) at −5 °C with the freezing level at 5,000 ft, and you are above it. This is airframe icing, not a sensor anomaly, and it is confirmed visually on the struts and leading edges.
The 8 kt loss at constant power and altitude is the important number. It is the measurable summary of several penalties acting together: drag rising as the accreted ice roughens the surfaces, lift falling as the aerofoil shape degrades, weight increasing, and propeller efficiency dropping. In a light aeroplane, ice on the tailplane can also reduce the download that keeps the nose up, giving a pitch authority problem that arrives without warning.
Treat the 8 kt as a minimum, not a measurement. If the pitot heat is not on, the static source or pitot may be affected and the ASI is then not telling you the truth at all. And note what the aeroplane is not doing: the stall warning may not fire earlier even though the stall speed has risen, because the wing has changed. Fly speed margins, not attitudes.
The aeroplane is not certificated for this. You are outside the envelope the manufacturer designed for, and the correct assumption is that the situation will get worse while you are in the cloud.
2. Evaluate climbing, descending, continuing and turning back, and state your decision.
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Continue. Terrain ahead rises to 8,200 ft with an MSA of 9,900 ft and the sky above the pass is not visible. You would have to outclimb the terrain in an aeroplane whose climb performance is degrading by the minute, possibly entering cloud and losing all visual reference. Worst option.
Climb. The moist layer extends to 12,000 ft, so climbing puts you deeper into the supercooled water and, above 9,900 ft, into airspace where you must be sure of terrain clearance with no visual horizon. Without known tops, climbing to escape is a gamble with no defined end point.
Descend. Descending is the natural instinct, and it is limited immediately by the 8,200 ft pass - you cannot get below the ice without going through the rock. The one thing that makes descent useful is what lies behind you.
Turn back. Twenty NM behind, the terrain falls away and the surface air is +4 °C. That is a defined escape: the ice will start melting as the outside air temperature rises above freezing, and you keep ground contact the whole way. Turn 180°, descend as the terrain permits, tell ATC what you are doing and land at the valley aerodrome.
The decision rule is simple: when you cannot climb or descend out of the icing and you have no known tops, leave the icing horizontally. Do it now, while you still have the performance to fly the turn and the approach.
3. How do you fly the approach and landing with ice still attached, and what do you tell others?
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Assume the aeroplane is not the one in the manual. Stall speed is higher, the wing is less predictable, and power is needed where you would normally glide.
- Speed. Carry a significant margin over the normal approach speed - at least 10 kt, more if the ice is thick or asymmetric - and accept the longer float rather than a slow approach. Undershooting with an iced wing is fatal; overshooting is not.
- Flap. Use less flap, or none, and check that they run to the selected position. Ice can disrupt flap travel and, if it sheds asymmetrically, produce a sudden roll.
- Power and glide. Expect a higher power setting and a steeper, shorter glide; do not fly a tight circuit or a forced-landing profile.
- Landing. Choose the longest into-wind runway available, touch down firmly rather than trying to hold the aeroplane off, and allow a much longer roll-out.
- After landing. Have the airframe and, in particular, the tailplane inspected before any further flight; ice can hide in hinge gaps and control horns.
Transmit a PIREP: location, altitude, temperature, type and severity of icing and, most useful of all, the effect - 'moderate rime, 6 mm, 8 kt IAS loss at 9,500 ft in the pass'. That single call may stop the next pilot from flying into the same air.