Föhn and mountain wave over the Apennines: downdrafts and a cold-altimeter correction
February, 10:00Z. You plan LIRQ (Firenze Peretola) to LIPZ (Venezia Tessera), 160 NM, crossing the Apennines near the Futa pass before descending into the Po plain. Aircraft: PA-28, VFR, planned cruise 9500 ft, endurance 3 h 20 min. MSA in the Apennine sector is 8500 ft; ridge tops are 4000–6000 ft.
Winds and temperature from the briefing: 5000 ft 020/30G45, 9000 ft 020/45, 12000 ft 020/50. OAT at 9500 ft is -18°C. QNH 1026 at Milano, 1019 at Roma. The forecaster mentions lenticular cloud at FL180 downwind of the ridge, small rotor cloud and shredding scud on the lee slopes, and 'classic föhn' conditions with clear, dry, warm air over the plain.
There is no convective activity, no icing forecast, and the destination is forecast CAVOK. You have filed no IFR.
1. Using the wind and the terrain, explain why a mountain wave is expected and where the hazardous sink will be found.
Read the worked answer
The ingredients are all in the numbers. The Apennine ridge runs roughly northwest–southeast, and the wind at 9000 ft is 020°/45 kt – close to perpendicular to the ridge line and strong enough (30 kt is usually quoted as the threshold) to force the stable air over the crest instead of going around it. The air is dry and stable aloft, which is what allows it to oscillate rather than mix out.
The result is a standing wave: the crests are marked by the lenticular cloud at FL180, which stays in one place while the air rushes through it. Beneath each wave crest, on the downwind side of the ridge, there is a downdraft, and below that a rotor zone with roll cloud and the shredding scud the forecaster described.
With a northerly wind, the lee side is the south and southwest slope – the side you cross after passing the crest heading toward Firenze, and the side where the scud sits. The sink can be several thousand feet per minute in the rotor, far more than a PA-28 can out-climb, so the danger is not the crossing itself but the descent on the far side, at low airspeed, close to terrain. The föhn's warm, dry, clear lee-side air is also a warning: good visibility does not mean good conditions.
2. You cross the ridge at 9500 ft and, 3 NM downwind, the VSI shows 1800 ft/min down while the airspeed climbs from 105 to 120 kt. What do you do, in order, and why not simply pull back?
Read the worked answer
Recognise the event first. A sudden 1800 ft/min descent with a simultaneous airspeed increase is a downdraft, not a loss of power and not a stall: the aircraft is maintaining its attitude and the relative airflow has changed. Pulling back is the trap – it converts the excess airspeed into a steep nose-up attitude, and when the downdraft stops and the relative wind reverses at the crest of the next wave, that attitude becomes a stall close to terrain with no height to recover.
So: lower the nose slightly to hold the recommended turbulence/gust penetration speed and keep the attitude steady; accept the altitude loss rather than chase it; and configure for go-around – carburettor heat as required, mixture set. Simultaneously turn away from the ridge, toward lower terrain and, in a wave, usually across the wind rather than straight into it, so you do not fly back into the sink on the upwind side.
If altitude cannot be held above the MSA and the terrain ahead, the correct call is a 180° turn back to the high ground you came from, with the descent continued on the downwind side only once clear. The key discipline is that terrain clearance is the priority, and the only reliable way to regain it is to fly away from the ridge, not to fight the sink.
3. You level at 9500 ft indicated with the local QNH set. Estimate the true altitude and the real terrain clearance over a 6000 ft ridge, and say what you would do about it.
Read the worked answer
Use the ISA comparison. At 9500 ft the ISA temperature is 15 − (1.98 × 9.5) ≈ −3.8°C. You are at −18°C, so the air is about 14°C colder than ISA. Cold, dense air means the altimeter over-reads: your true altitude is lower than indicated.
The rule of thumb gives roughly 4 ft per 1000 ft per °C of ISA deviation: 9.5 × 14 × 4 ≈ 530 ft. A more exact proportional calculation – 9500 × 14/273 – gives about 490 ft. So quote 500–550 ft and say 'true altitude about 9000 ft, not 9500'.
Over a 6000 ft ridge that leaves about 3000 ft of true clearance instead of the 3500 ft on the altimeter – still comfortable, but the margin is 15% smaller than the instrument suggests, and it shrinks further if you are flying at a lower indicated altitude or the ridge is higher than charted.
What to do: add the cold-temperature correction to every published minimum and MSA on a cold day, and keep an extra buffer of a few hundred feet over the ridge rather than the bare figure. Note also that the pressure gradient works in your favour heading north into the 1026 hPa high, so a QNH left at the departure value would add rather than remove height – do not let that cancel the temperature error mentally.