Spot the failure.

Oct 20, 2026

See if you can spot the one thing that's failed. Be sure to watch for a few seconds before making your choice. What's failed?

AStatic port

A blocked static port's most immediate effect is on the altimeter and VSI, and it eventually affects the airspeed as you change altitudes. But notice the VSI shows slight changes, first a shallow climb, then descent. If the port were blocked it wouldn't show any changes. And the AI agrees with the attitude reflected by the ALT/VSI. Option D is the answer 🔍

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BVacuum pump

If the vacuum pump had failed, the AI/DG would start to fail, but looking outside, you can see they each agree with the view of the horizon. Also, the vacuum gauge shows pressure is in the green. Option D is the answer 🧭

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CEGT too low

When thinking about EGT readings, don't think of them as a measure of good vs. bad. The gauge is used to adjust the fuel-to-air ratio in the cylinders, and it's a trend device. As you lean, the temperature rises until you reach peak EGT, after which further leaning reduces temperatures. Option D is the answer 🛩️

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DAlternator

You got it 👍

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Pick an answer. You can try as many as you like.

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The explanation

D. Alternator

What the panel shows

The reel opens with everything working normally. A few seconds in, the red VOLTS annunciator comes on and the ammeter swings to the discharge side. Every other instrument keeps working. That combination is the signature of an alternator that has dropped offline, with the battery now carrying the entire electrical load.

How the 172S tells you

The ammeter shows current flowing between the alternator, the battery and the electrical system. With the alternator working, it shows either zero or a slight charge. When the alternator stops producing power, it shows the battery discharging.

The low voltage annunciator comes on when bus voltage falls below 24.5 volts. With the alternator working, the bus sits around 28 volts, so the light stays off.

Two independent indications agree here, which rules out a bad gauge. If only the ammeter moved, you might suspect the instrument. When the VOLTS light agrees with it, the problem is real.

Why it isn't the others

Static port: A blocked static port freezes the altimeter and VSI, and the airspeed becomes unreliable as you change altitude. In the reel the VSI shows small changes, so the static system is working.
Vacuum pump: The attitude and heading indicators agree with the horizon, and the vacuum gauge reads in the green. The 172S has two vacuum pumps, and losing one brings on an amber VAC annunciator, not VOLTS.
EGT too low: EGT is a trend instrument for setting the mixture, not a good-or-bad reading. A low EGT usually means a rich mixture. It also runs off a thermocouple, which generates its own voltage, so it has nothing to do with the electrical system.

Why not the battery?

The battery hasn't failed. It's doing exactly what it should, carrying the load the alternator used to supply. If the battery had failed instead, the alternator would keep the bus up and the VOLTS light would stay off. The battery won't last forever, though, so the clock starts the moment that light comes on.

What the POH says to do

The Cessna 172S POH procedure for VOLTS illuminating in flight (ammeter indicates discharge):

1. Avionics master switch OFF. 2. ALT FLD circuit breaker CHECK IN. 3. Master switch OFF (both sides). 4. Master switch ON. 5. VOLTS annunciator CHECK OFF. 6. Avionics master switch ON.

If VOLTS comes back on: alternator OFF, nonessential radio and electrical equipment OFF, and terminate the flight as soon as practical.