Let's Talk About ILS Categories: How I Learned to Love the CAT III
Fog. That beautiful, silent killer of VFR flight plans. One minute you’re on a nice visual approach with the runway clearly in sight, the next minute the airport has dissolved into a grey soup and the only thing standing between you and a controlled flight into terrain is a pair of radio beams and your absolute trust in technology invented before the internet.
That technology is the Instrument Landing System. And once you understand how it actually works, you’ll never stop being amazed that commercial aviation has the accident rate it does.
What Is ILS, Actually?
An ILS installation consists of two main radio transmitters. The localizer sits beyond the departure end of the runway and broadcasts a signal along the extended centreline — it tells you if you’re left or right. The glideslope is mounted to the side of the runway, about 300 metres from the threshold, and transmits a beam at roughly 3 degrees above horizontal — it tells you if you’re too high or too low.
Your aircraft’s ILS receiver compares the signals and deflects two needles on your instruments: one for lateral, one for vertical. Keep both needles centred and you’re on the perfect approach path. Simple in theory. Absolutely terrifying at 500 feet in zero visibility with 200 people who’d really prefer you got this right.
The Categories
ILS approaches are certified to different minimums based on equipment accuracy, airport infrastructure, and aircraft/crew certification. Here’s the hierarchy:
| Category | Decision Height | Runway Visual Range |
|---|---|---|
| CAT I | 200 ft | 550 m |
| CAT II | 100 ft | 300 m |
| CAT IIIA | 50 ft | 200 m |
| CAT IIIB | 15 ft | 50 m |
| CAT IIIC | 0 ft | 0 m |
CAT I is standard stuff — you need to see the runway environment by 200 feet or you go around. Most commercial airports and properly equipped GA aircraft can manage this.
CAT II halves the minimums. Requires upgraded avionics, specifically certified runways, and trained crews. You might get the runway at 100 feet. You might not.
CAT IIIA is where the autoland system stops being optional and starts being mandatory. You’re committing to a runway you may not see until you’re on final flare.
CAT IIIB at 15 feet decision height means you will not see the runway until you are functionally on it. The 50 metre RVR means roughly the length of an Airbus A320. That’s all the visibility you’re guaranteed.
CAT IIIC — zero/zero — barely exists operationally. More on that in a moment.
Why CAT III Is Actually Impressive
Here’s what a CAT IIIB approach actually looks like from the flight deck perspective.
The crew sets up the autoland system early in the approach. From around 1,500 feet, the autopilot flies the ILS precisely. At around 400 feet, both the autothrottle and second autopilot channel are fully engaged in land mode. The aircraft is now flying itself with triple redundancy. At roughly 50 feet, the flare sequence initiates automatically — the nose pitches up gently, the throttles retard, and the aircraft settles onto the runway within a few metres of centreline.
And the crew may have seen literally nothing until the mains touched. The approach lighting might have appeared through the murk at 200 metres ahead. The runway lights at 100 metres. The touchdown zone at contact.
The rest was math, radio beams, and the engineering certainty that three independent computers won’t all fail the same way at the same moment.
The Flight Sim Angle
In simulators, CAT III approaches separate the casual simmers from the people who’ve gone too far. Setting up a proper CAT III in the FlyByWire A32NX — LAND mode armed, both autopilots engaged, Low Visibility Procedures active, ATIS checked for LVP conditions — and then breaking through the overcast at 50 feet to find the runway exactly where the ILS promised it would be… it’s a different kind of satisfaction.
The sim doesn’t perfectly model real-world factors — crew certification, runway monitoring systems, approach lighting requirements, the specific maintenance records of the ILS installation. But it gets close enough that the procedure feels meaningful. When it works, it’s genuinely impressive. The aircraft plants itself on the centreline, every time, because math.
The thing simulators don’t model well is the psychological weight of actually committing to a CAT IIIB approach in real life. In the sim, you can pause and try again. In real life, the 200 people in the back are considerably more invested in the outcome.
Zero-Zero: The Holy Grail Nobody Really Wants
CAT IIIC — zero decision height, zero RVR — is technically achievable with fully certified aircraft, runways, and crews. In practice, if you have zero visibility at ground level, you also have a completely different problem: how do you taxi to the gate? The high-speed turnoff is invisible. The taxiway centreline is invisible. The gate is invisible.
The real bottleneck for zero-zero operations has always been the airport surface, not the landing itself. Which is why CAT IIIC exists mostly at the margins of aviation regulations and in simulator scenarios where someone (me, at 1 AM) decides to test it with fog density cranked to maximum.
So next time you’re on a flight and the captain comes on the PA to mention fog at the destination, relax. They’ve briefed the approach. The ILS has it. The autoland has it. The three computers have it. And if they really don’t have it — wrong category, equipment snag, the specific runway not certified — they’ll divert to somewhere that does. That’s what alternates are for.
Now if you’ll excuse me, I have a CAT IIIB into EGLL to brief. In the sim. At 1 AM. With extra fog and a coffee that’s gone cold.