Watch a MiG-29 line up and open the throttles, and something happens that no Western fighter does. The main air intakes shut. Two doors swing across the inlets, sealing them off, and the engines start breathing through rows of louvres that have popped open on top of the wing roots instead.

It looks like a malfunction. It is the aircraft working exactly as designed, and the reason is a Soviet assumption about how the next war would begin.

The assumption behind the door

Western air forces of the Cold War planned to fly from long concrete runways with clean surfaces, and largely assumed those runways would still be there. Soviet planners assumed the opposite. They expected their airfields to be cratered in the opening hours of a war, and they designed aircraft that could keep flying from what was left: a repaired strip, a stretch of taxiway, a dispersal surface with debris on it.

A jet engine hates that environment. The intake of a low-slung fighter is a vacuum cleaner pointed at the ground, and a single stone through a compressor can destroy an engine in seconds. Foreign object damage is not an exotic failure mode; it is the single most common way a fighter engine dies in peacetime, let alone on a wrecked airfield.

The MiG-29’s answer was to simply close the front door during the dangerous part of the flight.

A Fulcrum caught mid-cycle, with one intake door deployed and the other still open.

How it actually works

The system has two halves that operate together. A blocker door rotates up inside each main inlet, sealing the duct. At the same time, louvred panels on the upper surface of the leading edge root extensions open automatically, and the engines draw their air from above the wing instead of from in front of the nose.

Air taken from the top of the wing root is not as clean aerodynamically as air taken from a properly designed supersonic inlet. It is turbulent, it comes in at awkward angles, and it costs pressure recovery, which costs thrust. That is why the doors close only for takeoff, landing and low flying. Once the aircraft is up and away from the ground, the doors open, the louvres shut, and the inlets go back to doing their real job.

What it cost

Nothing on an aeroplane is free, and this system is expensive in the currency that matters most: internal volume.

The doors, their actuators and the auxiliary ducting all occupy space inside the wing roots, and on a fighter that space would otherwise hold fuel. The MiG-29’s modest internal fuel capacity and consequently short range is one of the type’s best-known weaknesses, and the FOD system is part of the reason. The aircraft traded endurance for the ability to operate from a bombed airfield, which was a perfectly rational trade in a doctrine that expected a short, violent war fought close to the front.

It also reflects a broader design attitude that Western pilots found startling when the Iron Curtain came down and they finally got to fly the thing.

The intake doors fit that philosophy exactly. They are automatic. The pilot does not manage them, does not select them, and in normal operation does not think about them. The aircraft decides when its engines are at risk and protects them, because the system designer did not assume the pilot had spare attention to give.

Why the newest Fulcrums do not have it

The MiG-35, the most developed member of the family, deleted the dorsal louvres entirely. In their place it uses retractable mesh screens inside the main intakes, the same approach Sukhoi took on the Su-27.

A screen is lighter, simpler, has fewer moving parts and frees the volume the auxiliary ducting used to occupy, which can be filled with fuel. It is a sign of how the assumptions changed. A modern Russian fighter is expected to operate from a proper airfield, and endurance now matters more than the ability to fly out of a cratered strip with rubble on it.

The doors were never a gimmick. They were a doctrine, made out of metal, and you can still watch them close every time an older Fulcrum rolls onto a runway.

The Wings of the Red Star episode on the MiG-29, narrated by Peter Ustinov, with period Soviet footage of the type in service.

A longer documentary on the aircraft and the thinking behind it.

Sources: Wikipedia for the MiG-29 airframe, engine and intake system; Air & Space Magazine, The Truth About the MiG-29, for pilot quotes.