How do aircraft avoid anti-aircraft artillery fire?

Having spent over 16 months getting shot at by enemy AAA, sometimes on an almost daily basis, and having friends shot down by it, allow me to respond.

I assume you mean how aircraft avoid getting hit by triple-A (anti-aircraft artillery) flak, there are fortunately many ways.

• First a foremost is to fly in an unpredictable way - like a white wing dove during hunting season. Fly fast and jink every six seconds, by sudden and quite significant changes in altitude and direction, repeatedly.

• Obviously if Intelligence says the largest AA-gun in an area, for example, is a 37mm with a maximum effective altitude of 15,000 feet, one can avoid it by flying at 20,000 feet. [Of course that will put you in a more lethal, Surface-to-air-missile (SAM) envelope.]
• Of the three kinds of AAA fire - radar guided, optical guided (including new thermal tracking), and barrage fire – the pilot has a lot of help against radar guided AAA with his on-board defensive ECM (electronic countermeasures) jammer & deceiver systems in addition to chaff, along with other nearby aircraft with their own powerful ECM and anti-radar homing missiles.
• Speed is life. The faster you fly, the harder you are to hit.
• Sometimes a pilot may need to fly extremely low, right on the deck. This will be below radar guided capabilities, and with a short and very high angular rate to track, it makes a gunner’s job nearly impossible.
• When flying higher altitudes and with slow firing AAA like an 85mm [effective to 35,000 feet or greater], lookout doctrine from a wingman is helpful. From a mile or so abeam, watching the big black explosions tracking up ever closer behind your wingman, a call for an immediate and extreme maneuver is called for, since he cannot see what is creeping up on him.
  

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During the air war over North Vietnam, while SAMs got everyone’s attention, it was AAA that by far downed the most aircraft.

Thousands of U.S. fixed-wing aircraft were lost to ground fire of antiaircraft artillery (AAA), surface-to-air missiles (SAMs), and fighterinterceptors(MiG)s. The great majority of U.S. combat losses in all areas ofSoutheast Asia were to AAA. …. Among fixed-wing aircraft, more F-4 Phantoms were lost than any other type in service with any nation. In total, the United States lost in Vietnam almost 10,000 aircraft and helicopters,excluding number of UAVs.
Aircraft losses of the Vietnam War

It has been mentioned that because of standoff weapons launched at great ranges, AAA avoidance is not a problem. But it is still a problem for the weapon, which flies relatively slow, does not jink, does not have the ECM capability of aircraft… and is not as smart as a pilot.

While AAA is becoming somewhat obsolete, being replaced with anti-aircraft missiles and soon lasers, there will still remain a very large number of anti-aircraft-artillery for many years to come. Nevertheless the means and methods of avoiding being hit by AAA remain transferable and viable against new age, anti-aircraft weaponry.

AAA was the initial, grisly response to being attacked from the air. It seems natural enough—you bomb me, I shoot you. The most likely image that comes to mind are the clouds of flak that Allied bombers had to penetrate to attack German positions and cities during WW2, with the massive number of huge guns and exploding shells. With the advent of surface-to-air missiles (SAMs), these massive gun batteries gave way to missile batteries for area defense, relegating the gun to a point defense weapon (along with smaller, mobile SAMs as they were developed). Slap on a radar and these guns are suddenly far more dangerous. But since the shells aren’t rocket powered and can’t turn, they aren’t going to reach out and grab you like a SAM will. The larger the gun, the higher it will reach, but you still need to have a lot of these and surprise the aircraft, because at the altitudes most strike aircraft fly, a slight change in heading every 20 seconds will make all of these shells simply fly by. The smaller calibers are just not effective at higher ranges.

The real danger is to helicopters and ground support airplanes. Coming into the range of a ZSU-23 will mess you up. They are quad-barrel, radar-aimed guns with a devastating rate of fire. The way to defend against these is to suppress the radar by either jamming it or hitting it with an anti-radiation missile. But since they generally travel in packs, it’s best to just avoid them.

The curtains of fire over Iraq in ’91 are also iconic of AAA fire, but those were mostly just displays of panic. The coalition had taken the Iraqi radar out of the equation and the gunners were simply spraying the night sky with lead; far more dangerous to the populace, with the falling bullets.
An F-117 pilot wrote later how he saw the seemingly impenetrable garden of destruction. He resolved to continue, essentially giving his life for the mission. Upon emerging from the maelstrom, he marveled that he hadn’t taken a scratch. A sudden anguish overtook him, though, as he realized he was likely the only one in his squadron to survive. His gloom built to a depression until he landed and saw the full ramp. No one had died. No one in the entire strike had even been touched.

There have been some interesting duels between gun and strike aircraft during the Iraqi conflict in the ’90s. I wish someone had posted the footage for these; it’s impressive.

• During Operation Northern Watch, an F-15E Strike Eagle was tracking a gun that was shooting at it. The towed, 57mm S-60 was sitting in a playground, little black puffs marking it firing about every 20 seconds. The trailing eagle illuminated the gun with a laser and the lead dropped a laser-guided inert 1000-pound “bomb”. They are basically just concrete in the shape of a bomb and work very nicely when you need to minimize collateral damage. There was a satisfying puff of dirt and when it cleared, the gun was upside-down and badly bent.
• A UAV marked some wild footage in the South of two figures shooting at passing airplanes. In the frame was another S-60 and from one side of the picture a figure sprinted to the gun, took a few seconds to load, and sprinted back off-screen. A few seconds later, from the opposite side, another person streaked to the gun, quickly fired it (without aiming), and ran like hell off screen. This process continued at regular intervals for several minutes. The UAV, alas, was unarmed. You could practically hear the Jeopardy theme, though.
• The coalition had done such a number on the Iraqi air defense system by the late ’90s that they were executing any crazy idea that could get ordnance near an airplane. They decided to park a multi-rocket launcher vehicle under the air defense lanes, jack up one side of the truck to enable it to point as high as they could get it (they are designed for ground attack), wait for some airplanes to show up, then launch away, everyone’s fingers crossed. So, every once in a while, you’d see a rocket zip past your nose. The flight would call it in and the next launch would include a couple laser-guided bombs. Most often, the truck was gone by then. It was a weird cat and mouse game, with the cat on Quaaludes and the mouse running around with a tiny switchblade and a “Bite Me” tattoo.