Upgrade: Lav smoke sensors

The first signs of trouble on Air Canada 797, a DC-9 flying at 33,000 ft. en route from Dallas to Toronto, were the wisps of smoke wafting out of the rear lavatory. Soon, thick black smoke started to fill the cabin, and the plane began an emergency descent. Barely able to see the instrument panel because of the smoke, the pilot landed the plane at Cincinnati. But shortly after the doors and emergency exits were opened, the cabin erupted in a flash fire before everyone could get out. Of the 46 people aboard, 23 died.

The FAA subsequently mandated that aircraft lavatories be equipped with smoke detectors and automatic fire extinguishers. Within five years, all jetliners were retrofitted with fire-blocking layers on seat cushions and floor lighting to lead passengers to exits in dense smoke. Planes built after 1988 have more flame-resistant interior materials.

DALLAS/FORT WORTH | Delta Air Lines Flight 191

Upgrade: Downdraft detection

As Delta Flight 191, a Lockheed L-1011, approached for landing at Dallas/Fort Worth airport, a thunderstorm lurked near the runway. Lightning flashed around the plane at 800 ft., and the jetliner encountered a microburst wind shear--a strong downdraft and abrupt shift in the wind that caused the plane to lose 54 knots of airspeed in a few seconds. Sinking rapidly, the L-1011 hit the ground about a mile short of the runway and bounced across a highway, crushing a vehicle and killing the driver. The plane then veered left and crashed into two huge airport water tanks. On board, 134 of 163 people were killed. The crash triggered a seven-year NASA/FAA research effort, which led directly to the on-board forward-looking radar wind-shear detectors that became standard equipment on airliners in the mid-1990s. Only one wind-shear-related accident has occurred since.

LOS ANGELES | Aeromexico Flight 498

Upgrade: Collision avoidance

Although the post-Grand Canyon ATC system did a good job of separating airliners, it failed to account for small private planes like the four-seat Piper Archer that wandered into the Los Angeles terminal control area on Aug. 31, 1986. Undetected by ground controllers, the Piper blundered into the path of an Aeromexico DC-9 approaching to land at LAX, knocking off the DC-9's left horizontal stabilizer. Both planes plummeted into a residential neighborhood 20 miles east of the airport, killing 82 people, including 15 on the ground.

The FAA subsequently required small aircraft entering control areas to use transponders--electronic devices that broadcast position and altitude to controllers. Additionally, airliners were required to have TCAS II collision-avoidance systems, which detect potential collisions with other transponder-equipped aircraft and advise pilots to climb or dive in response. Since then, no small plane has collided with an airliner in flight in the United States.