GPWS – Innovation That Has Saved Lives

Instrument flying was pretty crude in the late 1960s when Don Bateman began developing what would become the ground proximity warning system (GPWS). If pilots followed all procedures correctly every time, there was no danger of hitting the ground. But if pilots made a mistake, there was nothing to warn them of the danger. Pilots, believing they were correctly following an instrument procedure but were not, flew into the ground way too often. 

At that time air traffic control radar coverage was spotty, and few airplanes or FAA radar systems had the capability to see a Mode C transponder altitude report from an airplane. Without reliable and automatic reporting of an airplane’s altitude, controllers could do nothing to warn pilots that they were flying too low.

But most larger airplanes did have radio altimeters. The radio altimeter broadcast a signal straight down and using either Doppler techniques, or radar techniques, could measure the distance to the ground directly below the airplane with accuracy of just a few feet.

The primary purpose of the radio altimeter was to allow very low decision height minimums on ILS approaches. Following the commands of the flight director, or coupled to an autopilot, airline crews were flying Category II ILS approaches down to a decision height of 100 feet above the runway. The precision of the radio altimeter in measuring those final feet on descent made the procedure safe.

The radio altimeter also was a key element in automatic landing systems that were also being developed in the 1960s. The British were first with the Trident to create an autopilot of such precision and reliability that it would actually land the airplane automatically when the visibility was down to a few feet, or even zero. Boeing, Lockheed, and Douglas soon had autoland systems in their jets using equipment from Collins and Sperry – now Honeywell. The radio altimeter measured actual height above the runway during the final part of the descent and triggered the flare logic at the proper height for landing.

But for 99 percent or more of a flight the radio altimeter did nothing useful until Don Bateman realized he could put the information from the radio altimeter to work in a system that could warn pilots when they were flying too close to the ground.

To be as useful as possible the GPWS needed to know the absolute height above terrain provided by the radio altimeter, but it also needed to know vertical speed and when an airplane was on final approach to land. The vertical speed data told the GPWS whether the airplane was descending, the terrain was rising, or both, so the system could calculate a warning in time for the crew to react.

To prevent nuisance warnings the GPWS needed to know that the airplane was on final approach and thus, logically, was descending toward terrain below. On an ILS approach GPWS monitored the localizer and glideslope signals and would warn if the airplane deviated below the glideslope with a voice callout of “glideslope, glideslope.” On a non-precision approach the GPWS compared flap and landing gear position to know that the airplane was on approach, and monitored for high descent rates and speed of closure with the terrain. The warnings were not as precise as on an ILS, but GPWS could still warn if the airplane was descending too rapidly toward the ground.

Bateman was working at Sundstrand – which Honeywell has since acquired – when he received patents for the first GPWS. And almost no airline bought the system. GPWS was only useful after a pilot made a mistake, and airlines were paying pilots to not make mistakes, so they didn’t want to spend the money.

Then, in 1974, a TWA crew descended too early on a non-precision approach intoWashingtonDullesAirportand flew intoMt.Weather, killing all aboard the Boeing 727. It was clear that GPWS could almost certainly have prevented the accident. In short order, and under pressure from Congress, the FAA mandated that GPWS be required in airliners. The system worked and controlled flight into terrain accidents by airliners were all but eliminated in the United States.

But Bateman wasn’t done with terrain warnings. He realized that modern navigation systems in nearly all jets by the early 1990s made it possible for GPWS to “look ahead” for rising terrain. Because the navigation systems know where the airplane is, and where it is going, Bateman saw that it was possible to compare the flight path and altitude to an electronically stored map of terrain around the airplane’s position. He called the system enhanced GPWS.

Bateman and the Sundstrand engineers did much of the E-GPWS development flying in a King Air. I went out to the Pacific Northwest and flew the King Air at mountains in good visual conditions, and every time the E-GPWS gave the “pull up, pull up”  warning in time to miss the mountain that was filling the windshield. The standard GPWS with its primarily “look down” logic couldn’t warn soon enough of the rapidly rising terrain on the same flight path. I wrote about the new system, and almost nobody bought it.

Then, in 1995 an American Airlines crew became confused about their position and clearance and flew into a mountain ridge in Colombia. The next day a reporter from the Wall Street Journal called me and asked if there was something that could be done to prevent such an accident in the future. I described E-GPWS to him and my experience flying it at mountain ridges and told him that it could have prevented the accident.

The following day a story about E-GPWS was on the front page of the Journal. Airlines quickly announced plans to upgrade to the new system, and it soon became a requirement.

Now the White House has announced that Don Bateman is winner of the National Medal of Technology and Innovation for his pioneering work on GPWS, and also on wind shear warning systems. I can’t think of a more deserving winner of the award. Thanks to Bateman’s foresight terrain warning systems are now available to airplanes of virtually any size and capability and the controlled flight into terrain accident has become rare.

The White House is late in recognizing Bateman’s achievements, but the aviation industry did no better. It took new tombstones to prove that GPWS and E-GPWS save lives.

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8 Responses to GPWS – Innovation That Has Saved Lives

  1. Fred von Zabern says:

    We don’t recognize technology and achievement enough, which is what makes America great. The world is now using EGPWS, and hopefully we will continue to provide the world with innovative technology that it wants. Thanks for bringing this to our attention!

  2. Bob Briggs says:

    Good history lesson.
    Now, how do I buy a certified angle of attack meter to put in my Bonanza?

    • Mac says:

      Hi Bob,

      Safe Flight Instruments, the company who’s founder Leonard Greene invented angle of attack measurement and stall warning, does have equipment that will work on your Bonanza. However, even though the hardware meets all FAA standards, it takes a Form 337 to install it on your airplane, and that can take time. But if you are interested, contact Safe Flight and they can get you started.

      Mac Mc

  3. Pingback: Aviation Innovator Honored | High Altitude Flying Club

  4. Peter Rochester says:

    Great post.

  5. Tim Timmons says:

    Mac,

    Just checked my mailbox. You made the cover of SportAviation two months in a row! First the T-6 in October now the Bell 47. Good for you, you are an outstanding addition to the EAA team. Keep doing what you do!

    Blue skies,
    Tim

  6. Hans says:

    Mac,

    I agree with Tim, please continue with doing what you do! Thanks

    Hans

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