777-200LR Flight Test Journal: Archives

26 August 2005

Cruise control

Mariann Jansen, autoflight systems engineer

Jim Vanden Brook, autoflight certification leader

Doug Ormiston, autoflight systems engineer

Autopilot is just what the name implies - operating key airplane flight functions automatically. It's like the cruise control on your car. Only much more advanced. Let me give you an idea of what's involved. The Autopilot Flight Director System (AFDS) is the main component to autopilot. AFDS provides two major types of guidance: the automatic pilot and the Flight Director.

Automatic pilot sends out commands that move the control surfaces in the cockpit, and the Flight Director requires the pilot to manually control the airplane, but with some guidance. When flying the Flight Director, the pilot lines up and centers the pitch and roll command bars, which gives her the guidance she wants.

Both systems can have a selection of different modes. For takeoff, only Flight Director guidance is available. Once the airplane gets above 200 feet, and for the remainder of the flight, various autopilot modes are available. The pilots use the Mode Control Panel (MCP) to select the desired AFDS mode, altitude, heading and speed.

For example, there is a mode that just holds the airplane's altitude and one that allows the pilot to change heading. There also are what we call L-Nav and V-Nav modes, or lateral and vertical navigation, which are the most popular. These modes allow airlines to set up pre-programmed flight paths. The paths can always be changed once the flight is in progress.

The most complicated mode is "autoland," which is the automatic landing function. It's the one we spend the most time testing. It has to be very precise and there are a wide variety of problems that can turn up. It has to be able to handle different facilities, terrain and Instrument Landing Systems (ILS) characteristics. On the 777, ILS is currently the only guidance we allow in the autoland mode.

If the airplane gets within range of an ILS and the pilot wants to do an approach, she selects the 'approach' button on the mode control panel. The pilot must manually move the flap lever and the gear handle. Then, typically the pilot will arm auto-speed brakes and auto-brakes, which provide automatic braking on the ground. The airplane can come to a complete stop on the runway without any additional manual action.

We have to meet strict FAA autoland performance and integrity requirements, ranging from how closely we track the ILS beams, to where on the runway the airplane touches down. We have to anticipate almost any condition the pilots may encounter.

Autoland is not necessarily a mirror image of how the pilot would fly. The idea is to get pilots comfortable with what autopilot is doing so they don't have to make a lot of corrections just before touchdown.

A lot of our certification testing package is done using the flight simulator. We first demonstrate in actual flight test a good cross-section of the challenges the system might face in real service. Then, to prove the accuracy of our flight simulation, we use flight test data on configuration, winds, terrain and runway information to recreate the flight test scenarios. Once we know it's accurate, we use the simulation to run a statistical performance analysis covering tens of thousands of approaches, varying everything from airplane configuration, airport parameters and winds, to a selection of failures.

We listen to what the airlines tell us about the autopilot performance and capabilities and usually make a few changes with each airplane derivative. There are several exciting improvements in the works for upcoming models. That's the benefit of technology. It allows us to always improve our products.