FMS FMC and how airplanes know where to go.

Flight Management, how does the airplane know where it is, and where it ought to go. The pilot may want to be in charge, but his job is to manage the systems. There are many systems in an aircraft, and many come together in a single computer called the Flight Management Computer (or Flight Management System). The pilot can use the FMS/FMC on the airplane to help manage these systems.

The heart of the navigation system are the gyroscopes and accelerometers. The gyros are known as the Inertial Reference System (IRS). The IRS will be used to measure changes in flight orientation. The IRS will output heading, attitude and change being imparted. Gyroscopes will measure current conditions, accelerometers will measure the change being imparted on the current conditions.

Gyroscopes are great tools for use in aircraft. The horizon gyroscope will hold true through many oscillations of the aircraft, climbs, turns and dives it will usually show the blue side up. The bank gyro will also handle climbs, turns and dives. The directional gyro will maintain heading for hours.

Accelerometers will measure the forces acting on the aircraft in the various directions. As you were taught in instrument training, or perhaps in private pilot ground school, the seat of your pants isn't accurate at measuring change in coordinated flight. Accelerometers are like the seat of your pants, measuring g forces in three directions (forward/rearward, left/right bank and pitch). They will inform the pilot, or flight management system if the aircraft isn't in coordinated flight, or the increase or decrease of thrust is having and effect.

Integrating the accelerometers and the gyros is how the aircraft can measure where it is relative to where it started. When the aircraft is initialized by the pilot, the current latitude and longitude are entered or received from the GPS system. As the aircraft changes position, the accelerometers will measure the forces acting on the aircraft from the TUG as it pushes the aircraft back from the gate. When the aircraft is in flight, turns can be measured by combining the angle of  bank, and the "vertical" acceleration to measure the horizontal component of lift (HCL), and compare it to centripetal force, to measure the rate of a turn.

A couple posts ago, I was going to talk about Kalman filters. This is where the Kalman filter pays dividends. The Kalman filter will take data that isn't perfect, and make some sense out of it. Sometimes gyros or accelerometers will measure unreasonable values, some large, some small. The Kalman filter will make a best effort to use that information in a way that is reasonable (it may throw the data away, or it may smooth it, such that it looks like a normal reading).

The gyros precess. Since bearings and motors are not perfect, the gyro won't always hold the proper heading for the entire trip. A certified IRS should be accurate to about 650 meters in 1 hour. That means that the aircraft know where it is in the world with a 650meter sphere around it. Most modern aircraft will update the FMS with GPS information, allowing the IRS and the GPS to argue about who is more accurate.

The IRS will output all this information, and the FMS will work together to let the pilot know where the aircraft thinks it is. The FMS will talk to the autopilot, and allow it to make corrections to insure the aircraft gets to it's destination.

The FMS will display what it knows to the pilot through various displays. The primary flight display (PFD) will show the pilot the location it thinks it is, along with what is around the aircraft. The control display unit (CDU) will be the user interface where a pilot can enter flight plan, and other information. The ailerons, rudder and elevator will adjust to make the aircraft head to the programmed direction.

When the aircraft is initialized, the pilot will enter a flight plan. The plan will include airports and other waypoints that the aircraft will be flying to. The FMS will also contain the navigation database. The nav database is where all the waypoints are defined, and any important information about them. The nav database is how the FMS uses the IRS data to know if the aircraft is heading to the proper place in space or not.

 About here is where I need to talk about autopilots, and I am running out of space. I'll talk about autopilots in another post.

Keep me up on your thoughts.

ACARS - How Texting Works

Okey, texting with airplanes happens all the time. It is part of the whole process. The pilot needs to know stuff, and without tying up the air with a bunch of information the pilot may mis-interpret, or need to read later, the pilot and folks on the ground can communicate with a medium most of use use, in text.

In most aircraft, there is a keyboard and a screen up near both pilots. They have the ability to use this device to send questions to the ground, and the ground has the ability to send messages up to the pilots. Information can include weather, or flight plan changes, gate assignment. Almost anything can be sent to the pilots on the screen.

For the most part, this system works similar to a cell phone. There are ground stations all over the country. These ground stations listen on certain frequencies for a signal on a certain frequency. when these ground stations hear a message, they forward it to the assigned receiver. Each airline has assigned address(es). Delta doesn't want United hearing their messages, as much as Jet Blue doesn't want Southwest hearing their messages. Each aircraft has its' own address as well.

These ground stations are owned by various carriers, similar to cell phones. ARINC and SITA are the two major players in the world. There are some smaller carriers as well, and they are limited to certain regions in the world. The carriers don't typically inter-connect messages. If your airline is using ARINC all messages will be on ARINC equipment once they leave the operations center, until they get to the aircraft.

Different stations in the world use different frequencies so the aircraft don't overcrowd a single ground station. The ground station frequencies are similar to the VHF navigation and communication frequencies already used on the aircraft. Most of the ACARS frequencies are in the 129 to 137MHz range. Each ground station can cover about 200 miles on these frequencies.

If a pilot wants to send a message to a dispatcher in the pilot's airline operation center the pilot would tune to the nearest frequency that is on their chart, and enter the message on the keyboard. The message would get transmitted to the ground station and the carrier would forward the message to the operations center for that airline. When the dispatcher receives the message, they can enter a response. The dispatchers response will be forwarded to the carrier, and based on the last known location, the carrier will forward the message to the nearest ground station. The ground station will send the message to the aircraft.

There are a couple 'if's above. The communications protocol is quite robust, allowing for queued messages to stay queued until the ground station receives the message, and acknowledges it. If a ground station is out of service, or the aircraft is tuned to the wrong frequency, the message will sit on the aircraft, until the situation improves. If nothing else, the messages will be cleared when the aircraft power cycles itself (IE shutdown, and brought back up), no one wants to hear about something that happened yesterday.

There are automatic messages sent over ACARS as well. When the aircraft is first powered up, and the pilot initializes the computers a message will typically be sent to the operation center. This message will go into a database, and allow the airline to look at when things got started, what flight the aircraft is assigned to, and other such information. When the doors are shut, and the brakes are released an out gate time message will be sent to the operations center, and when the aircraft squat switches are showing no weight on wheels, an off ground message time is sent. The time messages that the operations center knows about and uses are called the OOOI (ooey) times, Out gate, Off ground, On ground, and In gate. There are other times, like in range that the gate wants to know about as well.

The pilots will use ACARS for many operational items. If ATC needs to divert and aircraft, the ACARS will be a way the dispatcher and the pilot can determine if there will be operational impacts to ATCs request. Will there be enough fuel to take the new route, or will the new route cause people to be delayed are all considered. If the pilot needs to know about weather ahead, some airlines have the capability to send messages to the aircraft if there are significant changes to the weather.

The ACARS unit will ding when a new message comes in. This ding is handy should the pilot be working a situation in the air, and need to know when the resources on the ground have more information. The ding can be a distraction when the pilots workload is high. Most airlines limit the ding to when the aircraft is above 10000ft. Messages can still happen when the aircraft is below 10000ft, but the ding will not distract them.

Next time you are flying, and you wonder where the pilot got all the up to date information, it probably came over the ARARS unit on the airplane.