Air Data and FMS

I missed a bit in my last post about FMS's. (I am shortening everything to FMS,
it is mostly the same thing, a system). Air data, what is that...

Air data is usually known as the pitot static system on smaller aircraft. Getting the pressure information into the aircraft involves a analog to digital (A to D) conversion.

All aircraft have pitot tubes to measure air pressure because of speed. Pitot tubes are closed tubes, and the airspeed indicator, or air data computer only can measure the pressure of the air trying to get into them. Larger aircraft may have multiple pitot tubes on them, for redundancy mostly. The airspeed indication is made by comparing the difference in pressure from the ambient air, and the pressure forced in the pitot tube.

The ambient air pressure is entered to the system through static ports. There are multiple static ports on most aircraft. The static ports also help measure altitude and rate of altitude change (vertical speed indication). Again the static system measures change in pressure, from ground level to altitude.

There is a relativly simple formula (thanks to wikipedia) for incompressible fluids  pt is total pressure, ps is static pressure, p is density. V becomes the fluid velocity, or airspeed for us.




Air is easily compressible, and that makes the formula a little more complicated, since you have to integrate pressure and stagnation values.

Static pressure is a little easier. The air pressure doesn't change at a constant rate as the aircraft flies at a higher altitude, but the curve is relatively constant. Thanks to engineering toolbox we can use:

  p = 101325 (1 - 2.25577 10^-5 h)^5.25588  

p is air pressure (millibars) and h is height above a fixed point in meters. To get the whole formula, you need to include temperature, and humidity as well, see the wikipedia entry if absolute MSL needs to be measured.   

A small transducer can measure the different pressures, and provide a voltage that the computer can read. Computers are good at math, even complex math, allowing us to have usable information on heads up displays, tapes and other graphical presentations.

The air data computer reads these transducers, and puts the data into a usable format that the FMS can use.

The FMS allows setting marks called bugs on the instruments. If the pilot wants to fly at 250kt indicated airspeed, they may enter a command on the CDU keyboard, it will display on the airspeed indicator. The FMS also allows setting heading bugs, and feeds the flight director when flying on a flight plan.

There are more things in the FMS as well, and as I have time, I'll keep adding to the blog.

Write, and let me know what you think

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.