Navigation and autoflight, this is really B-2 stuff and highly complicated. At least this is what we keep telling the greasy B-1 guy's so they won't get interested in it.

The most important thing in order to get from point A to point B is to know where the aircraft is respective to point A and point B. The aircraft needs to know where it is, it needs to know where point A is and it needs to know where point B is and to make things even more interesting it also needs to know what waypoints it should be using and what diversion alternatives it has and which departure and approach it should be using. Each airport has SID's and STAR's (Standard Instrument Departure and Standard Terminal Arrival Route), these standard routes tell the flight crew from a certain point how to approach the airport or in the case of a STAR how to get to a certain exit point from where the aircraft can start to follow the flightplan towards his destination.

SID for schiphol airport amsterdam.

amsterdam approach by night.

That aside, how does the airplane actually know where it is?
There are many different navigation systems available for aircraft of wich the most famous is G.P.S. or global positioning system. Most aircraft operating today do have G.P.S. installed but a standby navigation system is still mandatory. What if something happens to the signal? or what if there is a global G.P.S. failure? There have to be backups to the system, mandatory navigation systems on aircraft.

A.D.F. or Automatic Direction Finder, O.N.S. or Omega Navigation System (external link!), V.O.R./D.M.E. or VHF Omnidirectional Range / Distance Measurement Equipment (VOR/VOR, VOR/DME or DME/DME), A.H.R.S or Attitude Heading Reference System, and finally the dead reckoning system I.R.S. or Inertial Reference System. There are several more navigation systems including four beacon's system, O.N.S, LORAN, Inertial Guidance System etc but these are generally not used on commercial aircraft (O.N.S. is only found in really old aircraft and A.D.F. is also on its way out).

Most aircraft today operate with GPS, IRS, DME/VOR, VOR/VOR and DME/DME for FMS or Flight Management System navigation, ADF as a relative bearing aid (way too unacurate to be used as a pure navigation system anyway) and IRS as a backup system wich uses measured accelerations and decelerations and rotations and calculates a present position from the known start position.

The actual aircraft position that the 'flight management system' uses as a so called PPOS or 'present position' is derived from combining data from the I.R.S. and preferably the G.P.S., if the G.P.S. is unable to deliver data the next preferred data is coming from combining the I.R.S. data with the DME/DME data, then the I.R.S. data with the DME/VOR, then the I.R.S. data with the VOR/VOR etc. until only the I.R.S. if left (no other input from outside sources, this is called 'dead reckoning').
These examples are not on all aircraft different aircraft have different inputs of navigation, the present generation 737 for example has no VOR input into the flight management system so if it falls back on radio navigation (i.e. VOR/VOR, VOR/DME or DME/DME) it can only use I.R.S. data combined with DME/DME.

The I.R.S. system.

When the flight crew boards the aircraft the first thing they will do is allign the I.R.S. and tell the aircraft where it is. This can be done by simply selecting the G.P.S. position as being the right position or they can enter their own present position by entering the lattitude/longitude.
in the Inertial Reference Unit's are laser gyro's, accelerometers and offcourse the software to compute it and communicate it.
On some modern aircraft the air data computer is incorperated in this unit also but is split internally by a firewall.

The input into these boxes is the aircraft's present position and when u select the I.R.S. from the allign to NAV mode it will start to measure, calculate and update the position of the aircraft.
The reliability factor as a function of time is known by the system and is a set value, This is called Navigation Performance, on the 737 this means that there is a imaginary range around the aircraft of where it is with a 93 percent certainty.
So if the system calculates that the aircraft has a 93 percent chance of being in a circle of 0.2 nautical miles then the actual navigation performance (A.N.P.) of the aircraft in that state will be 0.2 nautical miles.

For different flight phases there are different requirements for this A.N.P.
Take off for example might be 1 nautical mile, cruise may be 2 nautical mile and approach 0.5 nautical mile.
If the A.N.P. is higher then the Required Navigation Performance (R.N.P.) then the F.M.C. will generate a warning saying that the aircraft has a 'reduced navigation performance'.

This may all sound a bit much but it comes down to this, close your eyes and start walking down the street, the first couple of seconds should be no problem because u know that when u started to walk u were on the center of the sidewalk staying in the middle but after a minute or so something tells u that u are not sure anymore that u are still in the middle of the sidewalk and that u should start to be carefull. This is basically that R.N.P. message.

The I.R.S. system is the main input for aircraft systems for compass heading. It can measure the heading of the aircraft and because it knows where the aircraft is it can calculate true north and magnetic north. Systems like Autopilot (FCC's), Radio Magnetic Indicator's and ofcourse the display units use this heading data. If u want to learn more about compasses see this powerpoint training on adjusting the magnetic (standby/emergency) compass:

magnetic compass adjustment

This powerpoint training is courtesy of mr. Rob Warning, a technical specialist working for KLM.

I've mentioned F.M.S. what is that?

F.M.S. or Flight Management System. This is really the main navigation center of the aircraft. We have established that the aircraft knows where it is, with F.M.S. u tell it where u want it to go. U enter the flight plan that u want into the F.M.S. Pilot's can enter divert route's into the F.M.S. for if the destination airport is unavailable for whatever reason and u need to divert to another airport however I have not tried this myself.

When u start with the F.M.S. u need to select a start airport and a destination airport ofcourse. On these airport there are usually more then one runway's so u need to select wich runway u are taking off from and wich one u want to be landing upon, after this u select the appropriate SID and STAR (see top of this section about SID's and STAR's) and u have your departure SID and arrival STAR. Now that u have that u can select waypoints to fly along the route and u can tie the end of the SID and the beginning of the STAR to your route and select the type of landing, runway, etc. Now the lateral flightplan is complete, u then proceed by entering the gross weight (F.M.S. needs to know this to calculate minimum speeds and to give an input to the SMYD, (stall management yaw damper computer), the outside air temperature (for engine RPM/N1 target calculation), the take off flap setting (for V speed calculation), the cost factor (how much fuel the aircraft may burn, a lot means quicker flying, a little means cheaper flying but later arrival), maximum flight level and the center of gravity (stab position calculation) u can also select cross winds and such but these are more operational issues then technical issues. The F.M.S. calculates at wich speeds the aircraft should fly at wich flap setting (for more on flaps, see the flight controls section), it calculates the V1 (action speed), V2 (minimum operating speed with a certain weight and flap setting), Vr (rotation speed, the speed at wich the pilot flying rotates the aircraft into the air) and Vref (landing speed reference), it also calculates the stick shaker speed and alpha floor protection. Alpha floor protection is sent to the autopilot (FCC) and is used for too low speed, this is usually due to a too high vertical speed mode selection. In the vertical speed climb the aircraft assumes the vertical speed that is selected on the Mode Control Panel (MCP) and starts to climb, if the engines are at the maximum of the climb thrust setting and the vertical speed selected is such that this is not enough then the actual aircraft speed will decrease resulting in a speed that is dangerously low. If the autopilot is engaged the alpha floor protection will steer the nose of the aircraft down to avoid a stall of the aircraft and the autopilot mode will change over to a altitude hold mode. Same goes for overspeed mode when the aircraft is in a high vertical speed mode descent, if the throttle's are fully retarded (idle) and the aircraft's speed is still too high, the aircraft will also level off and cause an altitude hold mode on the autopilot. Level change is a safer alternative for changing altitude. With vertical speed u can say at a certain time u will be at a certain altitude and with level change u can say I am flying along with a nice thrust setting and I don't care at what point I will reach my desired altitude. Therefore level change is usually used on climb and vertical speed is usually used on descent because on descent (especially on non-precision approaches, i.e. approaches with no glideslope) the flight crew wants to hold a certain descent speed to reach the runway threshold at the right altitude.

When these speeds are calculated u can see them on the speed scale on the left side of the PFD. The stall warning computer or in some aircraft 'SMYD' (stall management yaw damper computer) calculates the barber poles, these are red striped areas on the speed bar that the aircraft should stay out of. The top one is too fast and the bottom one is too slow, this speed changes ofcourse with different flap settings and different angle of attack.

When the flight plan loading is complete u can see a map on the display units showing the present position of the aircraft and the waypoints that are in the flightplan. When the aircraft then takes off u can navigate using these indications on the display units.

When flight crews start the take off roll they usually have the Flight Directors switched on. When they now engage the take off by selecting TOGA (Take off/Go around) on the throttle handle the flight director pitch bar will come into view on the display units. This bar first shows a slight nose down steering command to keep the aircraft on the runway and at Vr it will shoot up to a pitch angle that get's the aircraft into a climb angle.

I've inserted a video of me and my colleague Rob pressing the TOGA switches on a dual engine run.

U will see the throttle's being moved by the Autothrottle system, at the end my colleague presses the two switches to the side of the throttle levers wich are the autothrottle disconnect switches.

When this is performed on the take off run the autothrottle moves the throttle levers forward to a take off climb thrust position and there at a certain wheelspeed (60-80 knots) the autothrottle will actually declutch to avoid any false input or failure into the autothrottle computer to change the throttle position during this critical flight phase.
At this time the elevator will actually control the aircrafts speed as the engines just produce climb power.

This flight director that I've mentioned is not the same as an autopilot (many people confuse the flight director for an autopilot and say well... it's all the same thing). First of all the flight directors are indicators and the autopilots actually send steering commands to the flight control surfaces (they steer the aircraft). A flight director is an indication to the flight crew to get the aircraft to where it wants to go but the flight director does not use any surface position inputs (it doesn't know what the aircraft is actually doing), this means that if let's say that on the Mode Control Panel (M.C.P.) the VOR mode is selected and the direction that the flight director wants to go is slightly to the left it will indicate exactly that to the flight crew. So u can imagine that when the flight crew has the aircraft in a 30 degree bank turning to the left they would want to start getting the aircraft out of the turn and steer to the right (otherwise the aircraft will overshoot the heading) while the flight director still indicates that the aircraft should go left.

The autopilot calculates with sensor inputs what the aircraft would be doing in the near future and will actually steer the aircraft out of this turn to intercept the heading that was selected on the M.C.P.

Can this autopilot land the aircraft all by itself?
Yes it can however different categories of autoland exist and only some aircraft can actually land the aircraft completely to the end of the runway automatically. These are the categories for autoland and the conditions for wich a certain category is mandatory:

Cat 1 - 200 feet above ground level Minimum visibility ~550 meters
Cat 2 - 100 feet above ground level Minimum visibility ~350 meters
Cat 3A - 50 feet above ground level Minimum visibility ~250 meters
Cat 3B - 20 feet above ground level Minimum visibility ~100 meters (runway guidance)
Cat 3C Zero visibility. (taxi guidance. not used because emergency ground personnel can not adequately reach the aircraft).

These autoland categories mean that the aircraft can fly into airports where the visibility is reduced. If on a Cat 3A autoland approved aircraft the flight crew does not see the runway at 50 feet (runway visual range, R.V.R.) then they will have to make a go-around wich means throttle back up fly away again. This weather forecast is ofcourse known to the flight crew so if the R.V.R. is too low for the aircraft that they are operating they will not depart and the flight get's delayed or even cancelled.

For cat 3A aircraft u need two of most of the sources for navigation, two flight directors, two seperate power sources for these systems and so on, for cat 3B u even need 3 seperate systems and 3 seperate power sources. cat 3B is also possible for 2 engined aircraft, it requires for example 3 FCC's (autopilot computers), a DC to AC inverter on the battery connected to an electrical autoland bus to sypply the third independend power source. Operators with autoland aircraft have table's in the operators manual that show the flight crew and the ground engineers what must be operational to be able to make an autoland. If u don't meet the criteria for a cat 3B autoland then the aircraft may be downgraded to a cat 3A or even lower category. Airports themselves must also be equipped with certain availabilities to make the airport suiteable for certain autolands. For example, if the aircraft is cat 3B approved and has three FCC's calculating the approach (two of them act as autopilot whilst the third monitors both of them and if any of the three disagree's the cat 3B autoland is no longer possible) and the airport has only VOR guidance but no I.L.S. (see top of this section on VOR and a bit further down I.L.S. will be explained) then the autopilot cannot guide the aircraft to the runway so still no autoland is possible.

So what is this autopilot that I've been hearing so much about?

I've just explained the different autoland modes of the aircraft but what is an autoland? Surely the aircraft cannot land entirely by itself?

Yes it can, as soon as the aircraft takes off and get's to a safe altitude the flight crew can select the autopilot to on. The autopilot has different functions in wich it can operate, for example u can select it to an altitude hold mode (aircraft keeps current altitude) or a level change mode (aircraft will go to a selected altitude with a fixed climb thrust) but most importantly it can be used to give steering commands to the flight controls of the aircraft.

Now we know that the aircraft knows where it is and knows where u want it to go but what about this autolanding?

On modern airports there is autoland availability, there was talk about M.L.S. or microwave landing system but it's been very quiet around it's development. There are more systems around the world like L.A.A.S., P.A.R. (military), W.A.A.S, G.L.O.N.A.S. or E.G.N.O.S. but I will only explain the well established I.L.S. system or Instrument Landing System wich used ground beacons to land the aircraft.

A localiser beam is really a combination of two ground signals of two different frequenties aimed pointing away from each other at an angle.

When one of the frequenties is received with a bigger amplitude (louder) the I.L.S. knows that it is too much to that particular side of the runway and needs to fly either left or right to get the amplitude exactly the same (wich should be dead center). As the I.L.S. 'captures' this localiser it then expects to encounter a glide slope. The aircraft should now be flying directly center of the runway.

The glide slope works the same as a localiser but in a vertical field, in this situation however the I.L.S. will first receive the bottom frequenty because it is flying into the beam at the underside ofcourse.

This is what the I.L.S. expects so it will only engage the glide slope when the pointer goes from 'steer up' down beyond level towards steer down'. This means that the aircraft is now at a direct course towards the landing strip. If u go to an airport and stand at the landing strip u can see the aircraft lined up in the air all flying in their localiser and glideslope (glidepath).

The glideslope is sent from the runway threshold (beginning of the landing strip) because thats where the aircraft should be touching the ground and the localiser is positioned all the way at the far end of the landing strip because a cat 3B aircraft can use this localiser for roll-out guidance (keep it straight on the landing strip).

Then the spoilers or liftdumpers combined with the autobrake slow the aircraft down. It is also possible for the crew to make a 'touch and go' wich means that at the last moment they decide not to land, they slam the throttle's forward and take right off again. Spoilers will go down again, autobrake will not operate etc.

What if the aircraft flies too fast or too slow?

If the aircraft flies too fast it can be structurally damaged, it is not a good idea to operate outside of the aircrafts maximum specifications.

(typical general aviation flight envellope)

What is displayed on these display units on the flight deck?

There are different configurations in different aircraft but I will explain the EADI and EHSI and the PFD and ND display systems.
These are two primary flight display's

For people that want a more detailed information as to what is displayed go and have a look on my display page:

Display Unit Indications

With all this traffic up there how can aircraft operate without flying into each other? Airports around the world use a system called Air Traffic Control (ATC) wich directs the air traffic safely trough the air without allowing them to get to close together.

If the ATC system doesn't function properly or u are flying in a uncontrolled part of the world when two aircraft are flying towards each other at the same altitude, the Traffic Collision Avoidance System (TCAS) will give a warning to the flight crew, if the ATC system is fitted with a 'mode S' transponder it will even interrogate the other aircraft's mode S transponder and agree wich aircraft will descent and wich one will climb and each aircraft will indicate this in their flightdeck, using the PFD or EADI to indicate the flightcrew that they may not bring the aircraft into a certain pitch angle and on the vertical speed indicator to stop crew or to command crew to a certain vertical speed.

When the intruder aircraft enters the yellow area u will receive a TRAFFIC alert in the flightdeck, when the intruder aircraft enters the red area a resolution advisory will be given wich is at this moment still only up or down, not left or right yet altough a new TCAS system is being discussed wich has this option.

Air Data Computing.

TCAS and many other systems must obviously know at wich altitude the aircraft is flying. There are two main way's to determine altitude, there is the radio altimeter (below 2500 feet) wich sends a sawtooth signal towards the ground, receives the echo and measures the time in between sending and receiving, that time minus the timedelay in the system can be calculated to give an altitude. This radio altimeter works only at relatively low altitudes but this gives altitude alerting on the Enhanced Ground Proximity Warning System (EGPWS). The other way of measuring altitude is by the air data instruments. The air data instruments are usually divided in two main systems being left and right and both have a probe and a static port on the left and right hand side of the aircraft so any yaw effect (turning the nose) has no effect on the measurements.

The air data systems are isolated from each other so a failed air data system cannot affect the other. A air data system uses a pitot probe, a static port and a tat probe (true air temperature probe). The pitot probe measures the head on pressure of the air so let's call it Pt (total pressure) wich is the sum of Ps (ambient 'static' pressure) and Pv (ram air pressure). The static port is flush with the fuselage and measures only the Ps (static pressure) so the difference between the two is the ram air pressure wich can be measured to get true airspeed 'TAS'. When we know TAS and altitude and I.R.S. acceleration we can calculate the ground speed, with this ground speed and the altitude we can calculate the Calculated Air Speed or CAS wich is displayed on the display units (the standby instruments do not get I.R.S. calculations so on these instruments TAS is indicated).

The altitude is taken from the static port, we know the barometric pressure is a function of temperature and altitude, for example: at sealevel on a standard day (around 18 degrees celsius) the ambient pressure is 1013.2 hPa. With around 8.2 meters altitude increase the pressure will decrease 1 hPa. The temperature is measured with the TAT probe and the barometric pressure is set before departure to the correct value so the altitude can now be calculated.

So now we have the aircraft altitude, speed and local temperature. The probes and ports all input into the two air data computers (ADC), these computers perform the calculations and supply the FMS (flight management computer) with the air data information. These ADC's also supply the TCAS and ATC with this information because when the TCAS transponder interrogates the intruder airplane it needs to know it's altitude and speed, there is no point in trying to avoid an aircraft of wich u don't know where it is. Same goes for ATC (air traffic control), the ATC has radar to see the aircraft on a screen but the actual height, speed and identification signals come from a transponder from the aircraft so all that the tower can see is what the aircraft itself has calculated.

U understand that setting the correct barometric pressure is extremely important. The flight crew can set the barometric correction for QNH (question nautical height, local barometric pressure) and QFE (question field elevation sets the barometric pressure at 0 feet at arrival airport 'this will give the flight crew 0 feet upon landing' this is almost never used). QNH is used for take off and landing only, the aircraft uses the local QNH to have the correct barometric altitude for that area. When the aircraft comes above the transition altitude the flight crew will select STD (standard) wich uses the 1013 mb of pressure wich is standard day at sealevel so that every aircraft have the same barometric reference. This is ofcourse very important for TCAS and ATC to keep aircraft away from each other.
Most commercial aircraft now have 'reduced vertical seperation minima' approved installations wich allows aircraft to have a minimum seperation altitude of 1.000 feet. They are allowed to fly at high altitude only when the air data system is accurate enough, this involves many things to make sure that this is the case but the main thing is that there is absolutely no damage allowed at or around the air data sensors.

These air data sensors are not only connected to the ADC but also to standby instruments so that when the aircraft loses it's power it can continue to operate. Needless to say, these standby instruments must be functional before any flight and all the sensors are heated to prevent ice build up creating unreliable reading.

typical installation of ADC in aircraft systems.

Does an aircraft have a radar system?

Yes, most commercial aircraft have a system called weather radar wich detects weather conditions in front of the aircraft (rain density in the clouds etc.). The weather radar sends signals and receives the return signals just like any other radar system, it uses the time between the signals to calculate distance and it uses the amplitude of the returned signal for density (different colors for different densities).

This weather radar is in the nose of the aircraft behind the 'radome'

The weather radar antenna can move left and right but also up and down because the aircraft's nose can point up or down. The weather radar system receives inputs from the I.R.S. (Inertial Reference System), the antenna will still scan up and down a little bit to get a more accurate reading of weather ahead but it will correct for aircraft nose up and aircraft nose down.

Most weather radar systems also use a ground clutter suppression to ignore signals that return after touching ground objects. It does this by calculating movement speed of the object by the doppler effect. Ground targets are not moving relative to the random movement of water droplets, a discriminator circuit sets a speed threshold of approximately 0.75 meters per second, any movement above this rate must be a cloud return, any below must be a ground return and is removed from the display.

Doppler effect: if a car drives away from u the frequency of the sound waves decreases because the source of the sound waves is being drawn away from u, when the car approaches u, u will notice that the sound waves frequency increases because the source of the sound is moving towards u. When a car drives towards u, passes u and then drives away from u, u can hear the sound pitch increase and decrease when it is driving away. The aircraft knows it's own speed and that's how it can calculate wether an object is standing still or is moving.

If an area has around 70 percent movement of 5 meters per second this will classify as being turbulent wich is usually displayed in magenta. (the maximum range for these extremely accurate measurements is not the maximum range of the weather radar but more in the region of 50 nautical miles).

Most weather radar antenna's are flat plate's (a.k.a. slotted array antenna's) wich have a narrower emitting range and therefore has to transmit less power then a parabollically shaped antenna's. Here's a parabolically shaped one:

The slots on the flat panel weather radar antenna are spaced from each other, they are spaced exactly a half of the wavelength of the used frequency apart, this way the energy at the sides of the antenna dish should be exactly zero and the only way for the energy to go is directly forward. therefore the antenna may not be damaged and this is also why it is not completely round. Obviously the sizes and distance's of these slots are very critical.

The radome is ofcourse the first object to meet the air head on, this radome must be lightweight, aerodynamically shaped, electrically conductive (for communication purposes) and strong.

The radome must be electrically grounded because of the corona effect, for this reason there are also 'static dischargers' connected to all the extended area's of the aircraft, these dischargers make sure that the electrical potential of the aircraft can be discharged.
If the electrical potential of the aircraft would become too high, it would create static noise on the communication set's.
Apparently the radome is also used to keep a certain electromagnetic field around the aircraft that the antenna's need to be in if they are to be used. I have heared of one case where an aircraft was unable to communicate at all in flight but never had a problem on the ground. Turned out that the radome wasn't electrically grounded.
I have been looking a long time for some evidence on this but still haven't found it, if someone does know more about this please email me.

Back to:

How do aircraft work ?