Let's have a look at the PFD. There are different ways of displaying information to the flightcrews, In this overview I will explain only the PFD of a B737-6/7/8/9 configured to our company's options.
Many aircraft around the world still operate without a so called EFIS or Electronic Flight Instrument System. Before the EFIS system there was the ADI (Attitude Direction Indicator) and the HSI (Horizontal Situation Indicator). These are mechanical instruments.
Here they are up close.
In EFIS equipped aircraft we use so called EADI (Electronic Attitude Direction Indicator) and a EHSI (Electronic Horizontal Situation Indicator) or a PFD (Primary Flight Display) and ND (Navigation Display). These systems are made up to provide the same visual display as the old ADI and HSI display system to make it easier for flight crews to read the data.
As u can see a EADI looks similar to a ADI.
And a PFD looks similar to a EADI again.
Let's start with the basics.
The first thing u notice is the blue bit on top and the brown bit at the bottom wich represents the horizon.
The blue part represents the sky and the brown represents the ground, logic would suggest that the flightcrew keeps the blue side up.
This data is called the attitude of the aircraft, the aircraft's pitch is considered nose up and down, roll is considered quite literally the rolling of the aircraft. (or banking of the aircraft).
Here's a roll attitude displayed on the PFD.
The display system get's attitude information from the lasergyro's in the inertial reference system.
The ground will be displayed level because it's the aircraft that moves and not the ground.
The aircrafts attitude is depicted as a aircraft symbol (often referred to as 'the wings').
If the aircraft attitude goes nose up, the wings will rise into the blue area, if the aircraft goes nose down the wings will lower into the brown area, if the flightcrew wants to fly upside down, the wings will be displayed upside down for a bit and then quickly move into the brown area.
So now we know that the attitude of the aircraft and the horizon is indicated on the EADI but there is a lot of other information that is found on this instrument. First let's look at the airspeed.
Here's short clip of a pitot static test performed on a EADI of a B737 present generation aircraft.
In this example u can see the airspeed in the same location as on the PFD but the altitude is displayed on another indicator on the right to the EADI.
Notice the green pointer shooting up at the current airspeed indicator when the airspeed starts to increase.
This is the speed trend vector and gives a prediction of the airspeed in the next 10 seconds based on the current airspeed and accelleration.
Also when the airspeed increases u can see at the bottom left a white number appear. This is the mach number wich appears at around 0.40 mach. (mach 1 being the speed of sound)
The airspeed scale indicates the ADIRS (Air Data/Inertial Reference System) airspeed of the aircraft wich is CAS (Computed/Calibrated airspeed). This is a different airspeed then the one used for the standby instrument system, the standby airspeed indicates IAS (Indicated Airspeed) because the pitot line from the pitot/static ports is directly connected to the instrument.
Note that this picture is a rough indication as to how the system works, the Boeing 737 next generation actually uses air data modules wich convert the air data from the sensor to a electrical signal wich they send to the computers.
The static line is used for altitude, corrected for barometric pressure and the pitot line is used in combination with the static line to supply this IAS (Indicated Airspeed). The ADIRU (Aid Data/ Inertial Reference Unit) will correct this IAS with AOA (angle of attack) to get the CAS (Computed/Calibrated Airspeed).
Keep this in mind when u have to do a airspeed test using a pitot static testset on a Boeing 737-6/7/8/9, set the AOA vane's to zero if u don't want three different airspeed indications.
Also there is a big difference between Mach number and IAS.
The speed of sound is called Mach 1 and unless u are flying a concorde, this is not what we want. It's important for flight crew to know that the higher the aircraft flies, the closer a certain calibrated airspeed will get them to a structural overspeed.
When an aircraft climbs to higher altitude the mach number increases whilst the IAS (and CAS) remain constant.
Here's a present generation pitot static test indicating 299 knots while passing 4.100 feet. U can see the mach number is now 0.486.
A little bit later as the aircraft passes 14.000 feet, u can see the CAS is still 299 knots but the mach number is now 0.590.
Also note that the green block in front of the altitude is gone. This block is visual aid telling the flight crew that they are below 10.000 feet of barometric altitude relative to sea level. Later on u will see this block digitally displayed on the PFD on the next generation aircraft.
Now that we know where the airspeed indication comes from, let's look at the maximum and minimum airspeeds.
The maximum manouvring speed only comes in view when the flaps are fully up but the maximum and minimum manouvring speeds are airspeed areas where loss of manouvreability occurs.
This minimum and maximum and the manouvring speeds come from the SMYD (Stall Management Yaw Damper) unit.
This unit gets data from the ADIRU (Air Data/Inertial Reference System) for airspeed and attitude of the aircraft.
It get's an input from the AOA sensor to get the angle of the air relative to the wings. (for example, IRS gives u a pitch attitude regardless of the direction that the aircraft is moving, combine this with the AOA and we know the attitude and the aircrafts 'flight path' that we need to calculate the min and max speeds). A flight path is the actual path that the aircraft moves through the air, indicated in this next picture by the grey arrow.
These AOA sensors are also used for the stall warning system, this system operates a so called 'stick shaker' to actually shake the control collumn to indicate the flight crew that they need to push the stick forward. On some aircraft (particularly some aircraft with high T tails) there is even a stickpusher that forcefully pulls the control column to aircraft nose down to build up airspeed and to get out of the impending stall.
Pitch limit, this symbol represents 2 wings stalling. This symbol indicates the safe pitch limit for the aircraft. This is calculated in the SMYD (Stall Management Yaw Damper) and uses the Inertial Reference System to place the pitch limit somewhere on the pitch scale.
Pilots should not steer the nose of the aircraft in a pitch higher than the pitch limit.
It's also important to know the weight of the aircraft for minimum and maximum airspeed computing, this data comes from the FMS (Flight management System). The FMS knows the weight of the aircraft including the fuel so if the fuel quantity indication system is unserviceable the flight crew must monitor the fuel quantity themselves and keep the FMS updated for this reason.
It uses flaps up switches for input and flap position transmitters (not to be confused with position switches, a position switch is a discrete, a 0 or a 1. A transmitter sends position information so it can be used for dynamic speed limit calculation).
The SMYD also looks at the second SMYD (like most systems onboard the aircraft) to check it's calculations wich is performing the same task for the other PFD display.
The SMYD then sends this data to the displays and the FCC's or Flight Control Computers for autopilot limits (we don't want the autopilot to steer the aircraft into a underspeed or into a overspeed).
FQIS is the Fuel Quantity Indication System.
Then there is the selected speed at the top of the Aircpeed scale.
This selected airspeed can come from either the MCP (Mode Control Panel) or the FMS (Flight Management System) depending if the DFCS (Digital Flight Control System) is in a MCP speed mode or in FMC navigation.
When the selected airspeed comes from the FMC, the selected airspeed in the MCP goes blank.
Then there are the flaps manouvring speeds, these indicate the flap setting that the flightcrew should select more up while accellerating and more down while decellerating to maintain good manouevreability of the aircraft. The next picture is of a flap manouvring card from a 757 that I found on the internet.
These flaps manouevring speeds can be seen as a tick with a green number (flapsetting) on the speedscale and depict the next setting for the flaps.
There are different speeds that are especially important to the flightcrew. These are the reference speeds.
First there is the V1 speed wich is called decision speed, the flightcrew is advised not to abort the take off for an engine failure after this speed.
Then we have the Vr speed wich is the rotation speed, the speed at wich the flight crew will pull on the control collumn for liftoff.
Next there is the V2 speed, this speed is the minimum safe take off speed for climb with one engine missing.
Then we have the Vref wich is the reference landing approach speed. Often used by pilots as a base from which to calculate speeds to be used during landing, and calculated as a margin over the stall speed - usually 1.3�VS0.
VSO being the stalling speed or the minimum steady flight speed in the landing configuration. The actual landing speed can be selected on the FMS control display unit (FMS CDU) and this will be depicted as a green 'ref' on the speedscale.
The V1, Vr, V2 (V2 + 15 knots) and Vref (Vref +15 knots).
And here is the Vref indicated. (in flight only)
Now let's look more closely into the attitude information that can be seen on the PFD.
First we have the white bank scale on top of the PFD.
This one shows the bank angle of the aircraft (roll movement). The white pointer turns amber when the bank angle is more then 35 degrees.
Flight director bars. When u select a DFCS mode, with the flight director on, the FCC (Flight Control Computer) will generate flight director commands for the flight crew to follow.
Slip indicator, this little white bar indicates the amount of slip that the aircraft experiences.
FPV (Flight Path Vector), The flight path vector displays the flight path angle (up down) and the drift (left right).
Approach reference, this is a indication that appears when the flightcrew selects a ILS frequency in the VHF NAV control panel.
When the frequency is selected it will display here the selected frequency, the selected course and the DME distance to the ILS/DME station (if the particular ILS HAS a DME signal).
If the frequency on the other VHF NAV control panel is not the same, the frequency will be amber with a horizontal line through it.
If the approach course's entered in the MCP are not the same left and right then the course turns amber with a horizontal line through it like this.
The localiser and glideslope deviation scale, these are explained in more detail in the section about testing the aircraft with a ILS/VOR/MB testset, more about this testset on the ILS/VOR/MB testset page in the avionics tools section.
Marker Beacon. This one is also explained in more detail on the ILS/VOR/MB testset page in the avionics tools section.
Note that this picture is taken on a 737 present generation and not on a 737 next generation like the rest of this presentation.
Rising runway. The rising runway is a visual indication to the flight crew that the aircraft is closing towards the runway at the last moments before touchdown.
This rising runway will be in view when the localiser is captured and the RA (Radio Altimeter) is below 200 feet.
The company I work for doesn't have this option on our fleet of 737 next generations so here's one on a non-EFIS boeing 737 present generation.
Decision Height. (called 'approach minimums' on a 737 next generation). In a precision approach, before this decision height is reached the pilots may perform a so called missed approach. The Decision Height is dependant on the land category of the aircraft. 200 ft in Cat I and II landings, 100 ft in Cat IIIa and 50 ft in Cat IIIb. It would be 0 ft in Cat IIIc but Cat IIIc is not used.
The decision height can be selected to be in radio alitude or barometric altitude, in this case it's in radio altitude.
Note the the green arrow on the altitude scale is the barometric decision height indicator and doesn't react to the radio altitude decision height.
When decision height is selected to barometric, the readout appears at the bottom of the PFD and the DH altitude will have a green line at the pointer, indicating that it is now active.
Radio Altitude. The radio altitude is only active (and thus only displayed) below 2500 feet above ground level.
The Rad Alt displayed here is -4. This is quite normal, the ideal situation would be that the RADALT would be 0 feet when the aircraft just touches the ground with the mainwheels (and the nose where the radio altimeter antenna's are higher up in the air).
The radio altimeter antenna's are located just behind the nose landing gear. Here's the antenna's removed from the aircraft and hanging loose from their cable's.
(took this one last week, when the paint shop sandpapered the fuselage, with the radio altimeter antenna's along with it......)
GPWS annunciations. These annunciations inform the flight crew about the status of the GPWS. Nowadays we use EGPWS but this is explained in detail on the EGPWS page.
This annunciation can be WINDSHEAR or PULL UP.
Then there are the altitude indications, there's the selected altitude bug and the digital readout on top of the altitude bar.
The current altitude display shows the current barometric altitude from the ADC or ADIRU (wichever is installed).
The green crosshatch that u can see in front of the digital readout is shown when the altitude is below 10.000 feet.
When the altitude goes below 0 feet the crosshatch shows negative.
When the selected altitude is reached, the digital readout becomes bold and a altitude box is highlighted around the altitude readout in amber and flashes if the altitude wanders away from the selected altitude for some reason.
The barometric setting is seen below the altitude scale in green.
(See the Navigation and Autoflight section for the why and how of QNH, QFE and STD settings and why they are important.
If the setting is in STD, this barometric setting will be 1013 HPa (or milibar). The selected barometric setting will be displayed below STD in white.
Landing altitude. The flightcrew selects the departure and arrival airport into the FMS. The landing altitude displays the ground level on the altitude scale.
The landing altitude will display the departure airfield until 400 Nautical Miles or one half of the distance of the flight plan. Whichever occurs first.
Vertical speed indication. Most vertical speed indicators nowadays are IVSI's wich are instananeous vertical speed indicators. These IVSI's get a input from the IRS (normal input is Air Data Computer) for vertical accellerations, this creates a faster display of vertical speed then the regular air data vertical speed indication only.
If a vertical speed is selected on the MCP, there will be a selected vertical speed bug indicated on this scale.
TCAS, the Traffic Collision Avoidance System displays data on the PFD and on the IVSI's (Inertial Vertical Speed Indicator's).
Nowadays, the mode 'S' ATC (air traffic control) transponder communicates with other aircraft and relays this information throught to the TCAS. The TCAS uses this communication possibility to give RA's (Resolution Advisories) to the flight crew.
This means that the flight crew has to either descent, keep altitude or climb to avoid colliding with another aircraft.
The mode 'S' ATC transponder communicates with the other aircraft and decides wich one will descent and wich one will climb, based on altitude information, current rate of climb, terrain clearance etc.
On the PFD it displays a 'don't enter' area, indicated by a red box coming from the top or bottom or both depicting an area where the flightcrew should stay out off to steer clear of other air traffic.
On the IVSI's the TCAS will display a 'don't enter' area also in the vertical speed display. Also to steer clear of the intruder aircraft.
Then there is the heading display.
First, there is offcourse the current heading wich is the white triangle in the middle pointing down.
The green mag indicates that it's displaying magnetic heading, wich is the default for our fleet. Aircraft operating far north or far south may have this setting to TRU (true heading).
Then there is the selected heading wich is displayed in magenta.
Track heading indicator, this needle points the track of the aircraft.
Then there are the Flight Mode Annunciations wich can be selected using the MCP (Mode Control Panel).
In this picture we have the Flight Director selected to on but no mode selected on the MCP yet.
Here we have the Flight Director still as the main DFCS mode but here we have the APP (approach) mode selected.
The green box around these indications is a 'mode change highlight' and appears for 10 seconds after each engagement.
Here is the APP mode selected but not engaged yet, they are indicated in white one line below the engaged line.
The three boxes at the top are the autothrottle, roll and pitch modes.
And this area is the A/P status annunciator.
The next picture shows a autopilot in command (CMD), the autothrottle is armed (white), a heading select is set for the roll mode and a VNAV path (vertical flight path from the FMC) is selected for the pitch mode.
During flight one autopilot can be in command, the dual autopilot can be selected when the flight crew selects the APP (approach) mode (the aircraft may not be below 800 feet radio altitude for the second autopilot to be coupled).
The different mode's for the autothrottle are:
ARM: this just means that the autothrotle is armed and waiting for a input to start work.
The autothrotle is in ARM when the aircraft is on ground and u select the autothrotle system to ARM, after the THR HOLD (throttle hold), the autothrottle disengages during the climb to make sure that no fault in the autothrotle system can mess with the throttles in this critical flightphase.
The autothrottle is also in ARM when the throttles hit the aft stop when the autothrottle is in RETARD mode on descent.
N1: when the flightcrew select TO/GA (take off/go around) for take off, the autothrottle will select the N1 to take off thrust. If the flight crew selects TO/GA in approach the autothrottle will select the N1 to a go around thrust, if the flight crew presses TO/GA again the autothrottle will select the N1 to maximum go around thrust.
This N1 mode is only active when the aircraft is in either takeoff, climb or in the maximum go around thrust mode.
In this mode the autothrottle does not control the aircraft speed, the autothrottle just supplies a set amount of thrust and the pitch channel of the DFCS controls the aircraft's speed.
For the climb function the N1 is used in VNAV climb and LVL CHG (level change) climb. in these modes the N1 light on the MCP will illuminate. (when the mode is unchangeable the light on the MCP will be turned off).
FMC SPD or MCP SPD: unlike the N1 mode, this mode uses the autothrottle system (engine thrust) to control the speed of the aircraft.
VNAV for FMC SPD and V/S (Vertical Speed) mode or LVL CHG (Level Change) or Alt Hold for the MCP SPD.
In this mode the speed light on the MCP panel may come on (when the mode is unchangeable the light on the MCP goes off, in VNAV cruise, the light will remain off for example to indicate that the speed mode cannot be changed to the N1 mode).
The VNAV mode will be in a N1 mode until the aircraft acquires the selected altitude and levels off, the autothrottle then automatically goes into the FMC speed mode (in VNAV the FMC controls the aircrafts speed) and the speed window on the MCP will go blank indicating that the FMC is controlling the speed.
Like I've said, the speed light on the MCP will not come on to indicate that the speed mode cannot be changed at this time.
Note that in this mode the alpha floor protection is active, the alpha floor is the minimum speed that the autothrottle system allows the aircraft to be in.
GA: the GA (go around) mode is for when the flight crew wants to abort a approach and 'go around' for another attempt.
When the TO/GA (one switch for both functions) is pressed once, the aircraft will be in this GA mode. If the flightcrew presses this switch again the aircraft will go in the maximum go around thrust and the N1 light will illuminate on the MCP.
The normal GA mode (i.e. press the TO/GA switch only once) will not set the autothrottle in either N1 or Speed because the autothrottle has an internally calculated thrust value to achieve a 8 percent climb gradient and therefore does not follow the FMS SPD or MCP SPD selection (and therefore not a Speed mode).
RETARD: There are two retard modes, there is the descent retard and the flare retard (flare is the aircraft nose up moment right before touch down). When the aircraft reaches the TOD (Top of Descent), the autothrottle mode will be retard, the throttles move back to the idle stop slowly. When they reach the idle stops, the autothrottle mode changes to ARM until otherwise selected.
At glideslope capture, the autothrottle mode is automatically switched to MCP SPD, the DFCS pitch will be G/S and the DFCS will use the elevator to steer the aircraft on the correct vertical flight path and the autothrottle adjusts the speed for the approach and the Speed light will be on on the MCP. At 50 feet radio altitude the DFCS sends the flare discrete to the autothrottle and at 24 feet radio altitude the autothrottle switches to flare retard and switches off the N1 and Speed lights on the MCP.
THR HLD: after take off, the autothrottle disconnects itself from the throttles to avoid any faults that may be in the system to affect the throttle levers in this critical flightphase.
The FMC calculates N1 limits for different flight phases and sends this to the autothrottle for the thrust settings for these flight phases.
These TMA's (Thrust Mode Annunciation's) are TO(take off), R-TO (rejected Take Off), CLB (climb), R-CLB (reduced climb), CRZ (cruise), GA (Go Around), CON (maximum continuous thrust).
The roll and pitch channels can be selected to the DFCS A/P (autopilot), the basic operation of this autopilot is called CWS (control wheel steering), in this mode the flight crew operates the aircraft like there was no autopilot engaged. The steering of the aircraft is now achieved by force transducers and through the FCC (flight control computer).
When the flight crew selects CWS on the MCP or CMD (command) but not a mode to work on, the DFCS will go in the CWS mode.
If the flight crew had selected CMD on the MCP and select 'for example' HDG SEL' the DFCS will go to CMD and the roll channel would show HDG SEL like u see in the next picture.
The different roll modes are:
HDG SEL (heading select): pretty self explanatory really, u select the heading that u want to fly and select the HDG SEL switch on the MCP.
VOR/LOC: This mode will be selected to fly on a VOR radial or to fly on a localiser for landing.
LNAV: This is the roll mode for the FMC navigation. i.e. the lateral navigation mode.
Then we come to the last space wich is the pitch mode channel.
The different modes are:
TO/GA: this mode is active when the TOGA switches are pressed. In the take off mode, the flight directors first indicate nose down to keep the aircraft fixed on the ground and at Vr will shoot up to get airborne. When airborne, above a certain radio altitude the flightcrew can select the autopilot to CMD or CWS.
In the Go Around mode the aircraft will abort the landing and climb to the altitude selected on the MCP.
V/S: the vertical speed is used to set a specific vertical speed on the MCP, the aircraft then climbs or descents (wichever u selected) at this rate. If the aircraft is in climb mode and the airspeed becomes lower than 1.3 times the stall speed or approaches the maximum speed, the V/S mode will change to LVL CHG.
ALT ACQ: When the aircraft reaches a altitude that was set on the MCP, the DFCS pitch will first indicate ALT ACQ (altitude acquired) and then switch over to ALT HOLD automatically.
ALT HOLD: in this mode, the DFCS holds the altitude at the time of selecting. If the aircraft was allready climbing, it will overshoot the altitude and return to the altitude at wich the ALT HOLD button was pressed.
VNAV SPD: with the DFCS in VNAV the VNAV SPD mode is active when the aircraft is climbing to the FMC altitude until it reaches the MCP altitude (then it goes into ALT HOLD) the flightcrew flies at this level until they are ready to continue the climb to the FMC altitude and can then press the VNAV button again to return to the VNAV SPD mode.
VNAV PTH: the VNAV PTH becomes active when the FMC altitude is reached. At the TOD (top of descent) the VNAV PTH will remain active for the descent but for this needs the LNAV too. If there is no LNAV available, the VNAV PTH will disengage. At this time the flight crew can still select VNAV SPD for the descent.
MCP SPD: this will be presented when the flight crew selects LVL CHG (level change). The flight crew must select a new altitude on the MCP and then press the LVL CHG button to initiate this function. If the aircraft was in VNAV, the new MCP SPD indicated on the IAS/MACH indicator (and therefore the target speed) will be the FMC speed at that time.
If the LVL CHG was pressed whilst the DFCS was in any other mode, the IAS/MACH indicator will be the airspeed at that moment.
G/S: When the flightcrew selects APP (approach) this mode will become armed until the G/S is captured and then the DFCS will follow the G/S to the runway threshold.
FLARE: this is the last moment before touchdown, the DFCS pitch channel will steer the aircraft nose up right before the landing (~50 feet radio altitude).
It is technically possible to perform a automatic flare on a single autopilot but this isn't what it's meant for. At single autopilot approach the autopilot should be switched off at the Decision Height and landed manually, we don't trust any single system to completely control a automatic landing to ground level.
If u are more interested in wich unit will actually send the signal to the display (if u are standing in the rain and snow on a outstation in some godforsaken country trying to figure out why u are missing a indication on your display unit) go to the SDS 31-62-00 to find wich indication comes from wich unit.