EFIS: PFD and ND - ATPL 022 — Instrumentation

The last lesson closed with a promise: the electronic flight deck kept the ADI/HSI pairing. This lesson delivers it. The electronic flight instrument system (EFIS) replaces the mechanical faces with screens: the primary flight display (PFD), the ADI reborn with everything needed to control the aircraft on one page, and the navigation display (ND), the HSI reborn as a mode-selectable moving map. First the machinery behind the screens, then each screen in full.

The EFIS building blocks

An EFIS has four parts:

the display units — the screens themselves, built on the technologies of the last lesson;
the symbol generator — the computer that actually draws: it collects the data from the aircraft's sensors and systems, turns it into the symbology, and feeds the finished picture to the display units; the screens themselves are only as clever as the generator behind them;
the EFIS control panel — where each pilot tells the system what to show;
the remote light sensor — measures the ambient light on the flight deck and trims the display brightness automatically, so the picture stays readable from a dark night to sunlight without the crew chasing a knob: the glare limitation of the last lesson managed in real time.

Layout, transfer and failures

The typical layout puts two display units in front of each pilot: the PFD directly in front, the ND beside it, the flying picture where the eyes rest and the navigation picture one glance inboard. Because every unit is just a screen fed by a generator, the system can offer a transfer facility: if a display unit fails, its information is moved onto another unit, the crew choosing which picture matters more or the surviving screen cycling what it shows. The failure cases must be kept apart: when a display unit fails, the screen itself is dead, and the transfer facility moves its contents elsewhere; when a symbol generator fails, the screens are healthy but the artist is gone, and every unit that generator was feeding loses its picture at once. That is why redundant symbol generators matter: with a second generator available, the crew switches the displays to the other source and the pictures return, a far smaller loss than a dead screen.

Behind all of it stand the standby instruments: independent attitude, airspeed and altitude, needed for the day all the display units fail. They come with their own challenge, honestly stated: they are small, and positioned away from the normal scan, so flying on them is flying on a fraction of the usual picture, in a corner of the panel the eyes must be retrained to find. The syllabus wants both halves: why they exist, and why using them is hard.

The EFIS control panel

The purpose of the EFIS control panel is selection: what each pilot's screens show and against which references. The typical selections, listed for the exam:

altimeter pressure setting — the baro reference the altitude tape uses;
ND mode selector — which of the navigation display's modes is shown;
ND range selector — how far the navigation picture reaches;
ND data selector — what extra data is drawn: waypoints, facilities, constraints, data;
radio-navigation aids selector — VOR 1/2 or ADF 1/2 onto the display;
decision altitude (DA)/decision height (DH) selection — the minimum the PFD will show and mark.

The PFD: the basic T, electronic

The PFD (on some aircraft named the electronic attitude director indicator, EADI) presents a dynamic colour display of all the parameters necessary to control the aircraft, and its main layout deliberately conforms with the basic T principle inherited from the classic panel: attitude information in the centre, airspeed on the left, altitude on the right, heading and track lower centre, with the flight mode annunciation lesson 18 taught across the top. Around the T the PFD carries the speed references the classic panel never could: the take-off and landing reference speeds, bugged on the speed tape for the moments they govern; the minimum airspeed and the lower selectable airspeed, drawn as the bands at the bottom of the tape below which the aircraft must not be flown or slowed; and the Mach number, read out beside the tape rather than on it.

The attitude information at the centre keeps the design of the attitude indicator from module 4: an artificial horizon with a fixed aircraft symbol, and superimposed on it the flight director command bars, so the electronic page shows the same marriage of attitude and command the mechanical ADI did.

Speed, Mach, altitude, heading

The speed tape — a rolling speed scale with a numerical read-out of the current speed at its centre; the limiting airspeeds according to configuration marked on the scale, so the tape itself shows what may not be flown; a speed trend vector, an arrow predicting where the speed is going; and a bug for the selected airspeed.
Mach — a selected Mach number is presented with the speed bug placed on the corresponding IAS (indicated airspeed) on the speed tape, while the Mach number itself is shown as a numerical indication outside the tape. The bug rides the tape; the Mach value stands beside it.
The altitude tape — a rolling altitude scale with a numerical read-out of the current altitude; the altimeter pressure setting beneath it; a bug for the selected altitude; and a means of highlighting the altitude when certain criteria are met, the read-out changing its emphasis as the selected altitude approaches so a level-off cannot arrive unannounced.
Heading and track — a rolling compass scale with a numerical read-out of the current heading or track, and a bug for the selected value, the strip at the bottom of the T doing in miniature what the HSI's card did in full.

The edges of the PFD

Around the T, the PFD gathers the rest of the flying picture, and the syllabus wants the design and location of each:

the flight mode annunciators (FMAs) — across the top;
the vertical speed indicator — beside the altitude tape, which also carries the TCAS (traffic alert and collision avoidance system) resolution advisory commands lesson 29 drew on its scale;
the radio altitude from lesson 28 — shown low on the display in the final feet;
the deviation indicators — ILS (instrument landing system) localiser and glideslope scales or their RNP/PBN, GBAS or SBAS horizontal and vertical flight-path equivalents, the lateral scale beneath the attitude and the vertical scale beside it, next to the altitude tape;
the decision altitude/height (DA/H) — selected on the control panel, displayed beside the altitude information and announced at the minimum.

The ND and its four modes

The ND (on some aircraft the electronic horizontal situation indicator, EHSI) provides a mode-selectable colour flight navigation display: one screen, several ways of drawing the world, chosen on the control panel. The four typical modes:

MAP (or ARC).
VOR (or ROSE VOR).
APP (or ROSE LS).
PLAN.

Two appearances carry the family:

EXPANDED — puts the aircraft symbol at the bottom with a forward arc of compass above it, so the whole selected range stretches ahead;
FULL (or ROSE) — centres the aircraft in a complete compass rose, so the same selected range is split around the aircraft and only half of it lies ahead.

Same selection, different reach: that displayed-range difference is the exam's favourite corner of this topic.

MAP mode

MAP (or ARC) is where most of a flight is spent: the route drawn over a moving map, ahead of the aircraft. Listed for the exam, what MAP mode can display:

the aircraft symbol, compass scale and range markers;
the current heading and track, either one 'up' depending on selection, true or magnetic;
the selected heading and track;
TAS/GS (true airspeed/groundspeed) and wind direction and speed (W/V);
raw-data radio magnetic indicator (RMI) needles for VOR/ADF if selected, with the frequency or ident of the tuned facility;
the route and TO/next-waypoint data from the FMS (flight management system) module 8 built;
navigation-database data such as airports, waypoints or facilities as selected;
weather radar information;
TCAS traffic information, the surrounding traffic but never the RA commands, which belong to the PFD's vertical scale;
the TAWS (EGPWS) terrain picture lesson 28 promised to this display;
failure flags and messages.

VOR and APP modes

VOR (or ROSE VOR) and APP (or ROSE LS) are the raw-data modes: the display becomes an HSI-type display format, the course-and-deviation face the last lesson taught. Both show the aircraft symbol and compass scale, the current and selected heading and track (as in MAP, either one 'up', true or magnetic), TAS/GS and W/V, and the frequency and identification of the selected navigation aid. Each mode then adds its own needles:

VOR mode — the VOR selected course, deviation indicator and a TO/FROM indicator;
APP mode — the localiser selected course and deviation indicator, and the glideslope indicator.

Both modes can still overlay weather radar, TCAS traffic and TAWS terrain, and both carry failure flags and messages.

PLAN mode

PLAN is the odd one out. What it can display:

a north-up compass rose and range markers, the rose here being a fixed north-reference ring rather than a live compass;
the aircraft symbol oriented according to the aircraft's heading;
TAS/GS and W/V;
the route and TO/next-waypoint data from the FMS;
navigation-database data as selected;
failure flags and messages.

Its purpose is route review: north is up at all times and there is no compass information; the centre waypoint is whichever waypoint is selected on the FMS CDU (control and display unit), so scrolling through the flight plan on the CDU shifts the map view along the route, the aircraft symbol sitting wherever the aircraft actually is on the path. And one warning the syllabus states plainly: using PLAN as the primary mode in flight may lead to disorientation and loss of situational awareness, because a north-up page with no compass tells a crew nothing about what lies ahead of the nose.

Mode, range and situational awareness

The last skill is the combination of mode and range: choosing, for each phase of flight, the picture that keeps situational awareness highest. In the cruise, MAP at a long range shows the route, the weather and the traffic far ahead; in the terminal area, MAP stepped down to a short range keeps the turns and the constraints readable; on a raw-data approach, APP or VOR puts the needles centre stage; on the ground, PLAN reviews what was loaded before it is flown. Selecting the appropriate range and displayed data for the phase of flight improves situational awareness; leaving yesterday's selection on the screen degrades it.

What the exam asks

The part lists, the failure distinction, the tape designs and the mode contents carry the marks. The overview, following the learning objectives:

List the parts of an EFIS: control panel, display units, symbol generator, remote light sensor

Describe the display layout, the transfer facility, and the need for and challenge of standby instruments

Explain the difference between a symbol generator failing and a display unit failing, and the value of redundant generators

Describe the EFIS control panel and its typical selections

Describe the PFD/EADI: dynamic colour display, basic T layout, and the attitude design with FD command bars

Describe the speed tape, Mach presentation, altitude tape and heading/track information

Describe the design and location of the FMA, vertical speed with TCAS RA, radio altitude, deviation indicators and DA/H

List the four ND modes and distinguish EXPANDED from FULL/ROSE, including the displayed range

List and explain the information displayed in MAP, VOR/APP and PLAN modes, PLAN's purpose and its risks, and mode/range selection for situational awareness