B737 Navigation Systems: IRS, GPS and Radio Navigation

The Boeing 737 navigates on two inertial reference systems (IRS), two GPS receivers and a set of radio receivers: VHF NAV and ILS, DME, ADF and marker beacon. The flight management computer (FMC) blends them into one position, the displays show their raw data, and the ATC transponders and the weather radar complete the navigation chapter of the operations manual.

This article follows the 737 NG flight crew operations manual (FCOM). The 737 MAX shares the type rating, and where the FAA Flight Standardization Board (FSB) lists a navigation difference for it, the text says so. Displays are covered in B737 flight instruments and displays and the FMC in B737 FMC and CDU; the principles are in IRS alignment and modes.

Navigation equipment overview

The flight management system comprises the flight management computer system, the autopilot flight director system, the autothrottle, the IRS and GPS. The dual FMC installation is certified as a sole-source navigation system, so a 737 with two FMCs may operate outside radio navaid coverage.

On the NAVIGATION panel, the VHF NAV transfer, IRS transfer and FMC source select switches are set to NORMAL in the first officer's preflight. On battery power alone the left side survives: left FMC and CDU, VHF NAV 1 and ILS 1, left IRS, left GPS, marker beacon, ADF No. 1, transponder No. 1 and DME No. 1. On a flight director or autopilot ILS approach the cross bus tie relay opens at glideslope capture, isolating the DC buses so that a single failure cannot affect both navigation receivers and both flight control computers.

IRS alignment, modes and loss of alignment

Each air data inertial reference unit (ADIRU) has an inertial and an air data section. Each IRS has three sets of laser gyros and accelerometers and is the airplane's only source of attitude and heading, apart from the standby attitude indicator and the standby magnetic compass on the NG. The ISFD, basic on the MAX, computes attitude from its own internal sensors.

The IRS mode selector

A normal alignment is started by turning the selector straight from OFF to NAV, and it is possible only between 78°15' north and south. It takes from five to seventeen minutes depending on latitude; during it the ISDU's HDG/STS display shows the minutes remaining, holding at 15 until 14 remain and then counting down. TK/GS shows true track and groundspeed.

The crew enters the present position on the CDU or with the ISDU keys, and several checks guard the entry:

• An entry not within 4 NM of the origin airport brings the CDU message VERIFY POSITION.
• The white ALIGN light flashing means alignment cannot be completed: the entered position differs significantly from the previous one, is unreasonable, or is missing.
• Two consecutive entries of the same position that fail the second internal test light the FAULT light.
• On the ISDU, a lit white CLR key means an entry was not accepted.

Boeing recommends a full alignment before each flight. In the preliminary preflight the selectors go OFF then NAV, and the crew checks that the ON DC lights come on then go out and that the ALIGN lights are lit; UNABLE REQD NAV PERF-RNP may show until alignment is complete. On a short transit stop, selecting ALIGN while parked gives a 30-second fast realignment, zeroing groundspeed error; the latitude and longitude should be entered before NAV is selected again.

The IRS stores magnetic variation between 82° north and south and combines it with true heading to give magnetic heading. The limitations prohibit operations based on magnetic heading or track where the table's error exceeds 5°.

Power and the ON DC light

The left IRS is normally powered from the AC standby bus and the right from AC transfer bus 2. If AC is not normal, either switches to backup DC from the switched hot battery bus and its amber ON DC light comes on; on the ground a horn in the nose wheel well warns of battery drain. Backup DC to the right IRS ends after five minutes if AC is not restored. A momentary ON DC light during the alignment self-test is normal.

Loss of alignment and the transfer switch

If alignment is lost in flight, the navigation mode, with present position and groundspeed, is gone for the rest of the flight. Selecting ATT relevels the system for attitude after about 30 seconds of straight, level, unaccelerated flight. Heading must then be entered by hand and can drift by up to 15° per hour, so it is cross-checked against an operating compass and updated.

The IRS transfer switch moves the attitude and heading source of the flight instruments: BOTH ON L, for example, puts both sides on the left IRS. Whenever it is not at NORMAL, an amber INSTR SWITCH annunciation shows that both pilots' displays use the same IRU data.

GPS and position updating

The two GPS receivers are independent, and each gives the FMC and other systems an accurate position. Their operation is automatic; the only crew control is deselection on the NAV OPTIONS page 2/2. The amber GPS light means both GPS units have failed, or, on a master caution recall, that one has.

On the ground the FMC computes its position from GPS, or from the IRS if GPS is unavailable. Position updates from the navigation sensors are used in this priority order:

1. GPS
2. Two or more DME stations
3. One VOR with collocated DME
4. One localizer and collocated DME
5. One localizer alone

Radio updating can continue while GPS is in use. If all GPS data is lost the FMC reverts to radio or inertial updating; with the IRS as its only reference, it corrects the IRS position by the typical error it measured while updates were available. The FMC autotunes the DMEs, and a specific station can be inhibited on NAV OPTIONS page 2/2. VOR/DME updating needs a VOR tuned by the crew, and localizer updating occurs only once the ILS has been tuned manually.

The result is shown as actual navigation performance (ANP), the FMC's estimate of its position error with 95 % probability. The EFIS POS switch adds the IRS and GPS positions and VOR bearing vectors to the map. At the MAX's return to service in January 2021, EASA's simulator assessment of air data failures did not support RNP AR operations, so EASA added a limitation to its MAX flight manual prohibiting them.

VHF navigation and ILS

Two ILS receivers are installed, and the crew tunes VOR and ILS frequencies manually on the VHF navigation control panels; the TFR switch swaps the active and standby frequencies.

• Source switching. With the VHF NAV transfer switch at NORMAL the captain's displays use multimode receiver (MMR) 1 and the first officer's MMR 2; BOTH ON 1 puts both on MMR 1.
• Autopilot. The digital flight control system cannot use VOR or ILS data that is not displayed, so the VHF NAV control must match the engaged autopilot: CMD A uses VHF NAV 1, CMD B VHF NAV 2.
• Antennas. With a localizer frequency tuned, the receivers switch from the tail to the nose antenna when VOR/LOC is armed or engaged; if they do not, LOC mode is inhibited.
• Identification. The Morse identifier of a tuned VOR, ILS or ADF can be decoded and shown on the PFD and ND. It is not compared with the FMC database, so the crew still checks it; the audio control panel filter selects voice, range (identifier) or both.
• Disagreement. If the two tuned ILS frequencies differ for more than a minute, the PFD frequency turns amber with a horizontal line.

ADF and marker beacons

The ADF feeds bearing pointers on the displays and, on the NG, the standby radio magnetic indicator (RMI). On the ADF mode selector, ANT optimizes audio reception and gives no bearing. TEST runs a sequence on the displays: the fail flag shows and the pointer goes out of view, then the flag clears, and finally the pointer slews to 135° relative bearing.

If heading or track information is lost, the ADF pointers are removed from the displays and the RMI pointers no longer show correct magnetic bearings, although relative bearings may still be right. The NG's standby RMI, powered from the AC standby bus, stays on after the loss of all normal AC while battery power lasts.

Over a marker beacon the PFD symbol flashes with its aural code:

The marker beacon lights add FT, white, for the self-test. At the final fix inbound the pilot monitoring calls the fix (OUTER MARKER, VOR or NDB), the time, the altitude and "ALTIMETERS AND INSTRUMENTS CROSS CHECKED". In the United States the FAA no longer requires a middle marker, requires an inner marker only for CAT II operations without a published radio altitude minimum, and accepts a suitable substitute for the outer marker as the final approach fix of a non-precision approach (see ILS).

ATC transponder

The transponder mode selector offers ALT RPTG OFF, replying without altitude, and XPNDR, replying with it, besides the TCAS positions. Transmissions are enabled automatically when the air/ground system indicates air. The amber XPNDR FAIL light means a transponder malfunction or, where fitted, ADS-B inoperative. For ADS-B, the left GPS supplies position to transponder 1 and the right GPS to transponder 2.

Depending on its level of enhancement, the transponder can also send flight number, airspeed or groundspeed, magnetic heading, altitude and GPS position, and some airports track airplanes on the ground whenever the selector is out of STBY or OFF. TCAS modes should not be used on the ground for this. At shutdown the reply or mode selector, depending on variant, goes to STBY. The FAA's AIM asks for altitude reporting and ADS-B Out across the airport movement area, TA or TA/RA on entering the runway, and STBY or OFF at the gate. On the MAX the FSB lists transponder information among the additions to the new AUX display.

Weather radar

The weather radar performs weather detection and ground mapping only; it should not be relied on for proximity warning or collision avoidance. Its antenna sweeps a 180° forward arc. Rainfall shows in red for the heaviest, then amber, then green, and in MAP mode surfaces appear in the same colors from most to least reflective, picking out coastlines, mountains and cities.

View full schematic

• Turbulence. In WX/TURB, precipitation moving horizontally toward or away from the antenna at 5 m/s or more is painted magenta and indicates heavy turbulence. Detection is limited to 40 NM whatever range is selected, and it needs precipitation: clear air turbulence cannot be seen.
• Range. On the 640 NM range, returns are limited to 320 NM. On the MAX the FSB lists a revised ND and weather radar range selector (Level B), so its behavior is learned from the MAX FCOM.
• Display sharing. Terrain and weather cannot share a display; TERR deselects the radar, and if one pilot selects each, the displays update on alternate sweeps. The WXR switch goes off automatically if the EFIS control panel fails in flight.

The radar also provides predictive windshear (PWS). It starts scanning by itself when the thrust levers are set for takeoff, even with an engine off or the IRS not aligned, and in flight below 2,300 ft radio altitude; the sweep then narrows to 120°, and alerts are given below 1,200 ft RA. The caution and warnings are described in B737 TCAS, terrain and windshear alerting and windshear and microbursts.

On the ground, TEST with the thrust levers not advanced checks the PWS caution, PWS FAIL and PWS warning indications for about 15 to 60 seconds; deselecting WXR stops it. The radar is off during the preflight, with no returns on the map, and is switched off again after landing. The limitations ask crews to avoid operating it in a hangar, except in test mode, and while anyone is within the area the nose radome normally encloses (see weather radar).

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.