This section will therefore focus on the Pegasus FMS, how it determines position and the use of VNAV during an approach.
The FMC is a multi-sensor navigation system, and can access:
Although all 3 MMRs have an ILS receiver, only the left and right MMRs have operating GPS receivers.
The ADFs are not used by the FMCs, due to the magnitude and unpredictable nature of the errors inherent in NDB navigation.
To calculate position, the FMC requires at least one IRS operating in navigation mode and supplements this with data from the other sensors.
To optimise the navigation solution, the FMCs can automatically tune the VORs and DMEs, but for this to occur, at least one VOR controller must be in AUTO.
For DME/DME position fixing, it is not necessary for both controllers to be in AUTO, as each receiver is able to simultaneously process data from 5 ground stations.
If required, automatic updating of individual NAVAIDs can be inhibited using the NAV DATA page and it is also possible to permanently inhibit VOR/DME positioning fixing; however, DME/DME and inertial updating cannot be prevented.
In areas that do not use WGS-84 as the coordinate system, GPS Navigation can be disabled using POS REF page 3/4, but will default back to ON when power is next applied to the aircraft.
Although the FMC may not be using IRS position data as it’s primary navigation source, it will always uses IRS velocity data to generate the steering commands necessary to maintain the desired flight path.
The FMC groups external data into 3 system categories: IRS, GPS and RADIO (which includes the VORs and DMEs) and, for each category, calculates and displays an Actual Navigation Performance.
As previously mentioned, Actual Navigation Performance represents the limit of the potential error in the aircraft position and when computing ANP, the FMC will consider such factors as: sensor availability and accuracy, signal strength and degradation, and the geometry of each fix.
Actual navigation Performance is not static and will vary with time according to the type of system.
For the IRS triple-mix position, the ANP will increase at a fairly steady rate from the moment the IRSs are aligned and for most flights, an ANP of about 6.0 is normal after 2 to 3 hours of flight.
For the RADIO and GPS positions, the ANP will vary according to the quality and geometry of the available fixes. An ANP of between 0.25 and 1.0 is normal for the RADIO position and between 0.05 and 0.12 for the GPS position.
Selecting the system with the smallest ANP, the FMC uses this system as the primary means of updating the FMC position and will use the systems ANP as the FMC's ANP. As the GPS position will normally have the lowest ANP, GPS will normally be the primary means of updating the FMC position.
ANP is a theoretical value, and the actual difference between the sensor and FMC positions can be observed on POSITION REFERENCE page 2. To ease comprehension, the displayed Latitude and Longitude can be toggled and displayed as a Bearing and Distance by pressing Line Select Key 6 Right.
The FMC position is based on the predicted current position, whereas the individual sensor (IRS, GPS and RADIO) positions are based on the last fix.
As the FMC updates the CDU display about every half a second, there may be what appear to be small discrepancies between the GPS and FMC position. Typically and dependent upon groundspeed, the GPS position will be approximately one tenth of a mile behind the aircraft.
By analysing flight data and known system performance, Boeing have demonstrated what RNP can be achieved using the Pegasus FMS and autoflight systems.
Although not reflected by the displayed ANP, it should be noted that the RNP achieved by making full use of the autoflight system is approximately half that achieved by manually flying the aircraft.
In order of priority, the displayed RNP will be either a manual entry, a navigation database specified RNP value or a default RNP value.
If the displayed RNP is incorrect, the crew can manually enter an RNP and the value entered will remain until changed or deleted. Care is required though, as setting an RNP smaller than necessary may cause unnecessary crew alerts, whilst setting an RNP larger than required will result in inadequate alerting.
If left to its own devices, the displayed RNP is based on the stage of flight:
| Stage of Flight | Default RNP |
|---|---|
| Above FL155 and not on a SID or STAR | 2.0 |
| At or Below FL155, or on a SID or STAR | 1.0 |
| Final Approach | 0.5 |
Once IRS alignment is complete on the ground, the FMC calculates it’s present position based on GPS position data smoothed by IRS velocity data.
If GPS updating has been disabled, a Pegasus FMC will update its position to coordinates of the takeoff runway threshold when the EPR switch is pushed at the beginning of the takeoff.
To ensure that the update is correct during an intersection departure, the distance between intersection and threshold must be entered on the TAKEOFF REF page.
Based on the typical ANP for each sensor, the FMC will update its position when inflight using the following priority:
The one occasion that GPS alone may not be the primary source of data is during an ILS approach when, providing certain criteria are met, the localiser signal will be used to improve the pure GPS position and an ANP of better than 0.05 can be expected.
When calculating the aircrafts position, the FMC will use whichever system has the lowest ANP and this will normally be the GPS position.
If the quality of the GPS position is either momentarily or permanently degraded due to satellite unavailability, satellite masking or receiver malfunction, the ANP will increase (at a rate related to the ground speed of the aircraft) until it reaches the ANP associated with the next best navigational solution. Typically, the ANP will increase to that associated with the RADIO navigation position (somewhere between 0.3 and 0.5).
The reason that the ANP does not increase instantly to the ANP associated with the RADIO position, is that prior to the time GPS navigation was lost, the FMC will have a high confidence of knowing where it was and is likely to be in the immediate future. It is only as the aircraft moves away from this known position, that the ANP will increase.
For occasions when there are insufficient satellites for the MMR to provide GPS position data to the FMC, the FMC is permitted to continue navigation using the last good GPS position combined with IRS velocity data.
Known as position coasting, this feature allows the MMR a short period of time in which to seek additional satellites, before the FMC must revert to the next best navigation system, thus preventing transient UNABLE RNP alerts.
If both GPS and RADIO position data are unavailable, the FMC will revert to using IRS navigation.
For areas specifying an RNP of 10 or greater, IRS navigation may be used for about 6 hours, but for flight in terminal airspace, IRS-only navigation may only be used for a matter of minutes.
To alert the crew of conflicting data, the following messages can be displayed.
If the difference between the FMC position and the aiding sensor (GPS, RADIO or Inertial) position is greater than 12 nautical miles for 5 seconds or more, the CDU will display VERIFY POSITION.
If the default RNP value changes to a value smaller than one previously entered by the crew, the CDU will display VERIFY RNP – POS REF 2.
If the crew either enter an RNP value greater than the default RNP value or enter an RNP less than the current ANP, the CDU will display VERIFY RNP ENTRY.
With the exception of VERIFY RNP ENTRY message, these CDU messages will also be accompanied by the EICAS advisory message FMC MESSAGE.
If the calculated ANP exceeds the RNP, the FMC will generate the EICAS message UNABLE RNP to alert the crew. Whether this message is displayed as an advisory or as a caution depends upon the stage of flight.
If the aircraft is not on the approach, the FMC will generate UNABLE RNP as an advisory message. If the aircraft is on the approach, UNABLE RNP will be generated as a caution message, accompanied by the master caution light and caution beeper.
Although primarily developed for GPS navigation, Pegasus will also generate UNABLE RNP if the ANP exceeds the RNP during non-GPS navigation.
Failure of either a GPS sensor unit, antenna or wiring between the antenna and MMR will generate the EICAS advisory message L or R GPS (as appropriate). If a total GPS system failure is detected, the EICAS advisory message GPS will be displayed, replacing the EICAS advisory message(s) L or R GPS.
To meet the requirements for RNP approval in RNAV airspace, there are time limits set on the frequency of navigational position updates that affect what equipment may be used.
For flight in RNP-10 and RNP-12 airspace, the following limitations must be observed:
| System | FMC Primary | Time Limit |
|---|---|---|
| IRS | FMC(IRS) (IRS NAV ONLY) |
6.2 hours since last full ground alignment or 5.9 hours since last DME/DME update or 5.7 hours since last VOR/DME update |
| VOR/DME | FMC(RADIO) | No time limit |
| DME/DME | FMC(RADIO) | No time limit |
| GPS | FMC(GPS) | No time limit |
For flight in B-RNAV airspace, the following equipment limitations must be observed:
| System | FMC Primary | Time Limit |
|---|---|---|
| IRS | FMC(IRS) (IRS NAV ONLY) |
2 hours since last full ground alignment |
| VOR/DME | FMC(RADIO) | No time limit |
| DME/DME | FMC(RADIO) | No time limit |
| GPS | FMC(GPS) | No time limit |
The following equipment satisfies the requirements for P-RNAV operations:
| System | FMC Primary | Time Limit |
|---|---|---|
| IRS | FMC(IRS) (IRS NAV ONLY) |
NOT PERMITTED |
| VOR/DME | FMC(RADIO) | NOT PERMITTED |
| DME/DME | FMC(RADIO) | No time limit |
| GPS | FMC(GPS) | No time limit |
The following equipment satisfies the requirements for RNP-RNAV (RNP-0.3) operations:
| System | FMC Primary | Time Limit |
|---|---|---|
| IRS | FMC(IRS) (IRS NAV ONLY) |
NOT PERMITTED |
| VOR/DME | FMC(RADIO) | NOT PERMITTED |
| DME/DME | FMC(RADIO) | No time limit (overlay) |
| GPS | FMC(GPS) | RAIM Prediction required |
If the PF selects Speed Intervention during a VNAV PATH descent using a Basic or PIP-FMS aircraft, the FMC will remain in VNAV PATH and maintain the vertical path using pitch steering commands, whilst the autothrottle attempts to maintain the speed selected in the MCP Speed Window.
Although this behaviour is useful during the approach phase, crews tend to prefer using FLIGHT LEVEL CHANGE during the descent phase due to the greater flexibility available.
When designing the software for the Pegasus FMS, Boeing and Honeywell responded to requests by modifying VNAV Speed Intervention to work more like FLIGHT LEVEL CHANGE.
If the PF selects Speed Intervention during a VNAV PATH descent using a Pegasus-FMS aircraft, the FMC will switch to VNAV SPEED and use pitch steering commands to maintain the MCP selected speed.
If the selected speed is faster than that required to maintain the vertical path, the aircraft will deviate below the path and if the speed is slower, the aircraft will deviate above the path.
To allow the aircraft to follow the required vertical path during the approach phase, the FMC must switch to ON APPROACH logic.
With ON APPROACH logic active, if the PF selects Speed Intervention when on the VNAV PATH, the FMC will remain in VNAV PATH and maintain the Vertical Path using pitch steering commands, whilst the autothrottle maintains the speed selected in the MCP Speed Window - exactly the same as a Basic or PIP-FMS aircraft does.
The FMC switches from DESCENT logic to ON APPROACH logic when either:
If a 'Direct To' waypoint on the approach is executed, VNAV transitions to approach phase when the airplane passes the 'Direct To'.
If a new waypoint is added to the approach and executed, VNAV may not transition to approach phase.
There is no change of FMA to indicate that the FMC has switched to ON APPROACH logic.
However, when the switch occurs, the displayed RNP will change to indicate either the default RNP for the approach phase (0.5) or the RNP that has been coded into the navigation database.
If ON APPROACH logic is not active and the ANP exceeds the RNP, the FMC will display UNABLE RNP as an advisory message. With ON APPROACH logic active, the FMC will generate UNABLE RNP as An EICAS caution message accompanied by the master caution lights and beeper.
Geoff Middleton