FMC

Question 1

Do the FMCs operate independently?

Answer 1

The dual FMC installation is certified as a “sole source” navigation system. Aircraft equipped with two FMCs are certified to operate outside radio navaid coverage. The second FMC serves as a backup, providing complete navigational functions if the other FMC fails. With a dual FMC installation, one FMC is always designated as primary. This is controlled by the position of the FMC source select switch. In the NORMAL position, the left FMC is primary by default. Although the crew can enter information into either MCDU, the primary FMC is responsible for synchronizing this information with the secondary FMC and updating both MCDU displays.

Question 2

Where do the FMCs receive their inertial data from?

Answer 2

Usually, the FMCs receive inertial data from their on-side ADIRU (left ADIRU to FMC 1 and right ADIRU to FMC 2).

Question 3

How is FMC position data calculated with the loss of GPS inputs?

Answer 3

When external position (GPS, radio navigation) updating is not available, the FMC uses the IRS position as reference. When the IRS is the only position reference, the FMC applies an automatic correction to the IRS position to determine the most probable FMC position. This correction factor is developed by the FMC’s monitoring IRS performance during periods of normal position updating to determine the typical IRS error value. It is important to note that when external position updating is not available, navigation accuracy may be less than required. Flight Crews should closely monitor FMC navigation (ANP v. RNP), especially when approaching the destination. The accuracy of the FMC navigation should be determined during descent phase by using radio navaids and radar information if available.

Question 4

How is takeoff thrust reduced from maximum takeoff thrust?

Answer 4

Fixed derates can be selected on the N1 LIMIT page. With derated takeoff selected, the thrust setting parameter is considered a limitation for takeoff; therefore, thrust levers should not be advanced further except in an emergency. A further thrust increase following an engine failure could result in a loss of directional control while on the ground. Derated takeoff rating can be further reduced by assumed temperature. ~Assumed Temperature Thrust Reduction Takeoff A takeoff thrust less than the full rated thrust may be achieved by using an assumed temperature that is higher than the actual temperature. The desired thrust level is obtained through entry of a SEL TEMP (assumed temperature) value on the N1 LIMIT page or TAKEOFF REF page 2. The maximum thrust reduction authorized is 25% below any certified rating. Do not use assumed temperature reduced thrust if conditions exist that affect braking, such as slush, snow, or ice on the runway, or if potential windshear conditions exist. When the assumed temperature method is used with full rate, the reduced thrust setting is not considered a limitation. If conditions are encountered where additional thrust is desired, the crew can manually apply full thrust.

Question 5

Where does the FMC receive fuel quantity information from?

Answer 5

The FMC receives fuel quantity data from the Fuel Quantity Indicating System (FQIS) and from the engine or from manual entries on the MCDU.

Question 6

What are the sources of position data to the FMC?

Answer 6

The FMC determines the present position from the IRS, GPS, and navigation radios. The FMC uses its calculated present position to generate lateral steering commands along the active leg to the active waypoint.

Question 7

Which input has the highest-weighted priority for FMC position?

Answer 7

FMC logic selects the hybrid GPS inertial position as the primary update to the FMC position. During GPS outages, the hybrid GPS position will remain valid, coasting on a calibrated solution with very low drift rates. If all GPS data becomes unavailable and the coasting hybrid GPS solution becomes invalid, the FMC reverts to radio or IRS updating.

Question 8

What criteria must be met for LNAV to engage?

Answer 8

• LNAV engages when laterally within 3 nm of the active route leg • outside of 3 nm of the active route leg, LNAV engages if on an intercept heading of 90° or less and the intercept will occur before the active waypoint.

Question 9

How is a VNAV descent path calculated by the FMC?

Answer 9

The descent path starts at the calculated T/D point and includes waypoint altitude restrictions. The path is based on:

• E/D point
• idle thrust
• speedbrakes retracted
• descent wind speed decreasing with decreasing altitude (or loadable winds)
• applicable target speed.