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Climb

Normally in the climb the VNAV, LNAV and timing functions will be operative.

		A C T E C O N C L B		1 / 1
1L	C R Z A L T			A T M A C E Y	1R
	F L	3 3 0		6000A
2L	T G T S P D			T O M A C E Y	2R
	2 8 0 / . 7 2		2 0 0 4 . 3 Z / 1 9 N M
	S P D R E S T			E R R M A C E Y
3L	- - - / - - - - -			3 1 0 L O	3R
	- - - - - - - - - - -
4L				C L B - T N 1	4R
			90.3/	90.3%
5L	< L T E N G O U T R T E N G O U T >				5R
6L					6R

Figure 16.12

On the climb page (CLB) at 1L is the planned initial cruising altitude, if one exists and the climb is active, and at 1R is the current climb restriction. The suffix ‘A’ indicates altitude. 2L gives the economy speed for the climb and 3L any speed restriction, which defaults to 250 kt and 10 000 ft. Any other speed/altitude restriction imposed by ATC can be input to 3L from the scratchpad. At 2R is the ETA and distance to go to the next position. 3R gives the height error at the next point showing the aircraft will be 310 ft low. The climb engine N1 is displayed at 4R. The prompts at 5 and 6 L and R direct the pilots to the other climb mode pages. (RTA is required time of arrival, to be used if an RTA is specified by ATC).

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Cruise

In the cruise all three modes will normally be active.

		A C T E C O N C R Z			1 / 1
	1L	C R Z A L T	O P T	M A X	S T E P T O	1R
		F L 2 1 0	F L 3 4 2 / 3 6 8		-----
	2L	T G T S P D				2R
		. 6 81		2 0 5 6 . 2 Z / 1 98 NM
		T U R B N 1			A C T U A L W I N D
	3L	61.1/ 61.1 %			129 / 14	3R
		F U E L A T	K A T L
	4L	7.8				4R
	5L	< L T E N G O U T R T E N G O U T >				5R
	6L					6R

Figure 16.13

The cruise page (CRZ) has the current cruising altitude at 1L with the required cruising speed at 2L; in this case the economy cruise speed. At 3L is the computed EPR/N1 required to maintain the speed at 2L, with the predicted destination fuel shown at 4L. At 1C is the optimum and maximum cruising level for the aircraft weight and the ambient conditions. The next step altitude is displayed at 1R with the time and distance to make the step climb at 2R. 3R shows the estimated wind velocity and 4R shows the predicted savings or penalty in making the step climb indicated at 1R. The other cruise pages are accessed through 5R, 6L and 6R.

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Area Navigation Systems (RNAV) 16

Descent

As in the climb the LNAV, VNAV and timing modes are all operative.

	A C T	E C O N	PATH	DES		1 / 1
1L	E / D A L T				A T M A C E Y		1R
	2 0 1	3				6000A
2L	T G T S P D				T O M A C E Y		2R
	.7 2 0 / 2 8 0		2 0 0 4 . 3 Z / 1 9 N M
	S P D R E S T					W P T / A L T
3L	2 4 0	/ 1 0 0 0 0		MACEY/6000			3R
4L	V E R T D E V			F P A	V / B	V / S
	2 5 N I			3.8	6 . 2	2360	4R
5L					SPEED >		5R
6L	< F O R E C A S T					R T A >	6R

Figure 16.14

With the active economy path descent (ACT ECON PATH DES) page selected, the target Mach number and CAS are at 2L; at 1L is the end of descent altitude. At 1R is the next descent position and altitude; the suffix A indicates at or above. Position 3L contains the speed transition, which is 10 kt less than that stored in the database, and the transition altitude. If none is defined then it defaults to 240/10000. No input is permitted to this field, but the data can be removed. The next waypoint and altitude is shown at 3R, with the expected deviation from this required height displayed at 4L. At 4R FPA is actual flight path angle based on current ground speed and rate of descent. V/B is the vertical bearing i.e. the FPA required to achieve the required height at the next position, and V/S is the actual rate of descent. Access to associated descent pages is gained at 5R, 6L and 6R.

Principle Of Operation - Twin IRS, Twin FMC

In a twin IRS system the left FMC will normally receive information from the left IRS and the right FMC from the right IRS. The systems compare the IRS positions but if there is a discrepancy, they cannot determine in isolation which system is in error. The FMC must have the input of an external reference in order to determine the correct position. Using Kalman filtering, the external reference is compared with the IRS positions to determine the system position. At the start of a flight the IRS position will predominate but as the flight progresses, the IRS positions will degrade and the weighting for the external reference will increase, commensurate with the selection of external reference, and the range from that reference.

There are four possible modes of operation of a twin FMS system. In the dual mode, one FMC acts as the master and the other as the slave. The systems independently determine position and the positional information is co-related, to check for gross errors, before being passed to the EFIS. This means that the position presented on the EFIS may differ from that on each CDU. With independent operation, each FMC works in isolation with no communication.

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The information from one of the FMCs will be used to feed the other systems and there will be a difference in position between the two FMCs and between the EFIS and the non-selected FMC. If one FMC is inoperative then the functions can be carried out by the serviceable FMC. If both FMCs are inoperative then IRS information will be used directly in the EFIS but the automatic performance functions will not be available.

Principle Of Operation - Triple IRS, Twin FMC

Positional information and heading from the triple INS/IRS is fed into the FMC where the information is compared to check for any system having gross errors and then averaged. This position may then be compared with an external reference which may be DME/DME, VOR/DME or GNSS. The FMC uses Kalman filtering to produce position and velocity. This filtering may be done purely using the IRS information or using a combination of IRS and external reference.

When operating at latitudes in excess of 84° the FMC will de-couple the IRS with the left FMC using the IRS in the order left, centre, right and the right FMC in the order right, centre, left. Over a short period of time each FMC will change the FMC position to the appropriate IRS position. The reason for the de-coupling is that the calculation of change of longitude from departure is a function of the secant of latitude, which, at values approaching 90°, is increasing rapidly (e.g. sec 86°00 = 14.3356, sec 86°01’ = 14.3955). This means that a small error in latitude will result in a large error in the calculation of change of longitude. This would give an apparent large divergence between the IRS positions in terms of the longitude calculated, although in fact the actual difference in position would be small.

Kalman Filtering

Kalman filtering is the process used within a navigation computer to combine the short term accuracy of the IRS with the long term accuracy of the external reference. The model assesses the velocity and position errors from the IRS by comparing the IRS position with the external reference to produce its own prediction of position and velocity. Initially the IRS information will be the most accurate, but as the ramp effect of IRS errors progresses, the external reference information will become the most accurate. The weighting system applied within the model will initially favour the IRS information but as a flight progresses it will become more biased towards the external reference. Consequently the position will be most accurate after the position update on the runway threshold but will gradually decay to the accuracy of the external reference. The position information will again improve when the aircraft is on final approach using a precision system (ILS or MLS). The more complex the model used (i.e. the more factors are included) the better will be the quality of the system position and velocity.

DME - IRS Accuracy

The position accuracy of the IRS continually degrades throughout the flight, although the heading and ground speed maintain a high degree of accuracy. The measurement of position is subject to random errors which are dependent on the range and the cut of the position lines. The second problem is solved by the computer selecting DMEs positioned so that a good cut will be obtained. Slant range error is compensated for in the calculation, but the DME error is constant at +/-0.25 NM +/-1.25% of range, so at 100 NM the error will a maximum of 1.5 NM. At the start of a flight this error will be large compared with the IRS error, but as the flight progresses the IRS is degrading at around 1 NM/h. After several hours, since the DME error is constant, the DME fixing will be significantly more accurate than the IRS.

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