How technically does the ISFD help Fail Opertaional ops on NG

[Pin Head]

Additional inertial information but how else does it precisely help the NG for fail operational performance? Does it take the role of that '3rd' autopilot?

Thank you

Pin

[repulo]

The ISFD provides third attitude data to the two autopilots so that in case one attitude data is supplying corrupted or no data the system can still rely on two attitude sources. Also, during normal autopilot operation with the failure of one ADIRU and the old school analog sby att. ind. it is not possible to reangage the autopilot, which is possible having the ISFD, since it is providing digital att. data to the autopilot in use.

[NSEU]

http://www.pprune.org/tech-log/510322-b-737-experts.html

[Pin Head]

Thanks

So the idea of three autopilots of the old LAND 3 terminology is now finished and sent to the history books?

Land 3 = 3 APS
Land 2 = 2 APS

[repulo]

Yes. The 737 never had three A/P. If, during a CATIII approach LAND 2 illuminates, the approach will be continued. There are some internal degradations, but the system still consideres it safe to continue the CATIII.
If it was the other way the MSG NO AUTOLAND would illuminate.

[7478ti]

Note that LAND 3 DOES NOT specifically mean 3 APs. Instead it means "Fail-Operational" capability. Similarly, LAND 2 does NOT specifically mean 2 APs. Instead LAND2 means "Fail-Passive" Capability. Both triple Ch APs and Dual-Dual APs were demonstrated to be capable of "Fail-Op" Cat III, and were approved for fail-op Cat III. The modern B737NG with the newer EDFCS AP has been shown to be equivalent to a triple channel AP, for the purposes of "Fail-Op" LAND 3 capability.

For some types of failures (including on the B737NGs with EDFCS), a Fail-Op AP LAND3 mode may revert to Fail-Passive LAND2 mode, ...and when authorized, some operators may continue an approach following those failure conditions, following a LAND3 to LAND2 mode reversion.

This is because the AP has been shown to meet Fail-Op to Fail Passive reversion acceptability. Accordingly, for some operators and aircraft types, Op-Specs may allow the operator to continue an approach following a LAND3 to LAND2 reversion. However, the minima typically are different (need a higher RVR) when that happens. Fail Op Cat III (using LAND3) typically does not use or need any DA(H), since an "Alert Height" is used to assure AP availability even through rollout. Hence RVR limits are typically lower for Fail-Op systems, down to RVR300 ft. Whereas for Fail-Passive APs and FP Cat III operations, a 50' DA(H) is typically used [a required DH], with more conservative RVR minima typically set, now at about RVR 600 ft. (even though both of these minima are actually considered to be within the Cat IIIb range). Decades ago, when the Fail-Passive APs limits were first set for Cat III (e.g., B727) a 50' DH with RVR 700/700/700 was required, leading to Fail-Passive systems once being commonly identified with limits at Cat IIIa. But that constraint is long obsolete, since the 1980s, and now both LAND3 and LAND2 systems can operate to Cat IIIb minima, albeit LAND3 Fail-Op systems generally have considerably lower RVR limits, currently set at RVR300/300/300 ft.

When use of an AIII mode HUD and LAND2 Autoland are combined by an operator, to achieve an equivalent Fail-Op hybrid system, other minima may apply (e.g., minima credit using a Hybrid Fail-Op System).

See FAA AC120-28D for more details.

[Denti]

We used no DH with 75m RVR for LAND 3 and 50ft DH with 125/75/75m RVR for LAND 2 with both engines working. In the OEI case, even though the indication remained at LAND 3, we had to use 50ft DH with 200/125/75m RVR.