Arming both ap for approach

[7478ti]

Answers to aterpster (WR)

Q1:What was the target level of safety on those early auto-land systems? Were they fail operational?

A1:They all met AC20-57 or equivalent, as well as AC120-28 (as amended, depending on the year certified)... Some were Fail-Op, and Some were Fail-Passive. TLS was a fuzzy contrived (non-FAA) math concept in serious disrepute, for some very valid fundamental conceptual reasons at the time (and in fact still is in disrepute even now!, ...versus using FHAs, FMEAs, and SSAs, to meet the criteria of AC120-28D). Note: the B747 was the first WB jet to have a purpose designed Fail-Op AL to actually be certified (It was a triple channel SPZ-1 bought by a few European airlines - Note: most early US airlines bought the B747 with the dual channel SPZ-1 which only had FP AL) but all early B747 versions still did not have rollout capability, until the "rollout special conditions was signed", and then the rollout feature was added. However the L1011 had a very good Fail-Op AL system (FCS-110) and it preceded the B747 for Cat III Op-Spec approvals. The DC10 system (PB-100), while nominally having an AFM airworthiness approval for "Cat III" back to the early days similar to the L1011, didn't get operationally cleared for Cat III until many years later (~1977 as I recall), when we did the first approvals on AA's DC10s, with the upgraded -30 gains and revised AP boxes. The DC10 AL system wasn't nearly as good of a system as the L1011s AL, or even the B747s. But it wasn't until the B744 that the B747 finally fully caught up to the L1011 for having a reliable Fail-Op AL capability with a significant usable flight envelope, in terms of both robust atmospheric tolerance, and ability to cope with irregular terrain. The A300 AL also had problems along the way, and those issues remained for much of its service life, but it eventually did get cleared (albeit with restrictions) for both Cat IIIa, and later Cat IIIb.

Q2: BTW, I am curious which 737 type was the first 737 to come with two autopilots, and with the equivalent level of safety and operational capability as the early 767s?

A2: The early B737s were optionally provisioned for dual channel FP AL to TD, with both the SP77 and later the SP177. But it wasn't until the control laws and other functions were upgraded in the Classic (B737-300/400/500) to achieve roughly the same basic FP AL (safety and operability) capability to TD as the early B757/B767 LAND2 mode that it could in any way be considered as equivalent. (Note: Even then, the B737 Classic was still NOT Fail-Op at that point, and still did NOT have rollout capability). The B737 didn't actually get a rudder channel, alignment, rollout, and LAND 3 Fail Op capability, until mid-way through the NG series, with the introduction of the new R/C EDFCS AP. Now the B737NG EDFCS AP is a terrific AP, fully capable of reliable Fail-Op, even in horrendous winds and gusts, and irregular terrain, either all engine, or OEI.

Q3: Several years after TWA completed delivery of its 727-231s they placed a follow-on order for ten "advanced" 727s (727-231A). It had a souped up version of the same crummy auto-pilot as the earlier 727s. But, it could do what they jokingly called CAT III (50' DA, 1,000 RVR). It would flare, but not decrab nor did it have rollout guidance. The pilot group basically boycotted that feature.

A3: That B727 version was the dual Channel FP SP-50 Mod Block 4 AP, and SP150 AP. You're correct, in that it didn't have the alignment and rollout capability of the later APs. But it was nonetheless approved for, and safely and successfully flew Cat III FP, with both EAL and DAL (to a 50' HAT DH and eventually RVR700/700/700), and with other airlines too. [Historical note: I was on the very first B727 FP AL Cat III "proof of concept" flight back in the early 70s, ...as we flew multiple approaches to KOKC in N27 (which was specially modified with the recently upgraded AP, in actual Cat III fog, ...with Boeing's Mis Sekajima introducing various pitch and roll hardover and slowover faults into the AP, even during flare (and unknown to the subject pilot during the event), ...as "proof of concept" testing]. It is also noteworthy, that in it's Cat III ops history, the B727 AP certainly wasn't as good as later APs, but in its entire operating experience during that period, there were no reported significant safety events in low visibility conditions related to the AP or AL use per se. However, there were HF related events that occurred, but they typically were due to crews disconnecting the AP, and trying to hand fly to a landing with very marginal visual reference (e.g., the KORD event). I'm well aware of the TWA B727 FP AL controversy, and Larry DeCelles views, as well as Joe Oliver at DAL, and the many others who were advocating for taking various courses of action back in those exciting days! What memories!