IRS Drift Example

[Mariano]

Balt,

If the FMC does not update its position on activation of takeoff thrust, our company recommends that we continue with the takeoff and advise ATC once airborne. Apparently, this is preferred to rejecting a takeoff (even though we might be moving at only a few knots by the time we decide to reject), which requires we vacate the runway, address the issue, and get back in line or taxi back to blocks, in addition to filing the required FAA report(s) for any rejected takeoff, no matter the speed at which it was initiated (once takeoff thrust is set, if we reject, it is considered a rejected takeoff, regardless of speed).

Hardy,

Our -200s, do not have the capability to manually select RNP (or to view RNP and ANP - no such line in POS REF 2 page). As long as SIDs or STARs allow IRU/DME/DME sensors, and as long as we have them operational (and VOR receivers set to AUTO), we are allowed to fly them. We are not allowed to fly RNAV approaches, though. Why are we allowed to fly RNAV1 procedures without having the capability of being alerted if navigation performance degrades beyond 2XRNP, I do not fully understand (I am sure the FAA has an explanation ;-). Perhaps someone here knows the answer to this.

Best regards,

Mariano

[Balt]

Thanks Mariano, that's really interesting. I'm assuming that's for FAA ops only? I'm pretty sure in Europe where I did most of my flying, taking off without the SID requirements met and then causing unanticipated yet preventable workload with ATC would have resulted in paperwork (and tea without biscuits with the chief pilot...)

[Mariano]

I agree, Balt. Seems nonsensical. Nevertheless, I have never seen or heard of a case of position not updating when pressing N1/EPR (or TOGA).

That being said, there were cases of crews departing with the autothrottles deferred. Since they were not pressing the MCP N1/EPR button, FMC position never updated and unintended turns took place at 400 ft. AGL.

After a few of these, they added a step on the DDG Operations section for TMC deferred stating that the MCP N1/EPR button must always be pressed.

"The devil is in the details" ;-)

Best regards,

Mariano