B737 Classic NPA - MCP altitude setting policies

[Centaurus]

Scenario: VOR or NDB instrument final approach. Let's assume three or four "Not Below" hard altitudes between top of descent on this Jepps and MDA. Final to sea level airport starts at (say) 10 miles and 3000 ft. The Jepps chart conveniently publishes a three degree slope in a table form of distance and height for each mile of descent which keeps you above the step down altitudes. This table is designed to enable a stable approach slope rather than dive and drive technique.

Question: Once top of descent is reached should the MDA be set in the MCP and the the aircraft kept on the DME/altitude slope -OR -should each hard (not below) altitudes be set in the MCP altitude selector and the next altiutde wound in after passing the last. Assume autopilot and autothrottle engaged throughout. The B737 FCTM recommends the latter even though this requires greater workload and possibility of increased autothrottle activity.

Providing the charted DME/altitude table giving approx three degrees is above any step down altitude, then presumably this is just as safe in keeping you above limiting heights? With more or less continuous fiddling with MCP altitude selector passing each limiting step - one could argue there is more chance of finger (literally) trouble than setting the MDA and keeping on the published slope using VS mode? Request comments.

[Denti]

Historically we allways had the SOP once in ALT HLD at the top of descen of the procedure to dial in a different altitude and then use VS to descent on a constant "glidepath" until MDA while checking all altitudes on the way down and certainly obeying all "hard" altitudes. Once 500' below missed approach altitude that was set.

Nowadays even on the classic we use VNAV or VNAV guidance to fly NPAs. Once in ALT HLD at the top of descent of the procedure we set MDA and descent on VNAV path. 500' above MDA we set the missed approach altitude and at 50' prior MDA either initiate the GA or decide to land.

Altitudes are checked on the way down and hard altitudes are observed, of course they are build into the FMC-Waypoints used to fly the LNAV/VNAV Procedure. On LIDO charts those are depicted in a very easy to see manner and therefore easy to crosscheck.

[Rainboe]

By fiddling with the MCP altitude setting minor corrections, aren't you going to keep capturing? At that stage, I would be in Alt Hold approaching TOD for the final approach, then reset to at least 1000' above current altitude (say 4000' for 3000') to allow VS to become active. Approaching .3 to descent point, wind VS 700fpm, and get next position and altitude calls, not forgetting to reset correct go-around altitude, then simply adjust VS to half groundspeed and check high/low at each position down to MDA.

[Nightrider]

Rainboe, this contradicts any training or manual I have seen for years. Not even a SOP will show your procedure, unless it is from some historic outfit with no updating to the real world.

An NPA should be flown in a CDA-style procedure. Being on the platform height the ALT SEL should be set to MDA, rounded up to next available setting. Using VNAV or V/S (preferred, with the GS matching rate), reset the ALT SEL to G/A altitude and decide about continuation or G/A at the MDA.
Continue the approach path until reaching the MAP.

[Rainboe]

Let's assume three or four "Not Below" hard altitudes between top of descent on this Jepps and MDA.Am I missing something here? We are talking about a simple NPA using guide altitudes, not hard altitudes on the descent? The only hard altitude is MDA (and crossing beacon altitudes) unless you have sufficient visual cues at MDA. Therefore the procedure I outlined is standard for NPAs in British Airways and present airline. You are flying this in a CDA, adjusting your ROD s required to try and cross the crossing altitudes at the correct altitude to reach MDA at the right place on the correct GS. What is wrong? You do not want to capture MDA- it will mess up your approach if you go visual and you are captured, and if you have to go around, there is a mad scramble to set GA altitude. In fact, in all my career, I have never-ever set MDA as capture altitude. You want GA altitude set at this stage.
Not even a SOP will show your procedure, unless it is from some historic outfit with no updating to the real world.= Please! Standard BA NPA procedure, and every simulator check involving Alteon and Gecat (Oxford Aviation) instructors! This is standard UK NPA approach procedure! I am perplexed.

Question to you Nightrider- a lot of approaches have check altitudes every mile of DME. Are you going to set the Alt Sel for every one? I would so love to sit in on it!

[EMIT]

Centaurus,

The answer to your question will have to be found in your company SOP's.
Some outfits may set the MDA in the altitude window, others will set the missed approach altitude in the window, and have the MDA only bugged on the altimeter.
In this forum posters can only advise you how it is done in their company, we will now nothing about your company SOP's.

[Nightrider]

Rainboe: I am perplexed.
Me too actually. Being on the 737 since many years for several companies (as a contractor), it was and is always "Set MDA"; approaching MDA it is "AP disconnect, Speed off, FD off, set G/A altitude xxxxft, FD on".
As a matter of fact, most companies I worked for had a mandatory go-around policy in case of level off (alt capt) at MDA.


Rainboe: Question to you Nightrider- a lot of approaches have check altitudes every mile of DME. Are you going to set the Alt Sel for every one? I would so love to sit in on it!
No, only if hard altitudes are involved, like in stepped approaches into mountaineous places. And I completely agree, these check-altitudes are only to check and adjust the vertical speed to comply with the required descend profile.

Emit: In this forum posters can only advise you how it is done in their company, we will now nothing about your company SOP's.
Very true as we see here.

[Rainboe]

approaching MDA it is "AP disconnect, Speed off, FD off, set G/A altitude xxxxft, FD on".
Well that's an awful lot of faffing around approaching MDA! I'm normally giving my all to attitude, tracking, altitude, speed, desperately trying not to dip below MDA. I don't want to be involved with someone setting Alt Sel at that stage on me, tooling around with my FDs off and on, or Speed button! I'll stick to the only way I've been taught, as I described, and practiced by all the instructors I've known in the UK, and by BA itself and my current employer! Hardly '...some historic outfit with no updating to the real world.'!