dasleben
That's just, like, your opinion, man
The goal is to be safer.
Vertical path non-precision approach safety?
- Thread starter DE727UPS
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z987k
Well-Known Member
Which is why I want to be at MDA before the VDP.
Autothrust Blue
Welcome aboard the Washington State Ferries
If you could arrive at a constant descent rate, stabilized, at VDP, at MDA, you'd not have to touch a single control and you'd likely hit the threshold.
That's the point.
dasleben
That's just, like, your opinion, man
What's the rationale behind that? You're (hopefully) not going to start down until your VDP anyway.
Other risks include the potential for the PF to start down before (or after) the VDP in marginal weather, misjudging sink rates while visually transitioning from level to descending flight, or excessive sink rates during large thrust reductions low to the ground.
z987k
Well-Known Member
Unless of course you have to go missed because you arrived at the MDA that's lets say 2500ft agl and you're trying to find some MIRLs and you're kind of underneath but the visibility is poor so it's hard to tell. Or you could have had the last 3 miles at that altitude to have a good chance of picking a piece of the runway environment out.
I can't say how many times, I've been trucking along at MDA and suddenly.. Ah HA! Very likely that if I would have then continued at MDA to the MAP, we would have to go missed at the MAP, ragged bottoms and all.
So I can have time to look. I want more than 1 second at MDA.
Starting down before the VDP isn't necessarily a bad thing. You'll just need to shallow the descent, but iirc, a VDP doesn't include terrain separation unless you're on the vasi/papi(within 4NM, which since we're non precision, we very well may not be at the VDP) that may or may not even exist.
dasleben
That's just, like, your opinion, man
Yeah man, we used to do all this junk in light aircraft. Works good, lasts long time. Instant power available, very little directional inertia. Get in trouble, you're out of there in a hurry. I know the culture; you don't want to miss an opportunity to get in when you have even a glimmer of a chance. What I described above is "best practice" in something with slower spool time, plenty of inertia, and significant pitching moments with big thrust changes.
Starting down before the VDP is poor form in a long-body aircraft, by the way. Great way to take out approach lights, or worse.
Tommay85
Well-Known Member
Yes, but at the same time, being pitched down(as if you were on an ILS) gives you more of a shot at finding the lights than if you're pitched up holding an MDA. Unless you're coming in quickly and not pitched up, which is doable in a turboprop of course. 130 knots(our approach speed in the metro) is level flight with flaps half. With the peterbuilt semi truck nose, you're not gonna see anything if the vis and ceiling are that low, but on an ILS(or something with vertical guidance in this case), I'd be pitched 5 degrees nose down. Much better chance at finding some lights. The Baron and 99, it didn't matter. Other airplanes, the pitch attitude could make a difference in finding the runway environment. It does in the Metro at least.
In these arthritic piles of junk without approved procedures for vertical guidance on a non-precision, I try to hit MDA at a reasonable distance, but I don't like sitting there for miles personally.
Tommay85
Well-Known Member
What is the spool-up time with a bigger turbine engine? I was under the impression that as long as you're not at flight idle, you'd have plenty of power pretty quickly. I mean, even the PT-6 takes 5 seconds to get anything from flight idle. I don't think @z987k is unfamiliar with slow engine response...
dasleben
That's just, like, your opinion, man
It's generally not bad, but you're also changing the momentum of a lot more airplane in the process.
CF6s have good response. The old Pratt JT-9Ds less so, but still not terrible. Best was the triple-spool RB211. I honestly couldn't give you anything in terms of seconds.
z987k
Well-Known Member
Ya, but its not like anything sporting a pt6 has any inertia whatsoever.
Tommay85
Well-Known Member
Ah yes, I can imagine the inertia issue. Would it really take more than say, 50 feet to change direction though if you really manhandled it?
I'm not arguing or even debating, just curious, but we aren't exactly trying to stop a freight train here you know.
Tommay85
Well-Known Member
You need to get out of that thing and back into something more masculine!
dasleben
That's just, like, your opinion, man
I don't want to overstate it, but it's more an issue of ending up low, slow, and low on thrust with a high descent rate. That's the combination that'll lead to balling up the jet, which is why the industry has moved away from dive and drive.Ah yes, I can imagine the inertia issue. Would it really take more than say, 50 feet to change direction though if you really manhandled it?
I'm not arguing or even debating, just curious, but we aren't exactly trying to stop a freight train here you know.
Tommay85
Well-Known Member
Indeed, I was just curious if you had ever seen the how much altitude it took to suddenly arrest a descent. Not necessarily on an approach, just in general, like maybe an RA or something.
ppragman
FLIPY FLAPS!
I hate to be the bad guy here, but I actually have to agree with the airline jockeys. The truth of it is, with dive and drive you have to adjust pitch and power at least once (at Bottom of Descent) and on some wacky approaches you may have to adjust pitch and power multiple times. I prefer that the only power adjustment I make is the power reduction in the flare or pushing the power up to go missed. I wish we could have the OpSpec and technology to do that in the Navajos, PC-12, and Caravan we fly.
Tommay85
Well-Known Member
Totally. You can kind of plan it in your head and do the same thing, but there's been a couple "Oh crap, I'm WAY high moments".
TUCKnTRUCK
That guy
Seems like most fms / systems, with coupled autopilots will initiate MDA capture 10-20% of the ROD before the altitude, which means you start to level off 130-200 feet prior to MDA. On a constant descent approach, this means you will hit your Calculated or charted VDP before you hit MDA. A high percentage of the time, this means when the crew hits MDA, they are pretty far above glideslope, and if the approach is to minimums they will be high. In systems like the proline 21, this is often incorrectly fixed by the crew using approach and not Nav/Vnav for an MDA based approach. This means at MDA they must either select ALT, or manually take the aircraft. Inadvertently, they then usually treat the MDA based approach like a DH, and execute the missed at the MDA, or hit alt @MDA frequently going below.
The reality is, MDA based approaches need to go away for VNAV capable airplanes. DDA should be published, and standard for anybody flying a Vnav path to an MDA. For those equipped to fly to lnav/Vnav DH, lPV, or RNP approaches it wouldn't matter. For the rest it would turn it into something like an ILS.
The reality is, MDA based approaches need to go away for VNAV capable airplanes. DDA should be published, and standard for anybody flying a Vnav path to an MDA. For those equipped to fly to lnav/Vnav DH, lPV, or RNP approaches it wouldn't matter. For the rest it would turn it into something like an ILS.
Hacker15e
Who am I? Where are my pants?
Oh no! You might have to manipulate those controls that the engineers designed for your hands to be on to make the airplane do something!?
ppragman
FLIPY FLAPS!
So, I have people in the back. It would be nice if they had a constant ride instead of what they get on dive and drive. I get plenty of opportunities to manipulate the controls - especially with some of the places we go to, but the truth is a constant, stabilized, single power setting approach flown like an ILS is much safer than a "dive and drive" approach. On http://155.178.201.160/d-tpp/1309/01225LD16.PDF if I fly so that I hit every step down I level off 5 times on the approach, if I wait to let down so that I stay above all of the step-downs (what I usually do) I only have level off, but it would be nicer to have essentially a fake ILS everywhere I go. That'd be way better. Here's another approach with 4 step-downs: http://155.178.201.160/d-tpp/1309/01191LDAX8.PDF. Here's one with 6 places to level off: http://155.178.201.160/d-tpp/1309/01236LDBC22.PDF. This approach sure would be nice to have a glideslope on: http://155.178.201.160/d-tpp/1309/05414LDAD11.PDF.
The goal isn't to "fly the airplane" as much as I can, I get plenty of practice going into places without approaches with short runways in mountainous terrain. The goal is to provide the safest, most comfortable, least sketchy ride for my passengers. Everything else is filler.
Tommay85
Well-Known Member
Well, in their defense, it is smoother. The private pilot in the back won't accuse you of being a throttle jockey either.