Non-precision approaches and descent planning

  • Thread starter Thread starter Roger, Roger
  • Start date Start date
Roger said:
The problem with autopilot is...most of our planes don't have it. The autopilots that are installed (KrAP 140) are a) not authorized for single-engine operations and b) not authorized to do altitude captures below 800' AGL.
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Are your KAP140s installed in something other than C-172S aircraft? I'm assuming a twin, because you're talking about single engine ops?

The 800 AGL limitation does not apply during instrument approach operations in the 172S I fly. You can have it engaged as low as 200 AGL during approaches.
 
jrh said:
Are your KAP140s installed in something other than C-172S aircraft? I'm assuming a twin, because you're talking about single engine ops?

The 800 AGL limitation does not apply during instrument approach operations in the 172S I fly. You can have it engaged as low as 200 AGL during approaches.
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DA-42. Just to be clear...autopilot OPERATION is allowed down to 200' AGL, but altitude captures (as suggested in OA's post) are prohibited below 800' AGL

DA-42 AFM Autopilot Supplement said:
6. Altitude Select captures below 800 feet AGL are prohibited.
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Back to the OP
Roger said:
...You are being vectored at 3000'. ATC says, "fly heading 340, maintain 2400 'til established, cleared VOR-DME 31". Rather than immediately descending to 2400', then descending to 1700 when established on the FAC, you would maintain 3000' until about 2.5 minutes from the FAF (1300 feet of descending at 500 fpm), aiming to cross the FAF exactly at 1700 and continue to MDA without leveling off. If it helps at all, here is the approach chart I'm thinking of (assume the vector takes you in between KAPPS and ARGEN).
http://204.108.4.16/d-tpp/0904/00807VDT31.PDF
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I don't see any benefit in delaying the descent. Assume you are solid IMC at 3000' and ceilings are 2500'. Would it not be better to descent into VMC at 2400' as soon as cleared to do so?

Additionally, if you follow the steady descent procedure, not only are you increasing the workload of the pilot, you are reducing the chances of encountering VMC. If you were to descend to the MDA as per the approach, then you have at minimum, additional seconds to see the runway environment and continue the approach VMC.

Its been a long time since I've done instrument work, but am I wrong in remembering that if I can see the runway environment any time I'm on the approach, I can manuever as needed to land as long as I maintain sight of the runway environment? i.e. if flyiing at MDA and I see the threshold lights, I can then descend below MDA as long as I maintain visual w/ those lights? If I lose sight of the environment, then I must go missed immediately, correct? (I'm trying to get back into the IFR mentality)
 
escapepilot said:
Additionally, if you follow the steady descent procedure, not only are you increasing the workload of the pilot,
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How do you figure? Constant rate descent along a course/radial down to an altitude. Pretty basic instrument flying.

you are reducing the chances of encountering VMC.
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How do you figure? You're either going to break out at or before reaching MDA or you aren't. Dive and drive doesn't change the chances of that happening.

If you were to descend to the MDA as per the approach, then you have at minimum, additional seconds to see the runway environment and continue the approach VMC.
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...and additional time to re-enter IMC.

Its been a long time since I've done instrument work, but am I wrong in remembering that if I can see the runway environment any time I'm on the approach, I can manuever as needed to land as long as I maintain sight of the runway environment? i.e. if flyiing at MDA and I see the threshold lights, I can then descend below MDA as long as I maintain visual w/ those lights? If I lose sight of the environment, then I must go missed immediately, correct? (I'm trying to get back into the IFR mentality)
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Incorrect.

You need to constantly be in a position to descend using normal maneuvers and land in the touchdown zone (it's only required to land in the TDZ for air carriers, but a good idea regardless). You can't just pop down out of MDA because you can see the lights/runway.

-mini
 
escapepilot said:
Back to the OP
I don't see any benefit in delaying the descent. Assume you are solid IMC at 3000' and ceilings are 2500'. Would it not be better to descent into VMC at 2400' as soon as cleared to do so?
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Conversely if we can assume we are in solid IMC at 3000' with VFR below assume it is solid IMC to minimums with a temperature inversion. You might want to hold off your descent to have minimal icing exposure. Maybe lower altitudes that day have been turbulent and for passenger comfort you would rather a straight shot down from altitude through the lower levels.

As I mentioned earlier, both methods are 100 percent safe and as long as they are within regulation there is no reason to say you cannot do one or the other. I would agree if there was some safety factor involved, but I cannot come up with any. All I see is options in this situation, options that should be taught to your student and explained how they can be utilized.
 
:yeahthat: Good point. I think the one constant in aviation is that nothing works 100% of the time and we as CFIs should teach students how to deal with the situations.
 
escapepilot said:
:yeahthat: Good point. I think the one constant in aviation is that nothing works 100% of the time and we as CFIs should teach students how to deal with the situations.
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It always depends.

That's the answer to everything in aviation. Nothing is 100%.

Even hitting the ground isn't a 100% probability of kill (pK) :)
 
I had an interesting encounter once making a non-precision approach with a NORDO cubby. Cielings were a ragged 1200ft or so and standing on the ground it looked like rainy VFR to me. Perfectly fine weather for the cub.

Getting to the MDA as early as possible would afford you the best chance of integrating with VFR traffic that might not be expecting you.

Todd "Dive and Drive"
 
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