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When should i descend?

Controlling your aircraft, using the autopilot etc.

Re: When should i descend?

Postby SurferTim » Wed Aug 07, 2019 7:58 pm

Been there. Done that. Aim short.

You have a distinct advantage now compared to when I learned. Nobody is shooting at you.
Fly low, fly slow, land on a dime. Twin Otter.
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Re: When should i descend?

Postby wkitty42 » Thu Aug 08, 2019 4:40 pm

Robertfm wrote in Wed Aug 07, 2019 7:39 pm:I guess the important bit is measuring the distance to start descent that I put in the calculator.

that is a key element, yes :)
"You get more air close to the ground," said Angalo. "I read that in a book. You get lots of air low down, and not much when you go up."
"Why not?" said Gurder.
"Dunno. It's frightened of heights, I guess."
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Re: When should i descend?

Postby Robertfm » Thu Aug 08, 2019 4:59 pm

There is very little margin for error. I'm assuming PHi map is accurate. Just landed and for the last mile had to gain a couple of hundred feet to reach the runway.
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Re: When should i descend?

Postby tdammers » Thu Aug 08, 2019 8:22 pm

Don't plan your descent all the way down to the ground. In a typical GA VFR situation, what you usually do is you plan your descent down to pattern altitude (typically something like 1000-2000 ft AGL), such that you reach this altitude a little bit before reaching the airfield. You then enter the pattern while maintaining altitude, and dial in the right descent profile by turning base at the right moment.

Here's how that works.

If your downwind is spaced correctly, then you will be flying parallel to the runway, opposite the landing direction, at a distance equivalent to one minute of straight flight plus one 180° turn, at pattern altitude. (At first, you will want to do this by timing your pattern entry: e.g., for a midfield crosswind entry, you fly over the airfield perpendicular to the runway, and as you pass the field, you set a timer, fly straight for about 75 seconds, then turn left for your downwind). Now plan your approach. Suppose you're 1500 ft above the runway, and you want to descend at 500 fpm. That means your descent from pattern altitude down to the ground will take 3 minutes. The two 90° turns you need to make will take a total of 1 minute (if you execute them properly, at standard rate), and the base leg itself will take another minute. That's 2 minutes. If you want to fly a short 1-minute final, then you watch the runway, and when you're perpendicular to the threshold, you start a timer, wait 60 seconds, then simultaneously turn base and initiate your descent. That one minute you flew on the downwind will also be one minute on final, plus the 2 minutes for the turns and the base leg, gives you exactly 3 minutes. If you want a longer final approach, you can also extend the downwind further, say 3 minutes; in this case, you would fly the entire base leg and both turns at pattern altitude, and initiate your descent as you turn final. If available, you can also use PAPI lights to guide you down; if you give yourself a sufficiently long final, you can turn final at pattern altitude, which should give you 4 red PAPI lights; initiate descent when you have 2 white / 2 red.

There is plenty of margin for error, but the trick is to use it before you're on short final.

The first margin is when you approach the traffic pattern: if your descent is too fast, no big deal, you just reach pattern altitude a bit before entering the pattern. If you're still a bit too high, just join the pattern, you can always fly another lap to shed altitude.

The second margin is how you time your base leg. You can extend your downwind as needed to give you enough space to get down on the numbers.

The third margin, then, is the throttle lever on final approach. If you end up coming in too low, add more power to reduce sink rate; if you're coming in too high, reduce power.

The final margin is called "go-around". If you come in way too high, and even idling the engine doesn't get you the sink rate you need, just go around - set full throttle, fly runway heading, gain airspeed, retract flaps, climb to pattern altitude, and fly another pattern to try again.
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Re: When should i descend?

Postby Robertfm » Thu Aug 08, 2019 8:38 pm

Thank you.
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Re: When should i descend?

Postby Johan G » Fri Aug 16, 2019 7:33 pm

If you guys like both anecdotes and math, I can recommend a look at "Eddie's" Code7700.com
Both 3:1 and 60:1 are interesting rules of thumb. Also, 1 NM is very close to 6000 ft (about 1 % off).
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Re: When should i descend?

Postby WoodSTokk » Fri Aug 16, 2019 11:10 pm

Captain Joe explain it here:
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Re: When should i descend?

Postby Johan G » Sun Aug 18, 2019 9:13 pm

Oh, that was great! :)
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