Is my CFI right about this?

[UAL747400]

So awhile ago I did a dual cross country to Bemidji and back for 414(CFI). I like to use the 3/6 rule for descent planning purposes. If you don't know what that is, it is take your altitude to lose in thousands of feet and multiply that by 3. That gives you the distance out in which to begin your descent. Then you multiply your ground speed by 6 to get your rate of descent. You can reverse this if you have a rediculous ground speed and want to descend slower.

Well I did this and determined that we needed to descend somthing like 15 miles out and at 850fpm or something like that, I don't really remember. The way I like to descend, is to descend at the indicated airspeed I was cruising at, or if I feel like conserving a lot of fuel, descending at Vno at idle. Or just plain descending at idle and taking whatever descent rate I get while at cruise speed.(temp. and smoothness of the air permitting of course). I've only done the last one if I've forgotten to do descent planning, or just having a "lazy" day.

We were at 9500 so I had full power at cruise. I go to pull back the throttle and my instructor tells me to keep it at full throttle otherwise our descent planning will be wrong. HUH!!!!?!?!?!

1) This I don't undestand, we would end up accelerating and THAT in its self would throw everything off would it not? He wanted me to stay at full throttle until the man. press got to 25"(props stay at cruise setting of 2500). I'm flying an Arrow by the way.

2) Then he says that we have a constant speed prop so this won't hurt us. That would hurt us even more would it not? A fix pitch would be less likely to cause us to accelerate, not a constant. With a fix, it will start cavatating at little when you get to higher airspeed. The constant speed one will just keep adjusting pitch to keep taking nice big bites out of the air further causing you to accelerate.

So, I had planned on getting to TPA 5 miles out, entering on a 45 for the downwind and doing a normal traffic pattern to landing. What we end up having to do is make a 360 north of the field and then enter the pattern. His explanation for that was because I let the descent rate slow by a couple hundred feet a few times for a couple seconds. OK, so because I descended at 500fpm for like a minute instead of 700, that resulted in me being 2000 feet high at the point I chose??? Nevermind the fact that we were now going 180 instead of 125 ish.

I can see where he's coming from in the interest of time, that would probably be the quickest way to get in. I only see marginal gains in doing this at the cost of actually throwing your descent planning out the window and in the summer, you're most likely gona have to slow back down anyway because of turbulance.

I've NEVER been taught to descend in that manner. It's always been descend at your cruise speed, or descend with at a reduced powersetting at Vno. The whole thing was so different to me, that I pretty much quit talking and dang near couldn't even fly the airplane it bothered me so much.

My instructor is great and I was just gona shrug this off, because in my mind this is just not right. However, I wanted to see what other people thought about this. I might have forgotten a few things, so ask if you need clarification on anything. Thanks.

[Baronman]

I don't really know what your CFI wants...

But it's good that you're planning your descents...You'd be suprised how many guys (even captains) can't plan a descent without the computer. Not much to it really.

Anyway, in the Arrow I used to take out a couple inches of MP (from where it was at cruise) and begin my descent. I would descend at a higher airspeed than what I was cruising at. Planes are meant to go fast....If that means a higher rate of descent- so be it.

[UAL747400]

Yeah, I probably drug that out a little more than I needed to without getting my point across.

What my instructor was essentially having me do was; after I planned for a descent at cruise speed of like 125 he was having me keep the power in. That resulted in accelerating to 180 knots instead of pulling the power back to descend at my calculated rate of descent and speed.

What he should have done is stopped me during my planning and told me that we're gona descend at Vno or whatever and have me calculate it based on what speed we're about to descend at. Not plan for the GS I'm currently at, and then descent at a higher speed. That obviously would never work in a million years. He basically saying that if I plan for a descent at 125 and descend at 180 instead that it would still work out.

Now that I really think about it, the planning I used to do is plan for 500fpm. Say I'm cruising along at 135indicated with 180gs. So I'd add 11 knots to that, 146(Vno)-135. So plan for a descent at 500fpm at 191 and then set a descent rate of 500fpm and then adjust power to maintain Vno. That's what the POH says to do anyway, except they call for 1000fpm.

I think I tried the 3/6 or 6/3 rule once in multi and kinda stuck with it.

[CaptainChris87]

of course he's a UND CFI due to his oblivious and increased ego, he is ALWAYS right. :P

[UAL747400]

Yeah, I don't know. The reason I'm posting it in here, is he didn't seem to want to discuss it much.

[AngelFuree]

I see what you mean. That's retarded. The descent planning should be used incorporating the descent groundspeed. Keeping the power in results in a higher descent airspeed than planned and an inaccurate descent profile. He probably realized his mistake later on but didn't want to accept it.

[UAL747400]

Yeah, I was just totally thrown for a loop with the whole thing. I don't even really know if I'm describing everything correctly. When it was becoming obvious that we weren't gona make the point I wanted to, he still wasn't letting me reduce the power to make it. We weren't gona make it from the very beginning obviously. Even when it came time to slow to below 129, he still wasn't letting me reduce the power. So here we are SCREAMING into bemidji at 146 knots descending at like 20fpm trying to slow down to 129. We can't reduce that power below 25" and 2500 rpms during a descent dontchya know. I was just sitting there like "WTF?!?!?!?!"

[AngelFuree]

Holy sh t that's bad. Just move on I guess, if you're almost done. At least you know what is right. Seriously man...some instructors will defend their "knowledge" to death for fear of being wrong. The BEST and coolest instructor I had here was for 325. He would always admit when he was wrong, and wouldn't make a fuss about it or try finding excuses and he would always welcome my techniques, ideas, etc. Not only is it good because he's sharing his experience and knowledge, but learning from the students too which is how every instructor should be. Too bad he's not here anymore. At least he's flying for a cargo company now.

[UAL747400]

I almost positive he was trying illustrate something with that, but it was such a pain in the ass way to do it. He should have waited until the next leg to show me a powered descent starting at the proper point and NOT descending at a faster than planned airspeed. That would never work in a million years. Unless you descended faster of course.

[taildragger173]

I usually try to keep the power in for a descent unless I have to pull it back. I do this mainly because of engine wear and tear. Rapid pulls to idle from cruise are hard on an engine and when you start operating high performance airplanes you will have to stage cool your engine. While the Arrow doesnt require stage cooling, keeping cruise power set and slowly backing off the power as the manifold pressure increases is a good way to get in the habit. I also like going fast. You can make up a lot of time in a descent if you plan it correctly. I've been in a Cessna Cardinal going 160 in a descent and passed a Barron...now that was fun. Going fast in a descent often allows me to start descending farther out but at a shallower rate vs. an idle descent.

If I'm flying an airplane with a 130 kt Ground Speed, I may plan my descent for 150 kts and then work out the 3/6 rule for that. When I get to my specified distance, I just push the nose forward and let the airspeed build. As I get closer to the airport and to TPA, I'll start reducing power and then level off without a power increase to bleed off speed. First notch of flaps if you're in an airplane where you can add them before the white arc or I'll drop the gear (in the Cardinal I'll drop flaps 10, then the gear almost simultaneously) and then usually the speed bleeds off pretty fast. At that point I'll set a Downwind power setting which I know will give me my desired speed on downwind.

At the end of the day though, it boils down to technique. There is no right or wrong way to do something like this as long as it is within the specs of the airplane. Try it many different ways so that you can find something that works for you. It will lead to experience which will in turn make you a better CFI.

[TXaviator]

Originally Posted by taildragger173

Try it many different ways so that you can find something that works for you.

Highly illegal. There is ONLY the UND way.

[BuickCFI]

when i fly the arrow i never pull power back unless i get too quick to drop my gear in time for landing. then again i only do decent planning on cross countries, and i haven't flown a planned XC in the arrow for a long time, but i did take it to FAR and back the other week just cuz i could pilotage all the way!

oh, and with a fixed pitch prop if you don't pull power back 2 things can/will happen.
1. you will accelerate
2. you could exceed redline

[UAL747400]

Originally Posted by BuickCFI

oh, and with a fixed pitch prop if you don't pull power back 2 things can/will happen.
1. you will accelerate
2. you could exceed redline

Yeah, I know. I was just temporarily confused.