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 Post subject: Re: Citation 501sp
PostPosted: 29 Jul 2021, 21:22 
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You guys are in a lot deeper than I got on any type rating. Are you sure you're not overthinking this? :D Don't trip over the minutia and miss the big stuff. ;)

Mike and I went to the same school. They trained us to overthink stuff. But my dorm had better parties. :woot:

That's probably why Mike is more successful than me in business. I did eventually give up drinking, though.

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 Post subject: Re: Citation 501sp
PostPosted: 29 Jul 2021, 23:43 
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Username Protected wrote:
Mike, I don't think Cessna would make a mistake like that, let alone a mistake that has gone uncorrected for more than 40 years.

It happens. Manuals are not perfect.

Quote:
By the way, where are you getting "120 KIAS at 8500 lbs (per the checklist instructions) is well below 0.6 AOA, somewhere around 0.35 to 0.40"?

Manual says stall speed clean is 79 knots at 8500 pounds.

Manual says best glide speed is 120 knots at 8500 pounds.

Since 0.60 AOA is 1.3 stall speed, then 1.3 times 79 knots is 103 knots, not 120 knots.

So 120 knots has to be lower AOA than 0.60 since it is faster than 1.3 times the stall speed.

So best glide is either 120 knots as written in the emergency procedure, or it is 0.60 AOA (103 knots) as described in the AOA systems description.

The manual is not consistent.

Hint: the correct answer is 120 knots and an AOA of ~0.40 for best glide (furthest distance traveled). An AOA of 0.60 won't get you as far, but it will be close to the maximum endurance, most time aloft.

Mike C.

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 Post subject: Re: Citation 501sp
PostPosted: 30 Jul 2021, 01:56 
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Username Protected wrote:
Mike, I don't think Cessna would make a mistake like that, let alone a mistake that has gone uncorrected for more than 40 years.

It happens. Manuals are not perfect.

Quote:
By the way, where are you getting "120 KIAS at 8500 lbs (per the checklist instructions) is well below 0.6 AOA, somewhere around 0.35 to 0.40"?

Manual says stall speed clean is 79 knots at 8500 pounds.

Manual says best glide speed is 120 knots at 8500 pounds.

Since 0.60 AOA is 1.3 stall speed, then 1.3 times 79 knots is 103 knots, not 120 knots.

So 120 knots has to be lower AOA than 0.60 since it is faster than 1.3 times the stall speed.

So best glide is either 120 knots as written in the emergency procedure, or it is 0.60 AOA (103 knots) as described in the AOA systems description.

The manual is not consistent.

Hint: the correct answer is 120 knots and an AOA of ~0.40 for best glide (furthest distance traveled). An AOA of 0.60 won't get you as far, but it will be close to the maximum endurance, most time aloft.

Mike C.

I still don't see an inconsistency.

Power off glide with zero flaps is not the same situation as approach with flaps extended and carrying power.

The AOA could be reading 0.6 in both cases but the IAS will be different.
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 Post subject: Re: Citation 501sp
PostPosted: 30 Jul 2021, 05:31 
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Username Protected wrote:
You guys are in a lot deeper than I got on any type rating. Are you sure you're not overthinking this? :D Don't trip over the minutia and miss the big stuff. ;)


hey the flight director has to right, just follow it

Andrew


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 Post subject: Re: Citation 501sp
PostPosted: 30 Jul 2021, 07:54 
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Just consider how many pilots have run C-210s out of gas simply by using the POH.


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 Post subject: Re: Citation 501sp
PostPosted: 30 Jul 2021, 09:08 
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Username Protected wrote:
I still don't see an inconsistency.

Which is why such an error can live in the manual so long.

Quote:
Power off glide with zero flaps

120 knots at 8500 lbs with no flaps.

Quote:
is not the same situation as approach with flaps extended and carrying power.

Stall speed I gave was clean at idle at 8500 lbs. No flaps involved.

Quote:
The AOA could be reading 0.6 in both cases but the IAS will be different.

Correct, but not applicable to the discussion since the airplane configuration is the same.

Try one more time:

8500 lbs, flaps up.

Stall is 79 knots (per performance section table).

Best glide is 120 knots (per emergency procedure).

AOA of 0.60 is 1.3 stall speed. Works out to 103 knots.

AOA of 0.60 does NOT match best glide. Best glide occurs at a lower AOA.

Mike C.

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 Post subject: Re: Citation 501sp
PostPosted: 30 Jul 2021, 12:38 
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Username Protected wrote:
Correct, but not applicable to the discussion since the airplane configuration is the same.
Mike C.

It seems to me that if Cessna is saying the AOA gauge reads 0.6 at best approach speed, this is in the approach configuration, i.e. with approach flaps on. In fact, the manual talks about a 0.6 AOA reading in the context of best approach speed being 1.3 times Vso (stall speed in landing configuration), not 1.3 times clean stall speed. Could well be that if you fly an approach at 0.6 AOA with zero flaps you'll be doing 120 KIAS, but that doesn't mean the manual is inconsistent.

In any event, the best thing to do is actually go out and test it. Not sure that would be totally conclusive as perhaps the AOA on the plane is not calibrated to spec, but it might be a starting point.

Or maybe someone from Cessna could be consulted?

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 Post subject: Re: Citation 501sp
PostPosted: 31 Jul 2021, 00:22 
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Username Protected wrote:
Could well be that if you fly an approach at 0.6 AOA with zero flaps you'll be doing 120 KIAS

No, you can't.

AOA and ratio of speed to stall are tied together. 120 knots cannot be 0.60 AOA if stall is 79 knots.

That just the way a wing works.

In level, unaccelerated flight, this equation holds:

AOA = (stall speed / current speed) ^ 2

For an approach at 1.3 Vso:

AOA = ( 1 / 1.3 ) ^ 2 = 0.59 (rounded to 0.6 for Vref)

For a plane that stalls clean at 79 and is going 120:

AOA = (79 / 120 ) ^ 2 = 0.43

Best glide is not 0.60. It just isn't.

Mike C.
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 Post subject: Re: Citation 501sp
PostPosted: 31 Jul 2021, 01:23 
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Username Protected wrote:

AOA = (79 / 120 ) ^ 2 = 0.43

Best glide is not 0.60. It just isn't.

Mike C.

According to your formula, it's 0.43.

But what if the AOA gauge in the Citation is not calibrated according to your formula?

I'm going with what the manual says. If the AOA gauge is calibrated in potatoes and the manual says best L/D is at six potatoes, in case of dual flame-out, guess what? I'm flying at six potatoes.

I do appreciate your raising this point in the interest of safety. If I get to fly in my friend's 501 again, I'll see if we can test it.

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 Post subject: Re: Citation 501sp
PostPosted: 31 Jul 2021, 01:33 
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Username Protected wrote:
But what if the AOA gauge in the Citation is not calibrated according to your formula?

That would mean 0.6 isn't Vref.

So that doesn't make sense. Cessna didn't invent a new meaning for the AOA fraction. 0 is no lift, 1 is stall, that's the scale.

Quote:
I'm going with what the manual says. If the AOA gauge is calibrated in potatoes and the manual says best L/D is at six potatoes, in case of dual flame-out, guess what? I'm flying at six potatoes.

And you will be short of where you could have landed.

Mike C.

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 Post subject: Re: Citation 501sp
PostPosted: 31 Jul 2021, 12:04 
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Username Protected wrote:
You guys are in a lot deeper than I got on any type rating. Are you sure you're not overthinking this? :D Don't trip over the minutia and miss the big stuff. ;)

Mike and I went to the same school. They trained us to overthink stuff. But my dorm had better parties. :woot:

That's probably why Mike is more successful than me in business. I did eventually give up drinking, though.


I understand the relatively recent emphasis on AOA's by FAA. I went through Flight Safety for Lears and Citations, but a long time ago, and AOA's were not in the training programs.
.
I operated a Lear 24 from a 3,000 dirt strip, and never felt that an AOA was any help. None of the Douglas and Boeings I flew had AOA's.

And God forbid that anybody should need to know what the best glide AOA is in a multi-engine turbojet. ;)

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 Post subject: Re: Citation 501sp
PostPosted: 31 Jul 2021, 12:13 
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Username Protected wrote:
And God forbid that anybody should need to know what the best glide AOA is in a multi-engine turbojet.

Ask Sully about that.

Or the guy in the Citation who got DEF'ed.

It happens.

Mike C.

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 Post subject: Re: Citation 501sp
PostPosted: 31 Jul 2021, 12:20 
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Username Protected wrote:
And God forbid that anybody should need to know what the best glide AOA is in a multi-engine turbojet.

Ask Sully about that.

Or the guy in the Citation who got DEF'ed.

It happens.

Mike C.


If you can store that kind of minutia in your bean, and still know everything else your supposed to know, more power to ya. If there's anybody who wants to build an airplane in ground school, it would be an engineer. :D

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 Post subject: Re: Citation 501sp
PostPosted: 31 Jul 2021, 12:24 
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Username Protected wrote:
If you can store that kind of minutia in your bean

It is in the manual, Paul. All engine out emergency procedure.

The manual presumes very forgetful pilots given the tiny amount of memory items it requires.

And pilots often forget the memory items even so.

Mike C.

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 Post subject: Re: Citation 501sp
PostPosted: 31 Jul 2021, 19:04 
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Username Protected wrote:
I understand the relatively recent emphasis on AOA's by FAA. I went through Flight Safety for Lears and Citations, but a long time ago, and AOA's were not in the training programs.
.
I operated a Lear 24 from a 3,000 dirt strip, and never felt that an AOA was any help. None of the Douglas and Boeings I flew had AOA's.

And God forbid that anybody should need to know what the best glide AOA is in a multi-engine turbojet. ;)

Hi Paul,

Trying to minimize what I have to store in my brain was my whole reason for this exercise. The manual says best glide is 120KIAS @8,500lbs + 3KIAS for every 500lbs over 8,500. If I am be-bopping along in cruise, and both engines quit, I like the idea of only having to look at the AoA gauge and pitch for a single value, regardless of current weight. It saves me from knowing my weight at all times (though maybe i should be constantly calculating this while flying?) and having to calculated a glide speed during a very stressful time.

I also like the heads-up AoA indicator that is on the dash. It really helps me keep my head out of the cockpit on short final.


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