What would you do? (Checkride)

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Fly4Fun!

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Fly4Fun!
I have my commercial checkride next week. I have been studying my butt off... I finally feel up to the ACS standards on both the ground and flight portion.

However, I have an issue:

The aircraft I am taking my checkride in is an old Piper Arrow. The AFM only offers one chart for takeoff performance, which is max gross. With a 1.5 margin, at max gross, with temperatures that are most likely for an august afternoon, I am not safely clearing a 50 foot obstacles.

With all that said, the situation I have been provided by my DPE doesn't have us departing at max gross. On top of that, I do not nearly need full tanks.

I know about the 30/70 rule for obstacles, and the 50/70 rule for takeoffs without an obstacle, but those are rules of thumbs to back up know performance numbers.

I believe that the correct answer is that in the interest of safety, only the know performance numbers can be utilized, and that a departure with the given conditions (HOT) is unsafe. Departing bellow max gross will have a resultant increase in performance, however how much of an increase is a guess, which is (in my opinion) an unacceptable level of risk.

Am I looking at this wrong?
 
You're going for commercial. It's not a guess. If you have your actual weight you can do the math.

You can calculate the new distances with all the information you already have. Put that physics class you took that you never said you'd use to work.
 
Fly4Fun! said:
Am I looking at this wrong?
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Yeah.

I have an old arrow with a take-off performance chart that was probably created by Pythagoras himself. If I used the max gross (2,600 lbs) AND a 1.5 fudge factor for performance calcs, I would never take off from some of the airports in the hills (Blue Ridge, Poconos) I go to in the summer. Use actual weights from your w/b calc. Leave some gas behind. Do you have to clear a 50' obstacle at your departure airport? I'm not going for my commercial ride (yet), but think of it as a real world exercise. It's a practical. So use practical calculations, not theoretical ones.
 
As @Arnold indicated, if you don’t have a 50-foot obstacle, you dont need 50-foot-obstacle distance.

more to the point in my mind, is how does your actual takeoff performance compare to the book? Is a 50% safety factor over book numbers enough?
 
EdFred said:
You're going for commercial. It's not a guess. If you have your actual weight you can do the math.

You can calculate the new distances with all the information you already have. Put that physics class you took that you never said you'd use to work.
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Can you fill us in on that? I don’t recall any level of physics being a Commercial certificate prerequisite, nor do I recall that math anywhere in FAA guidance.
 
MauleSkinner said:
Can you fill us in on that? I don’t recall any level of physics being a Commercial certificate prerequisite, nor do I recall that math anywhere in FAA guidance.
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Sure, beyond the FAA learning. You know the takeoff distance and rotation speed for full gross. Based on that, you can determine the time it took to get to that speed. With that information you can determine the average acceleration for the take off run. With that information you can determine the force being applied over the take off run. Now you can plug your less than gross weight number back into the formulas used to determine that. You now have a new acceleration number. But it gets slightly better. Your V speeds also decreases based on the square root of the ratio of current weight to gross weight. So you can also calculate your new rotation speed. Coupled with the new acceleration rate, you can calculate the new take off distance. You can also determine with the new Vx based on current weight the horizontal distance needed to clear the 50' object.

Is it a lot of work? Yeah, maybe, but it shows to the DPE that you actually put some thought into the process.
 
EdFred said:
Sure, beyond the FAA learning. You know the takeoff distance and rotation speed for full gross. Based on that, you can determine the time it took to get to that speed. With that information you can determine the average acceleration for the take off run. With that information you can determine the force being applied over the take off run. Now you can plug your less than gross weight number back into the formulas used to determine that. You now have a new acceleration number. But it gets slightly better. Your V speeds also decreases based on the square root of the ratio of current weight to gross weight. So you can also calculate your new rotation speed. Coupled with the new acceleration rate, you can calculate the new take off distance. You can also determine with the new Vx based on current weight the horizontal distance needed to clear the 50' object.

Is it a lot of work? Yeah, maybe, but it shows to the DPE that you actually put some thought into the process.
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And it’s probably math that less than 5% of pilots will know. So it would be an unreasonable expectation for the overwhelming majority of pilots.
 
MauleSkinner said:
And it’s probably math that less than 5% of pilots will know. So it would be an unreasonable expectation for the overwhelming majority of pilots.
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I didn't say it was required or should be required for the check ride, but it's what *can* be done to determine a takeoff and clearance distance, and doesn't take more than 9th grade math skills. But if someone walked into a checkride with you and dropped that on the table, wouldn't you be like "alright, this person isn't it training to JUST pass the checkride."
 
EdFred said:
You also aren't a DPE. :)
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then you should have specified you were only looking for DPE's to answer. but instead you said "But if someone walked into a checkride with you and dropped that on the table........"

but you're probably the guy that also says "with you" when you change freq's.
 
Playing Dr Piper with fancy math? Untested results with life and limb at stake? Unsuspecting public in the back seats? Yea, DPE is going to love you tenderly.

D169B7AA-B0E0-4B59-8350-A65B7250D220.gif 80067C42-4AF3-4577-B0BF-FA80D4CA774B.gif
 
Average acceleration..?? Is acceleration always linear ?
 
MauleSkinner said:
I’m not a DPE either…I only do ATP and type ratings. If an applicant came to me with that, I’d consider it a total waste of my time and theirs unless they can tell me why it’s acceptable for them to play performance engineer.
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You do the FAA proud. :D
 
eman1200 said:
then you should have specified you were only looking for DPE's to answer. but instead you said "But if someone walked into a checkride with you and dropped that on the table........"

but you're probably the guy that also says "with you" when you change freq's.
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I thought Maule was a DPE and I was quoting and responding to him.
 
This is a "Practical" Test, So you are asking the wrong question. The Question is what would You do if this was a paying customer you were taking up? Of course you are not going to do it if is unsafe or if you are unsure if it is safe. If you are sure it is safe, How are you sure it is safe? Have you practiced Max performance takes offs and know how much runway and obstacle clearance you can consistently need under those conditions with an adequate safety margin? If not, what is your alternate plan?

Practical test question/task often don't have "a Right answer" only "Satisfactory Answers"

Brian
CFIIG/ASEL
 
texasclouds said:
Playing Dr Piper with fancy math? Untested results with life and limb at stake? Unsuspecting public in the back seats? Yea, DPE is going to love you tenderly.
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None of this is untested....basic aerodynamics (ref: Aerodynamics for Naval Aviators) needing nothing more than simple arithmetic. Don't know about you, but I'll bet every student who took ground at a college or university took a basic Aerodynamics course and did this calculation as homework.
 
Fly4Fun! said:
I believe that the correct answer is that in the interest of safety, only the know performance numbers can be utilized, and that a departure with the given conditions (HOT) is unsafe. Departing bellow max gross will have a resultant increase in performance, however how much of an increase is a guess, which is (in my opinion) an unacceptable level of risk.
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FWIW, that's the approach I would take. Good Luck on your flight check!
 
Kritchlow said:
Average acceleration..?? Is acceleration always linear ?
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Unfortunately, most people have never taken Calculus 101 and don't understand that 1st derivative of acceleration is velocity (speed). Constant acceleration means increasing (linear) velocity. Linear acceleration means the velocity is increasing exponentially.

If you're interested (and I know almost no one here is interested) the second derivative of acceleration is the first derivative of velocity which is distance.
 
murphey said:
Unfortunately, most people have never taken Calculus 101 and don't understand that 1st derivative of acceleration is velocity (speed). Constant acceleration means increasing (linear) velocity. Linear acceleration means the velocity is increasing exponentially.

If you're interested (and I know almost no one here is interested) the second derivative of acceleration is the first derivative of velocity which is distance.
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And the first derivative of the volume of a sphere is its surface area. :)
 
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The basic physics that Ed is calculating is found on the circular slide rules that the Airforce issued to pilots for use in whatever plane they were flying. I had one that I used for years, donated it to the College Park Aviation Museum.

I found it reliable for not only takeoff performance, but rate of climb for our C 172's.

Most E 6 B apps have that function too, for any of the modern electronic flight information users who do not have the real device.

My 'antique' devices were gifted to me by a WW 2 pilot, who was no longer flying, and hated the equipment to go to waste.

I used these devices for my Commercial and Instrument check rides, as well as the written exams.

What source do you recommend for the OP? My check ride was in an Arrow, similar to his.
 
geezer said:
The basic physics that Ed is calculating is found on the circular slide rules that the Airforce issued to pilots for use in whatever plane they were flying. I had one that I used for years, donated it to the College Park Aviation Museum.

I found it reliable for not only takeoff performance, but rate of climb for our C 172's.

Most E 6 B apps have that function too, for any of the modern electronic flight information users who do not have the real device.

My 'antique' devices were gifted to me by a WW 2 pilot, who was no longer flying, and hated the equipment to go to waste.
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But you, like Ed, are not going to impart the actual knowledge of how to do it to idiots.
 
murphey said:
Unfortunately, most people have never taken Calculus 101 and don't understand that 1st derivative of acceleration is velocity (speed). [...]
If you're interested (and I know almost no one here is interested) the second derivative of acceleration is the first derivative of velocity which is distance.
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Hmmm. Acceleration is the first derivative of velocity (not the other way around). The first derivative of acceleration is usually called "jerk" (the rate of change of acceleration).

Velocity is the integral of acceleration... and distance would be the second integral of acceleration.
 
woywoyboy said:
Hmmm. Acceleration is the first derivative of velocity (not the other way around). The first derivative of acceleration is usually called "jerk" (the rate of change of acceleration).

Velocity is the integral of acceleration... and distance would be the second integral of acceleration.
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arrggggggg....that tells you how long it's been since I taught Calc.
 

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For the OP…
91.103 says
(b) For any flight, runway lengths at airports of intended use, and the following takeoff and landing distance information:

(1) For civil aircraft for which an approved Airplane or Rotorcraft Flight Manual containing takeoff and landing distance data is required, the takeoff and landing distance data contained therein; and

(2) For civil aircraft other than those specified in paragraph (b)(1) of this section, other reliable information appropriate to the aircraft, relating to aircraft performance under expected values of airport elevation and runway slope, aircraft gross weight, and wind and temperature.
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do you have “other reliable information” that you can apply? Have you done a takeoff under similar conditions and determined how your actual perform compares to the book numbers? That would be the easiest.

Personally, I’m fond of sporty’s Take Off Performance computer…it uses either manufacturer data or your actual takeoff as a baseline to determine takeoff distance for other conditions, and addresses more nonstandard conditions than most light airplane AFM/POHs.
 
The original subject is getting lost in the trivia here.

What is the OP required to do? No 150% rule of thumb is going to get him through his exam with the DPE. Those rules are a waste of performance at sea level, and killer at high altitude and hot days

What source do you suggest, he needs something legal.
 
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