Takeoff Distances For Weight Less than that specified in POH Chart

15 Answers

1. 
   

   
   

   Lucas on Nov 02, 2013

   Dear bwoodjamin
   You can extrapolate by doing the proportion method and you will find a value that is slightly higher than the one actually required for takeoff. Using higher values (worst case scenario values) is always good practice. First because of the increased safety margin and second because most trainers are not exactly new and their performance will always be slightly decreased.
   Lets say the takeoff distance at 2300 LBS is 1300 feet and we would like to calculate the distance for a weight of 2100 LBS. We can say that 2300 : 1300 = 2100 : x and therefore x = 1300 * 2100 / 2300 or x = 1186.95.
   This value is actually higher than the actual required takeoff distance of the 1070 feet shown by an N model 172 POH that you can find at:

   http://alameda-aero.com/wp-content/uploads/2013/05/Cessna172NPOH.pdf

   So again by using the proportion method you are being conservative.

   By the way the link above is for the full POH of an N model 172. If you need a different model (say a P model) you will want to google: Cessna 172P POH pdf and you will find plenty.

   Now, if the values you are trying to find are for the actual private pilot written exam, that will not work as the written requires extreme precision. On the other hand the written exam would not ask such a question. They have plenty of interpolation questions but they would ask you the value at 2100 and the chart would give you the value at 2000 and 2200.
   For example 1100 at 2000 and 1220 at 2200. All you would need to do here is divide the difference by two and add it to 1100 to get an answer of 1160.

   Finally if you need more help passing the knowledge test consider downloading our free demo at:

   http://passfaaexams.com/free-pvt-written-demo-download/

   This will give you an idea of what our courses can do for you and whether you like our teaching methods.
   The full course is a thirteen hour interactive ground school that will help you pass the knowledge test but also impart a great deal of information on everyday flying topics and is a perfect companion for a self-studying aspiring pilot.

   Cheers
   Lucas

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2. 
   

   
   

   Lucas on Nov 02, 2013

   Dear bwoodjamin
   You can extrapolate by doing the proportion method and you will find a value that is slightly higher than the one actually required to takeoff. Using higher values (worst case scenario values) is always good practice First because of the increased safety margin and second because most trainers are not exactly new and their performance will always be slightly decreased.
   Lets say the takeoff distance at 2300 LBS is 1300 feet and we would like to calculate the distance for a weight of 2100 LBS. We can say that 2300 : 1300 = 2100 : x and therefore x = 1300 * 2100 / 2300 or x = 1186.95.
   This value is actually higher than the actual required takeoff distance of the 1070 feet shown by an N model 172 POH.

   So again by using the proportion method you are being conservative.

   By the way If you need a different model (say a P model) you will want to google: Cessna 172P POH pdf and you will find plenty.

   Now, if the values you are trying to find are for the actual private pilot written exam, that will not work as the written requires extreme precision. On the other hand the written exam would not ask such a question. They have plenty of interpolation questions but they would ask you the value at 2100 and the chart would give you the value at 2000 and 2200.
   For example 1100 at 2000 and 1220 at 2200. All you would need to do here is divide the difference by two and add it to 1100 to get an answer of 1160.

   Finally if you need more help passing the knowledge test consider downloading our free demo at:

   http://passfaaexams.com/free-pvt-written-demo-download/

   This will give you an idea of what our courses can do for you and whether you like our teaching methods.
   The full course is a thirteen hour interactive ground school that will help you pass the knowledge test but also impart a great deal of information on everyday flying topics and is a perfect companion for a self-studying aspiring pilot.

   Cheers
   Lucas

   0 Votes 1 Votes  1 Votes

3. 
   

   
   

   Kris Kortokrax on Nov 03, 2013

   The effects of changing the gross weight are not described by a linear function.

   If you want a much better approximation, go to the Pilot’s Handbook of Aeronautical Knowledge, pages 10-13 and 10-14. There you will find that a 10 percent change in takeoff weight will result in about a 21 percent change in takeoff distance. You will also find the rationale for the approximation.

   Compare the approximation against the numbers in the 172N POH and you will find them quite close.

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4. 
   

   
   

   Brian on Nov 05, 2013

   “The effects of changing the gross weight are not described by a linear function.”

   This. I’m not sure the formula he’s using either; perhaps it’s being confused with the formula used to correct Vspeeds for weight? That formula is: Vspeed corrected for weight = Sqrt (Current weight / Gross weight)

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5. 
   

   
   

   Lucas on Nov 05, 2013

   “You can extrapolate by doing the proportion method and you will find a value that is slightly higher than the one actually required to takeoff.”

   Ok so this guy is trying to self study to pass the Private Pilot knowledge test. He is not applying to NASA.

   Next I can imagine he should read and fully understand “Aerodynamics for Naval Aviators” or he just wont cut it for the test, will he?

   I always try to aim my answers at the students level and at the question itself rather than show I know more than the other instructors that come to this site.

   I said the value would be slightly higher and if he uses the worst case scenario he will be safe for the actual flying and for the test I explained the written will not ask such questions, only interpolation questions. There is no simpler answer to his question.

   Over the years I have had plenty of students, coming from other flight schools or clubs, ready to give up because they thought becoming a Pilot was just too hard. After a time it became apparent what had happened: they were not bad students, they just had bad teachers.

   After some simple down to earth explanations they decided to continue flying and still do so safely today. I am pretty sure that they’re understanding of aerodynamics is still the same as it was when they passed the PPL and that did not require them to know that the weight to T/O distance is not described by a linear function. They do know how to calculate the take-off distance and also how to add a good margin to stay on the safe side and at the end thats all that matters to examiners such as myself.

   Cheers

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6. 
   

   
   

   Brian on Nov 05, 2013

   “I always try to aim my answers at the students level”

   I thought you were here to freely advertise your site. Perhaps I’m mistaken though.

   Learning to fly by learning to ‘pass a test’ using as many short cuts (regardless of their accuracy) as necessary is probably a good reason why accident rates have taken no major change in 70 years. This mantra of, ‘oh it doesn’t matter if it’s right, it only matters if it’s good enough to pass the test’ is the problem. Here, let me sum up your post while removing the inaccuracies:

   —

   Dear OP,

   Interpolate. By the way, check out my website….

   Cheers
   Lucas

   +1 Votes 1 Votes  0 Votes

7. 
   

   
   

   Kris Kortokrax on Nov 06, 2013

   “Ok so this guy is trying to self study to pass the Private Pilot knowledge test. He is not applying to NASA.”

   His question had nothing to do with passing the knowledge test. As you said, that question would not appear on the knowledge test. He was looking for a rule of thumb to estimate takeoff distance for a weight other than gross weight.

   “Next I can imagine he should read and fully understand “Aerodynamics for Naval Aviators” or he just wont cut it for the test, will he?”

   Why must everything be geared to passing a test? Do you believe that the poster is incapable of reading and understanding Aerodynamics for Naval Aviators? The general answer is indeed found in Aerodynamics for Naval Aviators.

   “I always try to aim my answers at the students level and at the question itself rather than show I know more than the other instructors that come to this site.”

   Again, do you believe that the people asking questions on this site are stupid and cannot understand anything but the most simplistic answer, which you must provide? Just because someone corrects wrong information which you have provided does not mean that they are trying to show that they know more than you (although they might). We are more concerned with providing the correct information than trying to safeguard your self-esteem.

   “I said the value would be slightly higher and if he uses the worst case scenario he will be safe for the actual flying and for the test I explained the written will not ask such questions, only interpolation questions. There is no simpler answer to his question.”

   Your approximation (which provides an incorrect answer that is higher than the runway length required) only works if one is estimating distance for a weight less than max gross. If one were to obtain a special flight permit to ferry an airplane across the Atlantic and used your proportion method to estimate takeoff distance for a weight greater than max gross, his estimate would be less than the distance actually required.

   Your priority seems to be providing a simple answer as opposed to the correct answer. Looking at Aerodynamics for Naval Aviators one can derive a simple general formula for estimating takeoff distance. Using the numbers from your example, 2100 / 2300 = .91304.
   .91304 squared = .83364. .83364 X 1300 = 1084. The only step added to your proportion formula is to square the result of the weight difference, which is then used to obtain a much closer estimate. This correlates with PHAK’s 10% weight change resulting in a 21% distance change. This does not require extraordinary intelligence to comprehend and use.

   “They do know how to calculate the take-off distance and also how to add a good margin to stay on the safe side and at the end thats all that matters to examiners such as myself.”

   They do not know how to calculate the correct distance if they are using your proportion method. You could simplify things further by just having them use the gross weight distance. That would have a bigger margin of safety.

   I looked for the names of examiners on the FAA website for FSDO’s surrounding your business address on your website. I did not see your name. For what kinds of practical tests are you authorized by the FAA as an examiner?

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8. 
   

   
   

   Lucas on Nov 06, 2013

   Wow this is seriously pathetic.
   So now this student pilot is going to ferry a plane across the ocean overweight, without a license and without asking advise from a fellow pilot who has done this before.
   Are you for real?

   “hello, I am self-studying for the private pilot written”
   What part of this question does not ask advise on passing the written?

   Just because I am part of a company that helps people pass their written tests with simple solutions, rather than insanely complicated answers I don’t think you guys should slander us. If a student asks me specifically the exact formula for calculating the T/O distance in a precise configuration, because his goal is to fully understand the concept, I will spend the time with him, but if he asks for advise on how to pass the written “hello, I am self-studying for the private pilot written” (AGAIN) the answer I provided is the only one I will ever give. Once he passes the written we can move on to more serious matters.

   For your other question I have been a 141 chief pilot since 2002 first in florida, then in NJ and now I go where I am needed. I am authorized to conduct private, Instrument, commercial, CFI (also Initial), CFII and MEI.

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9. 
   

   
   

   Kris Kortokrax on Nov 06, 2013

   Nowhere did I suggest that a student would be ferrying an airplane across the ocean. What I said was that if a pilot ferrying an overgross airplane (one with extra fuel tanks legally installed and approved by the FAA with a special flight permit) relied upon your method to determine the required takeoff distance, his estimate would come up short.

   Your definition of slander appears to be that an individual disagrees with you. Webster defines slander as “to make a false spoken statement that causes people to have a bad opinion of someone.” The statement I made is not false. Your estimation of takeoff distance is wrong.

   The “insanely complicated” correction merely involves squaring the difference between the current weight and the max gross weight, prior to multiplying it by the distance required at gross weight. Why would you eschew that simple step and cling to a method which produces an incorrect answer?

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10. 
    

    
    

    Mark Kolber on Nov 07, 2013

    bwoodjamin, which model 172 are you using?

    As I recall, at least since the P model, the 172 POH has included takeoff distances for multiple weights. For example, in sequential pages, the 172S POH has takeoff distance tables for 2550, 2400 and 2200 lbs. You can interpolate between them just as you would for the altitude and temperature parameters within a chart. (I’m assuming you know how to interpolate since it’s required for a number of questions)

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11. 
    

    
    

    bwoodjamin on Nov 07, 2013

    I am using a digital copy my instructor provided me with for a 172R. It only has the Short Field Takeoff at 2450.
    I do understand the interpolation method, and I am perfectly capable of taking in information with a critical and pragmatic approach. Any feedback and opinion only serves to make me a better pilot in the long run.

    Safety of flight comes first, and that means that I should be as well informed and capable as possible. My goals are not to just pass the tests, but to excell in what I do along my chosen career path! Perhapse I should have given yall a more clear introduction instead of just throwing the question up there.

    Now, I have a long way to go to get there, and find myself presently stuck on the ground due to lack of funds. I figure I can learn quite a bit and build my knowledge just as much by studying and asking the hard questions, not simply trying to find the bareboned answers. I want to be challenged, and stretch my mind as much as possible.

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12. 
    

    
    

    bwoodjamin on Nov 07, 2013

    One more thing as a note of background.. about 9 years ago I took a course at a local community college and passed the written exam. Im building off of what I recall from that time, and using the old textbook as a study-aid along with the FAR/AIM, and whatever other resources i have available to me. ooooh! I just found the PHAK this thing gives alot more info than my textbook. awesome! thanks Chris Kortorax

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13. 
    

    
    

    Brian on Nov 07, 2013

    “I just found the PHAK this thing gives alot more info than my textbook.”

    The FAA offers most of it’s textbooks online. The PHAK is a good one and Airplane Flying Handbook (AFH). For details on specific areas try these:

    Aviation Weather Services: AC 00-45G – http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/6a07efaa9a68c1a8862576c70052e9e5/$FILE/AC-0045G.pdf

    Aerodynamics for Naval Aviators: http://www.faa.gov/regulations_policies/handbooks_manuals/aviation/media/00-80T-80.pdf

    The PHAK and AFH will be your primary source of information. However, these two sources will give you detailed information about specifics you might wish to know on specifics of weather and dynamics. They are pretty boring reads otherwise, but their tables of contents and index’s are quite robust and, as such, make great reference material.

    Good luck in your readings! It will pay off in the end. As an aside, you might consider the option of flying remote controlled aircraft. They fly the same except for the power to weight ratio difference. I learned on these first and found the concepts transitioned quite well into my flight training.

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14. 
    

    
    

    bwoodjamin on Nov 08, 2013

    Thanks for those links Brian, i will definitely use those, And its interesting that you mentioned flying RC. In Highschool I built and flew several gliders, and in fact my major physics project involved flying them and performing calculations of lift and forces of flight, Glideslope angle and stuff.. I know alot of it must have stuck.. cg and balance for example are inherent in building these gliders. I remember the first time I was forced to actually throw this thing I spent hours building and how it suddenly ballooned up into the air because the CG was placed too far aft… The resulting oscilation as the planes inherent stability corrected for it etc.. I can definitely relay much of the information I learned from doing that into training now.

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15. 
    

    
    

    Brian on Nov 08, 2013

    Well, if the physics is interesting to you have a look at the book, The Illustrated Guide to Aerodynamics. It’s a wonderful book that really helps you understand the dynamics of an airplane in simple english. If any one topic in the book excites you then reference it in AFNA (linked in my first post) and you can get a bit more about the analytical (math) side of the picture.

    Keep in mind, AFNA only deals with simplified calc equations in the form of algebra. If you enjoy deep analytical texts let me know. I can offer a few to get you started. These aren’t needed to learn to fly, of course, but if it interests you it can be fun to read.

    Also NASA has quite a robust aerodynamic index. You can have a look at it here: http://www.evolutionaryflighttraining.com/resources/aerodynamics — This is my own personal site where I pulled in the information they provide; the second row of tabs at the top offer you various courses to read through if you so desire. All of these are simply pulled from grc.nasa.gov however.

    Enjoy, I look forward to your future questions! 🙂

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