Experienced Pilots: Do you always calculate takeoff distance?

Discussion in 'Pilot Training' started by Chrisgoesflying, Jan 11, 2021.

  1. Chrisgoesflying

    Chrisgoesflying Pre-Flight

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    If you fly out of an airport with a long runway, let's say 6,000 feet and you're flying your standard Cessna 172 (or similar), do you actually calculate your takeoff distance prior to takeoff or do you just up an go (after doing your pre-flight inspection and run-up obviously)?

    To follow up on that, if you do calculate it, and let's say based on your calculation you should have a takeoff roll of about 1,500 feet, would you simply use that as your abort point, basically saying if you're not in the air by 1,500 feet down the runway, you abort the takeoff since you still have 4,500 feet remaining? Do you add some sort of padding to the number (in case the wind has changed suddenly for example)?

    If you don't calculate it, what do you use as your abort point and what do you look out for to make that decision? The 50/70 rule on such a long runway really doesn't make any sense to me - I mean, if it takes you 3,000 feet to reach 70% of your Vr, something is majorly wrong, yet based on that rule, you'd still take off.

    This summer, I had an issue on takeoff and I'm wondering if I could have avoided being in that situation. I fly out of a long runway with a small two seater that is usually up in the air within about 1,000 feet. One hot and humid afternoon in August, I went flying at max T/O weight (which I rarely do, but that day I took someone who is on the heavier side). I did my run-up, all seemed normal. Lined up on the runway, advanced the throttle, rpm was right about at static max rpm, I rolled down the runway, feeling a little sluggish but airspeed came alive, Vr came along further down the runway than it normally would, but I chalked it off to being hot, humid and at max weight. Anyway, once in the air, I quickly noticed climb rate is really bad, I glanced at the rpm and it was about 300 below what it should be. I managed to still climb out to about 400 feet and hold altitude with the power I had and ended up landing on one of the airport's runways without an issue.

    Long story short, I had a stuck valve and couldn't make full power. How could I have avoided taking off in the first place? Now, every time I takeoff I obviously have this in mind and kind of also confirm my rpms are rising as I gain speed, basically doing: full power, confirm static rpm, confirm oil temp/pressure in green, confirm airspeed alive, confirm rpm raising, confirm Vr and rotate. What else can I do to ensure I make sound decisions while still on the ground?
     
  2. Sal M CFI

    Sal M CFI Filing Flight Plan

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    I follow a recommendation I learned from a CFI a long time ago. Have 70% of your takeoff speed reached by at least 1/2 way down the runway. If not, abort. It's engrained in me and easily to calculate. Now, if I have a really short field, then I review take off charts, density altitude, etc. But I fly out of NY and FL so we generally have no issues and I rarely fly out of short fields.

    NOTE, I make everyone of my students announce 70% TO speed, 50% runway every time, and even when I am giving flight review.
     
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  3. RussR

    RussR Pattern Altitude

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    The problem with this rule of thumb is that it really only applies on a runway that is short for the airplane you're flying, given the weight and density altitude, as @Chrisgoesflying mentions:

    If you're in a 172 on a 5000 ft runway on a standard day at sea level, and you roll halfway down the runway before getting 70% of your takeoff speed, you should have aborted a long time ago, not waited until then.

    So it requires a little more than just a rote application of the 70/50 calculation.
     
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  4. midlifeflyer

    midlifeflyer Touchdown! Greaser!

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    I only fly light singles and I will admit "no" to "all the time." I have a good idea (actually, it's in my checklist) what my takeoff performance should be at max gross at two different altitudes, sea level and "reasonably" high density altitude. That gives me a decent estimate about when to worry on the takeoff roll and when to do a calculation because those estimates are close.

    Absolutely agree with @RussR on the proper application of the 70/50 rule.
     
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  5. Sal M CFI

    Sal M CFI Filing Flight Plan

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    Actually no, it requires a pilot to think about how far the airplane will be down ANY runway at a given airspeed. I hear them call out "airspeed" alive, but that's it. What's that mean? The instrument is working, great. Where are you when you call that out?
    The reason to think about how far down at 70% is to actually "think" about how far down you are. It's an excercise in understanding what the plane is doing and what it should be doing.
     
  6. MauleSkinner

    MauleSkinner Final Approach

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    70% of takeoff speed by 50% of your calculated/estimated takeoff distance is how the 50/70 rule would be applied on longer runways, although I probably wouldn’t abort if I was 2-3 knots slow at that point like I would on a minimum length runway.

    Sounds like the RPM indication would have been a valid abort criteria before that, though.
     
  7. kyleb

    kyleb Final Approach

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    Answer: No. My airplane is capable of 1,000' takeoffs in any loading and density altitude situation I have ever encountered. If I have the skill to land the airplane at an airport, the airplane has the performance to get me out of the same strip. Now, if I was at higher elevations, some math might be necessary, but I haven't yet found the circumstance where I didn't have plenty of margin to skip that step.
     
  8. schmookeeg

    schmookeeg Administrator Management Council Member

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    Sounds like you couldn't avoid it prior to departure, as it was a thing that manifested on applying take-off power.

    What might have been missing was the tachometer in your engine instrumentation scan on departure.

    I don't compute runway lengths generally, I have a set "minimum runway length" for airports that I use. My minima have enough margin built in to absorb the weather and loading scenarios I typically fly. If I'm using a runway below my minimum, I'll do the math and set fuel loading appropriately.

    $0.02 :)
     
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  9. Grum.Man

    Grum.Man En-Route

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    No I don’t calculate it but that’s also because I don’t go places I think getting back out of if so marginal that I have to calculate it. I’ve flown my planes enough that I know what they are capable of and am in tune with them enough to know when something isn’t right long before it becomes an issue. In your scenario a glance at the tachometer would have thrown up a red flag immediately to abort if I was missing 300 rpm from normal.

    Granted I fly primarily east of the Mississippi so the most I would ever see for elevation at takeoff is about 3500’. If I flew to the Rockies and into an airport with a 6k’ DA I would probably run the numbers as that’s an environment I’m not familiar with.
     
  10. George Mohr

    George Mohr Line Up and Wait

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    Very much like CG calculations, It's more about knowing what is 'out of the norm' for you and your airplane. Examples are helpful. In my 182, even heavily loaded, I know that I can easily get airborne in 1000 ft at my normal density altitudes. So unless I'm outside my normal operations, i.e. its very hot, or I'm at a high DA airport, or the runway is shorter than 2000 ft, I know I'm good. My previous airplane was a 160hp warrior. Same idea applies, but the number change dramatically. So if the performance factors do not fall comfortably into the range of your normal experience, do the calculation.

    For an abort rule of thumb, I want to be flying by mid-field. The 70% airspeed at half field length is a good one, but not conservative enough for my personal taste.
     
  11. EdFred

    EdFred Taxi to Parking

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    Nope. T/O ground roll is 750' at max gross/ max performance. Landing roll is 600' So my start stop distance is 1350'. Add 15% for grass, add another 20% for the (worst) DA I (usually) fly out of. That gets me to about 1850', but I'm never at gross when I'm in and out of grass strips. As long as I have 2000' I'm good, and no need to calculate every time. When I have taken off at gross, it's been on a 5000' (or more) paved runway, and I'm not trying short field technique. If I'm not off by a certain point at those fields (and it isn't going to be even close halfway) I know something is up, and I'm shutting it down with well over 3000' remaining. So for the most part the 70/50 rule is bollocks, and a complete waste of time, since the gear is already pulled up and I'm climbing well before I am halfway down the runway.

    But I also have been flying the same plane(s) for the past 18 years, and when I've flown a new to me model, it's always been some place where the 70/50 rule still isn't going to make any sense.
     
    Last edited: Jan 11, 2021
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  12. jordane93

    jordane93 Final Approach

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    If I’m loaded heavy and it’s a short runway I’ll do a calculation. Most of the time it’s just me and one other person and a 5500-6800ft runway so it’s never an issue.
     
  13. WannFly

    WannFly Final Approach

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    very similar to what @Grum.Man said. I set up a aborted take off point before my roll when visiting a new airport and if i dont see at least 45-50 kts by that time, there is something wrong. over generalizing, there are exceptions.

    i fly the same plane, know my numbers and dont go places with very short runways (there are exceptions, like high winds down the runway, very cold and -ve DA etc)
     
  14. EdFred

    EdFred Taxi to Parking

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    You're in Fargo, those aren't exceptions, but the norm. :D
     
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  15. OkieAviator

    OkieAviator Pattern Altitude

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  16. wsuffa

    wsuffa Touchdown! Greaser!

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    IT depends for me. When I know the plane type well and the area well, then no, I don't. If the field is marginal length or at a materially higher density altitude, I'll generally run a check. If it's a new plane type for me, I'll generally check. With exceptions, of course: if it's a 10,000' runway and I'm flying a single-engine six banger my thought process is different than it is with a 1500' runway with trees at either end.

    And then there are the one or two airports that I've been to that are wedged in between high-tension power lines. Then it's not just a matter of getting off the field, it's a matter of clearing the (not proverbial) 75-100' obstruction off the end. Only once have I been uncomfortably close to the power line on takeoff....
     
  17. eman1200

    eman1200 Touchdown! Greaser!

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    nope. I take a deeeep breath and let out a big long WEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEE!!!!! if my wheels aren't off the ground by the time I'm done weeee'ing, it might be time to abort.
     
  18. MauleSkinner

    MauleSkinner Final Approach

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    I disagree...high winds in Fargo are generally as far from any runway heading as possible. ;)
     
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  19. MauleSkinner

    MauleSkinner Final Approach

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    Do you play piano, too, or just tinkle on the keys?
     
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  20. RussR

    RussR Pattern Altitude

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    Well I think we're actually in agreement then - it requires a little more than just rote knowledge, it requires some thought. My issue with simple statements is that some people (I've seen it), just recite it back even if they're on a 10,000 ft runway. When, if you're in a 172, if you're not airborne in the first 5000 ft, then you have other problems and shouldn't be continuing the takeoff regardless of the rule of thumb.

    Like most rules of thumb, it requires a little more thought than just rote memorization.
     
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  21. EdFred

    EdFred Taxi to Parking

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    That's why I didn't bold the "down the runway" part of the statement. :D
     
  22. MauleSkinner

    MauleSkinner Final Approach

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    The Earth is round, not flat. :p
     
  23. keen9

    keen9 Pre-takeoff checklist

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    Sounds like your loss of performance on the takeoff roll was within the error of everything that goes into the calculation (winds, rolling resistance, weight, etc). The only way you are going to diagnose that small of a performance loss is using a per cylinder EGT on your runup.
     
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  24. chemgeek

    chemgeek Pattern Altitude

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    I have some "pre-calculated" takeoff parameters: if I have 3000 feet or runway and the DA is 3000 feet or less, then there is no legal loading of my aircraft that would preclude initiating a safe takeoff. Outside those parameters, I'll consult the POH and estimate the runway needed to evaluate safety.

    When in the takeoff roll, I have two checkpoints: (a) initial takeoff roll rpm should be at the typical value; (b) if I have the aforementioned 3000 feet of runway and DA<3000 ft, I need to be off the ground and have a positive climb rate at the half-way point. That is very conservative, but if I can't meet this requirement, something is really wrong.

    During my one and only real-life takeoff abort, I hit all the objectives except the positive climb rate at the runway half-way point. An unexpected mountain downdraft gust and/or wind direction change was preventing climb out of ground effect, despite a prior departure a minute ahead of me with similar performance characteristics having no problem. On a second takeoff attempt in the opposite direction a few minutes later, there was no issue departing safely. Had I persisted on attempt #1 I would have been in the trees at the end of the runway. Wind can be squirrelly at mountain airports, even with little mountains like we have in NY.
     
  25. Domenick

    Domenick Line Up and Wait

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    I once took off in my Warrior (-161) from Douglas, WY (DGW), at full gross or perhaps a bit over, on runway 29 (6534', elev 4933) and ground temp pushing 100F. I wish I had calculated my takeoff distance. No problem getting airborne in less than half the runway, but once out of ground effect, climb was anemic. Followed hwy 93 for a few miles until I was sure we would stay in the air.

    I suppose calculating TO distance and ROC would not have changed anything, but it would have clued me to what to expect.

    And, yes, when in doubt, I do calculate TO distance and ROC. I use one of those sliding TO calculators and a phone app.
     
  26. Grum.Man

    Grum.Man En-Route

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    Against my better judgment knowing how debates go on POA this simply is not true at least on an airplane with a fixed pitch propeller. There is a static run up allowance in the POH. The tolerance is usually about 100 rpm. If the rpm doesn’t reach that upon full power something is wrong. A 300 rpm difference from normal should have been a huge red flag and easily noticed.
     
  27. MauleSkinner

    MauleSkinner Final Approach

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    **and/or other reliable information if POH data is not sufficient.

    just to pick a little nit. ;)
     
  28. jayhawk74

    jayhawk74 Pre-Flight

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    I taught my students to compute the amount of runway needed max weight, 3000 ft altitude (taught on the East Coast), and 95 degree temp. Once they knew that number that would be the minimum length needed for any condition. If they wanted to do a runway below the minimum then do a calculation for that runway and conditions.
     
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  29. MauleSkinner

    MauleSkinner Final Approach

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    And how did you teach them to apply that to the OP’s malfunction?
     
  30. aftCG

    aftCG Pre-takeoff checklist

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    Short answer is yes, I calculate it. I own one of those slide rule things that allows you to compensate for DA, runway slope, grass surface etc. I mostly use it on unfamiliar planes until I know what performance to expect.

    For planes I fly regularly I watch every takeoff until I have a solid base, then I use the runway markings, signs, etc on every takeoff. At max gross my Citabria better be off the ground by the aim point markings (500') at my home airport or there is something wrong. Solo it better be off in 350'. In either case I also expect to be at 1000' by the far end of our 5000' runway.

    A fully loaded C172 down near sea level better be off the ground in 1000'. Do I abort if I get there and there is 4000' remaining? No, but I should be thinking in my head that "oh, I guess DA is higher than normal, so another couple hundred feet is expected" and if it takes much longer than that I would be prepared to abort (haven't had to in 25 years but it's part of the briefing). If the plane seems to be running fine and DA is not an issue then I should be considering things like trim not set or flaps were left down, or the dog is still tied to the right main.

    Last June I ferried a Cardinal RG from NC to WA, with the owner and his wife, along with all their baggage (they flew in from out of the country). That plane was loaded to max for every takeoff. Some of the airports were miserably hot and humid, others were at very high elevations. I insisted that every airport we used be 5000' long and I can remember the takeoff roll from Casper, WY was very long. I noted the runway markings when the mains got off the ground and looked it up on Google satellite later. Lets just say I wasn't surprised.

    RE: the slide rule runway distance calculator things which I keep in a pocket of my knee board. It very accurately accounts for DA, head/tail wind, slope, grass, wet - all of it, and fills in nicely where POHs tend to get vague. But to count on it you need to use it for a while in normal situations. Only a fool would pull it out for the first time at Leadville on a hot afternoon.

    I once rented a C172RG (aka "Gutless") and took two of my sons to see my ailing father. Our takeoff coming home was in the afternoon heat around 100F in the Oregon desert. There was a decent headwind and tall obstructions weren't much of a concern. Still, everyone knows the 172RG is a dog and I wanted to know what to expect, or even if it was going to get off the ground at all. Full tanks and three meat eaters put the plane at max gross. My slide rule said something like 1100' which I laughed off as fiction. The plane left the ground in exactly 1100'. I'm positive the headwind was a big factor but I was surprised at the number (and the accuracy of the slide rule). Not saying much for the climb rate but the VSI was above zero and the departure was uneventful.
     
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  31. idahoflier

    idahoflier Cleared for Takeoff

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    A lot of good replies already, the only thing I would add is if things aren't looking right then abort the takeoff. I think a lot of pilots get in a mindset that once the takeoff roll has started they have to get in the air. Similarly pilots feel a commitment to landing. If things don't look right, ABORT!
     
  32. Tarheelpilot

    Tarheelpilot En-Route

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    I’m not saying it’s true but my first thought was “that pilot either didn’t do a run up or doesn’t know what to look for during the run up.”
     
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  33. keen9

    keen9 Pre-takeoff checklist

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    OP said the runup was normal. Minus 300rpm is in the climb where it is already too late to not takeoff.
     
  34. David Megginson

    David Megginson Cleared for Takeoff

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    No, I don't calculate if the runway is 2½x or more longer than my likely liftoff distance (a 3,000 ft paved runway easily fits that bill for a PA-28-161 at lower elevations). Being close to gross weight, higher-than-normal density altitude, risk of a tailwind (variable), obstructions off the end of the runway, or a rough surface are factors that might convince me to run the tables even for a longer runway.

    The reason for the 2½x rule is that that gives me lots of stopping room if I'm not at the standard 70% of liftoff speed by halfway down the runway (pick a landmark halfway down while you're still sitting on the threshold, so that you know). OTOH, if someone plans to go on to the airlines, it's probably a good habit to establish early — as someone told me once, every takeoff for a 747 is a maximum-performance, short-field takeoff. :)

    In fact, I practice the abort all the time. Whenever my plane has had maintenance, I do a high-speed taxi until the wheels just barely leave the ground—so 100% of takeoff speed instead of 70%—and then abort the takeoff as part of the post-maintenance testing—that lets me wiggle the controls and confirm the surfaces are all responding properly under load. The 3,300 ft runway at my home airport leaves plenty of room for that without straining the brakes or breaking a sweat (or even having to backtrack from the end of the runway to the last turnoff).
     
  35. Sal M CFI

    Sal M CFI Filing Flight Plan

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    :)
     
  36. midlifeflyer

    midlifeflyer Touchdown! Greaser!

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    That's interesting. I'm exactly the opposite. If I can get out, I can get in. I think one of the make/models I fly may have a longer landing distance than takeoff distance but by far most of them have takeoff as the limiting distance.
     
  37. David Megginson

    David Megginson Cleared for Takeoff

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    If you like analogue toys (vs calculating stuff on your phone or tablet), Aircraft Spruce still has stock left in Europe on this classic for quickly estimating takeoff distance:

    https://www.aircraftspruce.eu/apr-pocket-computer-daa-304.htm

    (I think I bought the last one in North America a year ago, because they've been removed from the Canadian and US catalogue ever since. :) ).
     
  38. kyleb

    kyleb Final Approach

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    Drake the Outlaw
    In the Winter, light, I can get the RV-6 off the ground in 300'. My ability to land it without smoking the brakes or tires is about twice that. I leave a little margin on the table in the landing phase so I don't drop it in too badly.
     
  39. David Megginson

    David Megginson Cleared for Takeoff

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    I have no experience with the RV-6, but perhaps your approach speed is too high? Or maybe it just has to do with high power loading on the wings.
     
  40. kyleb

    kyleb Final Approach

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    The airplane has a pretty high power to weight ratio at light weight, so acceleration is strong. No doubt I'm leaving landing performance on the table by not dragging it in at 55 knots with substantial power, but there is no reason for me to "play" that close to the margin. Also, I don't brake crazy hard to achieve that stopping distance. A more aggressive pilot could do better. But there's no need for me to be that aggressive and accept the increased risk.

    Here's a picture... Starting at the blob at the bottom, if I initiate a takeoff run from there, I'm off the ground (light weight, low density altitude) before the first dot. About 270' per google. Landing on the runway end marker (the second dot), I can make the first turnoff (the third dot) with medium braking. That's about 660'. Theoretically, I could take-off, fly 50ish feet, and land in a total of just under 1,000'.

    upload_2021-1-11_16-37-55.png
     
    Last edited: Jan 11, 2021
  41. Dana

    Dana Pattern Altitude

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    Dana
    I don't think I've ever actually calculated a takeoff roll. Well, I must've, back when I was learning to fly, but not since. But then, all of the planes I've owned had takeoff and landing distances under 400', so there aren't many airports I can't get in or out of. The shortest runway I regularly go into is 1400' of grass, and I usually operate from a 900' strip of grass between the paved runway and taxiway at my home field. That said, I live near sea level and have never flown into any airport over 2000' elevation. None of my planes (all antique or experimental) have had a POH to get "book" values from either, though.