Aviation Knowledge Question

Crashnburn

Crashnburn

Pattern Altitude
Joined
Aug 8, 2018
Messages
1,776
Location
Erie, CO
Display Name
Display name:
Crashnburn
I’m studying Aviation Knowledge in preparation for restarting flight training. I’m looking at a Landing Distance table. A note says”Add 20%of the ‘total to clear 50 foot obstacle’ to both the total to clear and the ground roll for dry grass. “

How does the distance from the obstacle to the touchdown point affect the stopping distance on the dry grass? I know it adds a safety margin, but is that note correct, or should it say 20% of the ground roll? Thanks
 
My guess would be by making the correction based on a percentage of the distance over an obstacle they were able to use a round number, 20%, which is easy to calculate in your head. If it was based on a percentage of the ground roll distance then the percentage would have been higher, and likely not close enough to an even 10% to be rounded to an even 10%.
 
Thanks. That’s possible, but you’d always have a margin if you round it up to the next highest 10%. I can see why you add it to both distances.
 
Hmmm... sounds like it’s a correction for dry grass vs hard surface? Which would make sense, less stopping power on grass than a hard surface.
 
Tools said:
Hmmm... sounds like it’s a correction for dry grass vs hard surface? Which would make sense, less stopping power on grass than a hard surface.
Click to expand...
Yeah, but why add 20% to the 50 feet to touchdown distance? That distance is a constant because it has nothing to do with the runway surface.
 
Oh, I see... ya, no clue.
 
Can ya post a link to that table?
 
I know what you’re saying and this is my interpretation of that. I’ll use easy round numbers for my interpretation. Ground roll is 500’, distance to clear 50’ obstacle is 1500’...add 20% to both for dry grass. The 1500’ needed is the total runway distance needed with the obstacle and ground roll combined. So it’s not 1500’ for the obstacle and then the ground roll distance....you would need to add 20% to both distances. But again, that’s your total runway distance needed for both scenarios (obstacle vs no obstacle). If that makes sense. I’m sure someone will correct me if I’m wrong.
 
RussS said:
I know what you’re saying and this is my interpretation of that. I’ll use easy round numbers for my interpretation. Ground roll is 500’, distance to clear 50’ obstacle is 1500’...add 20% to both for dry grass. The 1500’ needed is the total runway distance needed with the obstacle and ground roll combined. So it’s not 1500’ for the obstacle and then the ground roll distance....you would need to add 20% to both distances. But again, that’s your total runway distance needed for both scenarios (obstacle vs no obstacle). If that makes sense. I’m sure someone will correct me if I’m wrong.
Click to expand...
I understand the 20 % needs to be added to both distances because ground roll is part of total landing distance.

But the formula calls to add 20% of the total distance to each distance when you’re only on dry grass during the ground roll, when you’re braking.
 
Crashnburn said:
I understand the 20 % needs to be added to both distances because ground roll is part of total landing distance.

But the formula calls to add 20% of the total distance to each distance when you’re only on dry grass during the ground roll, when you’re braking.
Click to expand...

Is there another section for wet grass? That would make sense. Wet grass obviously much longer distance needed than the 20%

I feel like something is missing
 
RussS said:
Is there another section for wet grass? That would make sense. Wet grass obviously much longer distance needed than the 20%

I feel like something is missing
Click to expand...
No section for wet grass
 
Crashnburn said:
I understand the 20 % needs to be added to both distances because ground roll is part of total landing distance.

But the formula calls to add 20% of the total distance to each distance when you’re only on dry grass during the ground roll, when you’re braking.
Click to expand...
What airplane is this and can you link a chart?
 
Sorry, this is from The Flight Manual 2: Ground School. It doesn’t identify the plane, and it’s printed on dead trees.
 
Crashnburn said:
I’m studying Aviation Knowledge in preparation for restarting flight training. I’m looking at a Landing Distance table. A note says”Add 20%of the ‘total to clear 50 foot obstacle’ to both the total to clear and the ground roll for dry grass. “

How does the distance from the obstacle to the touchdown point affect the stopping distance on the dry grass? I know it adds a safety margin, but is that note correct, or should it say 20% of the ground roll? Thanks
Click to expand...

The chart does not have to be logical, it just has to give you the right answer when you follow the instructions.

The FAA written, like most standardized tests, is as much a test of reading comprehension as anything else. They are testing your ability to read and follow directions. It may be deliberately tricky, or simply based off a real world aircraft with a similar chart.
 
  • Like
Reactions: YKA
Ah... In other words, the ground roll is about a 50% increase, or a 20% total distance increase. It’s simply their method to use the latter.

As mentioned, successful chart usage is HEAVILY reliant upon correct “notes” interpretation.

And it frequently isn’t very obvious.
 
It’s not hopeless and the reading comprehension part makes sense. Sorry my question took up so much bandwidth, and thank you all for your time and answers.

A different chart says ground roll is approximately 70% of the total landing over a 50 foot obstacle distance, so one could work backwards to see what percent the dry grass adds directly to the rollout distance.
 
Crashnburn said:
A different chart says ground roll is approximately 70% of the total landing over a 50 foot obstacle distance, so one could work backwards to see what percent the dry grass adds directly to the rollout distance.
Click to expand...
Lemesee, 70% is only a bit more than 2/3, so, say, 1= airborne portion of total (paved) landing distance and 2 = ground portion of it. So 2/3 of 3 is the paved ground rollout distance (2nd chart). Then, 1/5 of 3 (1st chart) = 3/5 = 60%, and 60% of 3 should be a bit less than 2/3 of 3 (2nd chart), right? Because grass is slipperier? The difference being the answer you seek, methinks. @EdFred probably has already made a chart to confirm this, of course.

EDIT: Upon further review this call on the field has been reversed. The ball carrier (me) stepped out of bounds at the 60% yard line. No time remains on the clock, game over.
 
Last edited:
That note is badly worded.

If you add 20% to the ground roll, then the distance to clear a 50' obstacle should only be the difference of ground rolls added to the 50' paved obstalce, not 20% more than the paved 50' distance.

Orrrrrrr....add 20% to the 50' distance and the difference between that and the paved distance is the increase in takeoff roll.


Once you break the ground, it doesn't matter what the surface is.
 
dmspilot said:
View attachment 93294
Click to expand...
Sure. Makes sense.

Take 5,000' @ 41°F.
  • The distance to land over the obstacle is 1,195'.
  • 20% of 1,195 is 239'. That's you additive.
  • The "normal" no-obstacle ground roll is 495'
  • On grass, it's 495+239=734'
  • The "normal" distance to land over the obstacle is 1,195'
  • On grass it's 1,195+239=1,434'
 
midlifeflyer said:
Sure. Makes sense.

Take 5,000' @ 41°F.
  • The distance to land over the obstacle is 1,195'.
  • 20% of 1,195 is 239'. That's you additive.
  • The "normal" no-obstacle ground roll is 495'
  • On grass, it's 495+239=734'
  • The "normal" distance to land over the obstacle is 1,195'
  • On grass it's 1,195+239=1,434'
Click to expand...

I read that note like 6 times before you posted the actual math. After looking at the math and re-reading the note, it finally makes sense what they were trying to say. Still contend it was badly worded though.
 
EdFred said:
I read that note like 6 times before you posted the actual math. After looking at the math and re-reading the note, it finally makes sense what they were trying to say. Still contend it was badly worded though.
Click to expand...
Perhaps, but I expect context to give meaning. I think it's also the only way it makes sense. What's the alternative? 2,168' whether landing over an obstacle or not?
 
I think I worked out the math.
Total Takeoff Distance = X
Distance to touch down = 0.3X
Ground Roll = 0.7X

For grass, add 0.2X so
New Total Landing Distance = 0.3x + 0.7x + 0.2x
The grass can't affect the glide distance to the touchdown point, so
New ground roll = 0.7x + 0.2X = 0.9x
The increase in ground roll then is: (0.9x - 0.7x)/0.7x =~ 0.286
I'd round it up to 30%, but that's just me.

Thanks all for your participation and inspiration.
 
Crashnburn said:
I think I worked out the math.
Total Takeoff Distance = X
Distance to touch down = 0.3X
Ground Roll = 0.7X

For grass, add 0.2X so
New Total Landing Distance = 0.3x + 0.7x + 0.2x
The grass can't affect the glide distance to the touchdown point, so
New ground roll = 0.7x + 0.2X = 0.9x
The increase in ground roll then is: (0.9x - 0.7x)/0.7x =~ 0.286
I'd round it up to 30%, but that's just me.

Thanks all for your participation and inspiration.
Click to expand...
Glad yo got an answer to your question. I have no idea if you are right or not. Me, I would just use the chart.
 
Oh, I’ll use the chart. I just couldn’t understand why you’d use the glide distance as part of the calculation for how much dry grass adds to the rollout distance. The later knowledge of the fixed glide to rollout distances helped me understand why it’s OK to use the total landing distance to calculate how much dry grass will extend the rollout.
 
Really wet grass can stop you extremely quickly...once those tires sink in deep, you need about 4 feet.
 
It’s an FAA test chart designed to determine you know how to interpolate and apply the chart instructions. If you were to work a problem with a PA not given in the fields, non standard temperature, a head wind, and land roll on grass, you would use a bunch of the 150 minutes you have for the written knowledge test.
 
Crashnburn said:
Oh, I’ll use the chart. I just couldn’t understand why you’d use the glide distance as part of the calculation for how much dry grass adds to the rollout distance. The later knowledge of the fixed glide to rollout distances helped me understand why it’s OK to use the total landing distance to calculate how much dry grass will extend the rollout.
Click to expand...
Let me ask a question. Using your formula, do you get the same answer as the chart gives you regardless of altitude/temperature? If so, you have successfully drilled down into the mathematic formula and related considerations the people who test flew the airplane and created the chart used to give their numbers. A valuable academic exercise for those whose minds work in that direction.
 
That’s a good exercise to try. I’ll let you know my results. Still, absence of a negative is not proof of the positive.
 
You must log in or register to reply here.
Back
Top