Trivia quiz: Landing light

[Flying Bear]

You're in your C172 or equivalent spam can, making a landing at the normal POH approach speed of 65 kts. You checked the POH before the flight, and concluded you will use 1,500 ft of runway, assuming you're at maximum certified weight.

Nobody wanted to go fly with you today, so you're 400 lbs below maximum weight.

- Will you use more or less than 1,500 ft of runway?

- Is there something you could have done better?

[MadDog]

I would turn on the landing light....oh no that just shut down the engine...... crap....!

[Tanner_J]

I'm confused to hell out of this topic. The whole title threw me off thinking it had something to do with the actual light...maybe you should change the title...

[Dmmoore]

Originally Posted by Flying Bear

You're in your C172 or equivalent spam can, making a landing at the normal POH approach speed of 65 kts. You checked the POH before the flight, and concluded you will use 1,500 ft of runway, assuming you're at maximum certified weight.

Nobody wanted to go fly with you today, so you're 400 lbs below maximum weight.

- Will you use more or less than 1,500 ft of runway?

- Is there something you could have done better?

As we are landing 400 pounds under max operating weight (lighter than max) we can expect to use less runway "IF" we reduce the over the fence speed to 62 knots. At 65 knots the 172 will float a bit and use up a bit of runway that the lighter weight should have made possible.

[Dmmoore]

Originally Posted by Tanner_J

I'm confused to hell out of this topic. The whole title threw me off thinking it had something to do with the actual light...maybe you should change the title...

That's the trouble with English You can use a flash light (electric tourch for our British friends) to light your way, if you take something out of your aircraft it becomes light but it doesn't glow.

You can be on the right road but not be on the right (correct) road.

And my favorite; "Turn left right here!

We won't talk about homonyms.

[screaming_emu]

Originally Posted by MadDog

I would turn on the landing light....oh no that just shut down the engine...... crap....!

Hehe, a bit off topic but that reminds me of something a student of mine did once.

We flew to KWWD from KBWI and were practicing landings there. On the first landing my student completely forgot his before landing checklist so we didn't have the aux fuel pump on. No big deal...once we hit the ground, I'll take over bring the plane to a stop and not let him go until he figures out what is wrong. Uneventful landing and my student immediately sets up for a touch and go, flaps to takeoff and full power. He was a little too enthusiastic with the power and the engine just stops. This happens in the DA20 which is why you need the aux fuel pump on too. So I take control and pull off on the first taxiway and ask "ok, what happened?"

His response?

"I forgot to turn the landing light on!"

[Flying Bear]

Originally Posted by Dmmoore

As we are landing 400 pounds under max operating weight (lighter than max) we can expect to use less runway "IF" we reduce the over the fence speed to 62 knots. At 65 knots the 172 will float a bit and use up a bit of runway that the lighter weight should have made possible.

That would appear to be the right answer!

Wonder how many actually take that into account though... Wasn't told about that, neither in performance class* nor by any of my flight instructors... "We need less runway when we're light" + "Final approach speed is 65 kts" = oops...


* For Tanner_J: That's aircraft performance in ground school, not ballet class...

[3WE]

Ummm....I'm thinking that you could land and stop a 172's in less than 1000 ft almost 100% of the time....and that's the numbers given in the book...

Could have done better? Yeah, a lot better....sounds to me like you took the actual landing distances and added factors that you didn't mention.....Distance over a 50-ft obstacle, distance for an average hold off....maybe some safety buffer.

Now, given that it's hard to find runways shorter than 2000 ft, does factoring in 400 lb under gross have much meaning?

Finally, I would worry a lot more about takeoff....are there surrounding obstructions/terrain....is it 98 degrees and humid.....are you at a 6000 ft MSL airport???

Takeoff effects are usually much more significant than landing ones because 1) it's easier to loose altitude than gain it and 2) alititude affects your powerplant which you need a lot more for takeoff than landing.

[Dmmoore]

Well, given the parameters set forth in the original post, total runway was given as an example only.

In the 172 (between S/L and 2K feet) I have never had a landing roll that needed to exceed 1000 feet but total runway required was always somewhat longer.

I rarely operated the 172 out of fields shorter than 2K feet.

[Flying Bear]

Well... 172SP, SL @ 20 deg C, no wind, dry grass, 2550 lbs, landing distance over 50 ft: 1613 ft. Landing distance required by regulations (1.43 times the above): 2305 ft

Same conditions at 2150 lbs: 1484 ft and 2120 ft respectively.

In other words, if you think you are legally ok to land on a 2150 ft airstrip because you're light, but don't use the proper approach speed, you're kidding yourself. Your performance will be worse than at max weight, not better. It should be 61 KIAS at 2550 lbs and 56 KIAS at 2150; not the universally agreed upon 65 KIAS regardless...

I suppose you are right that it shouldn't matter in practice... But the extra safety margins should be for unforseen circumstances, not for screwing up your performance calculations!

Anyway, was just curious how many would give the "you'll land shorter 'cuz you're lighter" answer...

(Crossed posts with Don...)

[LRJet Guy]

Single pilot, single engine, no radar, no autopilot, no flight director, no FMS. Declare a friggin emergency....... That's what I'd do.

Seriously though. Who the hell calculates landing distances in a 172?

[Flying Bear]

Originally Posted by LRJet Guy

Seriously though. Who the hell calculates landing distances in a 172?

Us student pilots do...