Climb over obstacle plus distance traveled

J

jamesthunder88

Filing Flight Plan
Joined
Jun 27, 2014
Messages
5
Display Name
Display name:
Mike
I'm having trouble with a math problem. This is posed on the commercial checkride the examiner I'm using does.

3067ft runway
Field Elevation is 4154
29.62
30°C
1200 ft off runway, 150ft building.

Vx on this plane is 82 and with the performance charts I've established that the aircraft will take off at 2100ft, climb to 50ft at about 3000 ft. Which leaves me 100 ft to clear the obstacle and only 1200ft to do it in. The correct answer is clearing it by about 320 feet, but when I've asked my instructor if he could help reproduce the answer we crash into the building by 20 ft too low.

Could anyone lend a hand?
 
1. distance remaining/speed = time to the obstacle.

2. time to the obsticle * rate of climb = altitude gained.

3. highlight of the obstacle - (initial altitude + altitude gained) = height by which you will clear the obstacle.

Plug the numbers from the POH and your problem in to these equations and you will get the answer. Make sure you are using POH tables correctly, and you are not making any basic math mistakes.
 
timwinters said:
:rofl:

That was my first thought also...but probably isn't what his instructor is looking for.
Click to expand...

:dunno: I'd think they would have to take it. It's always one of the things I look at briefing a departure, "Where is my minimum climb route back around?" I'm not worried about climbing over a building, I'm concerned with avoiding it.
 
Who, in their right mind would cut a takeoff performance calculation so close in reality?

However, 320ft is a lot, but you have too much variance in your answer 320 vs -20.
 
Jaybird180 said:
Who, in their right mind would cut a takeoff performance calculation so close in reality?

However, 320ft is a lot,
Click to expand...

Not if the wind shifts significantly at 50' AGL.

I agree with your first sentence!
 
Add nitrous oxide to assist in your takeoff :)
 
jamesthunder88 said:
I'm having trouble with a math problem. This is posed on the commercial checkride the examiner I'm using does.

3067ft runway
Field Elevation is 4154
29.62
30°C
1200 ft off runway, 150ft building.

Vx on this plane is 82 and with the performance charts I've established that the aircraft will take off at 2100ft, climb to 50ft at about 3000 ft. Which leaves me 100 ft to clear the obstacle and only 1200ft to do it in. The correct answer is clearing it by about 320 feet, but when I've asked my instructor if he could help reproduce the answer we crash into the building by 20 ft too low.

Could anyone lend a hand?
Click to expand...

Welcome!

You know the answer how? I don't believe it. You've got a climb gradient of 5.5% from liftoff to 50' (rise over run). Maintaining Vx for another 1267' ought to give 120' of total climb which is 30' too low.

EDIT: I lost track of how many times I had to edit this. Finally grabbed a pencil and drew it out. Whew...

dtuuri
 
Last edited:
dtuuri said:
Welcome!

You know the answer how? I don't believe it. You've got a climb gradient of 5.5% from liftoff to 50' (rise over run). Maintaining Vx for another 1200' ought to give 117' of total climb which is 33' too low.

dtuuri
Click to expand...

You cheated him of 67 feet.
 
jamesthunder88 said:
I'm having trouble with a math problem. This is posed on the commercial checkride the examiner I'm using does.

3067ft runway
Field Elevation is 4154
29.62
30°C
1200 ft off runway, 150ft building.

Vx on this plane is 82 and with the performance charts I've established that the aircraft will take off at 2100ft, climb to 50ft at about 3000 ft. Which leaves me 100 ft to clear the obstacle and only 1200ft to do it in. The correct answer is clearing it by about 320 feet, but when I've asked my instructor if he could help reproduce the answer we crash into the building by 20 ft too low.

Could anyone lend a hand?
Click to expand...

I come up with 120.38 feet at the obstacle. You miss the top by 30 feet. Go around it.
 
dtuuri said:
You know the answer how? I don't believe it.
Click to expand...

The question was from my commercial multi oral. Being nervous and moving on to the question, "would you try to take off?" He walked me through the problem and got the answer of clearing it by 320 ft, another student did the checkride a few days later, had the same problem, had him walk him through the clearance portion and also arrived at 320 feet.

My commercial single is today and I'm expecting a similar problem, I just wanted to try and get an answer and then see his way of doing it again.
 
Action Hero answer to lighten the mood: "I'd switch to blasters/missles and blow the obstacle out of my flight path."
 
I don't see anything about computed climb rate from the POH/AFM performance data in your discussion. That would be essential to solving the problem.
 
Last edited:
jamesthunder88 said:
Also rate of climb is 700 fpm, 0 wind.
Click to expand...
At what IAS? You'd then have to turn the IAS into TAS based on PA and temperature, and TAS into GS (although you say wind is zero which means GS=TAS). With GS and climb rate, you can compute climb gradient.
 
I keep a transit and laser range finder in my flyaway kit for just these kids of obstacles.
 
I agree with 30 feet too low. After liftoff, he's 50 feet agl at 900 feet yieding a 3.1798 degree slope. At 2167 feet from liftoff he's only a little over 120 feet agl.
 
Electric said:
1. distance remaining/speed = time to the obstacle.

2. time to the obsticle * rate of climb = altitude gained.

3. highlight of the obstacle - (initial altitude + altitude gained) = height by which you will clear the obstacle.

Plug the numbers from the POH and your problem in to these equations and you will get the answer. Make sure you are using POH tables correctly, and you are not making any basic math mistakes.
Click to expand...

I think there was a step missed between one and two. I converted to seconds so it looked something like this.

1: 1200/88/60 = 0.227
2: 0.227 * 700 FPM = 159
3: 150 - (50 initial altitude + 159 ft gained) = Clearing the obstacle by 59 feet

I'll check back in after the checkride with information on how he (the DPE) does it. But the above seems pretty straight forward.

EDIT: 88 is Vy, which is the incorrect speed for trying to clear this obstacle but is the number I used during the multi.
 
Last edited:
Ron Levy said:
At what IAS? You'd then have to turn the IAS into TAS based on PA and temperature, and TAS into GS (although you say wind is zero which means GS=TAS). With GS and climb rate, you can compute climb gradient.
Click to expand...

Indicated is 82 knots
 
jamesthunder88 said:
Indicated is 82 knots
Click to expand...
Good. Then it sounds like you have all the information you need to solve the problem, as well as enough guidance on how to do it. You need to know how much altitude you'll gain at 700 ft/min moving 1200 feet forward at whatever TAS you get at 82 KIAS at whatever pressure altitude 4204 MSL is with an altimeter setting of 29.62 and +30C. "The rest is left to the student as an exercise."
 
thehulk06 said:
Add nitrous oxide to assist in your takeoff :)
Click to expand...

50/50 NOx-Nitromethane spray = Reciprocating rocket engine. You can spray an extra 300 HP pretty easily. If you set it up right, you could set a time to climb and recip altitude record.
 
Henning said:
50/50 NOx-Nitromethane spray = Reciprocating rocket engine. You can spray an extra 300 HP pretty easily. If you set it up right, you could set a time to climb and recip altitude record.
Click to expand...

Sounds like a good way to go for the fastest-time-to-overhaul record too. :rofl:
 
flyingcheesehead said:
Sounds like a good way to go for the fastest-time-to-overhaul record too. :rofl:
Click to expand...

No, not really, 300hp is 300hp regardless the fuel used to develop it. Nitromethane is a less energetic fuel than gasoline, the thing is that it carries it's own oxygen so you can stick a lot more of the fuel in per volume of air. It's actually going to be gentler on your engine.
 
You must log in or register to reply here.
Back
Top