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stephen2688
1st Apr 2017, 03:05
The screen height of 35ft that a jet must be flying at when at the V2 speed, where is this imaginary screen, is it at the end of the runway or clearway (TODR)?

pattern_is_full
1st Apr 2017, 03:29
The 35' screen is at the end of TODA - takeoff distance available - therefore includes clearway if there is any.

Don't confuse TODA (what the runway can provide - a fixed distance for any given runway) with TODR (takeoff distance required - what the aircraft needs at a given weight and environmental condition, and thus variable for every flight).

And always make sure TODA > TODR. ;)

john_tullamarine
1st Apr 2017, 05:02
is it at the end of the runway or clearway (TODR)?

Subtle points of note -

(a) end of runway is TORA

(b) end of clearway is TODA

(c) TODR, for a specific aircraft, is the figure which comes out of the AFM calculations and must not exceed TODA (as noted)

The 35' screen is at the end of TODA

(d) A very common error; I guess mainly due to the typical BFL drawing one tends to see in the textbooks. Screen is at the end of TODR. If there be excess available in the TODA compared to TODR, that's fine, but it's not relevant to the specific takeoff calculation once the V1/VR ratio is selected.

If the particular situation results in TODR = TODA, then screen will be at the end of the TODA but that is a specific instance rather than the general case. An extreme way to visualise the distinction is for an aircraft with TODR = 2000 ft departing from a 14,000 ft runway .... hitting 35 ft at the end, while probably being great fun, is not relevant to the routine takeoff exercise.

B2N2
1st Apr 2017, 05:16
In case you're wondering what that looks like Stephen:

http://farm3.static.flickr.com/2390/1966090559_6f1b153a0a.jpg

https://s-media-cache-ak0.pinimg.com/564x/a5/af/97/a5af979373991408ebdca7cfb8b5f2be.jpg

RAT 5
1st Apr 2017, 11:39
And now, folks, (drum roll) for your further entertainment pucker you lips, get ready with your oo's & ah's and OMG's, the next contestant will do this on only 3 engines. (loud applause) from those not running for the exits.

john_tullamarine
1st Apr 2017, 11:43
RAT5 ... tick :E

galaxy flyer
1st Apr 2017, 14:37
Noting the magic of telephoto lenses!

Too Few Stripes
1st Apr 2017, 21:27
EASA OPS CAT.POL.A210 (AMC1)

TAKE-OFF OBSTACLE CLEARANCE

(a) In accordance with the definitions used in preparing the take-off distance and take-off flight path data provided in the AFM:

(1) The net take-off flight path is considered to begin at a height of 35 ft above the runway or clearway at the end of the take-off distance determined for the aeroplane in accordance with (b) below.

(2) The take-off distance is the longest of the following distances:

(i) 115 % of the distance with all engines operating from the start of the take-off to the point at which the aeroplane is 35 ft above the runway or clearway;

(ii) the distance from the start of the take-off to the point at which the aeroplane is 35 ft above the runway or clearway assuming failure of the critical engine occurs at the point corresponding to the decision speed (V1) for a dry runway; or

(iii) if the runway is wet or contaminated, the distance from the start of the take-off to the point at which the aeroplane is 15 ft above the runway or clearway assuming failure of the critical engine occurs at the point corresponding to the decision speed (V1) for a wet or contaminated runway.

(b) The net take-off flight path, determined from the data provided in the AFM in accordance with (a)(1) and (a)(2), should clear all relevant obstacles by a vertical distance of 35 ft. When taking off on a wet or contaminated runway and an engine failure occurs at the point corresponding to the decision speed (V1) for a wet or contaminated runway, this implies that the aeroplane can initially be as much as 20 ft below the net take-off flight path in accordance with (a) and, therefore, may clear close-in obstacles by only 15 ft. When taking off on wet or contaminated runways, the operator should exercise special care with respect to obstacle assessment, especially if a take-off is obstacle-limited and the obstacle density is high.

B2N2
2nd Apr 2017, 04:05
You to reiterate how important performance calculations are...they're not just numbers.
They're lives...

galaxy flyer
2nd Apr 2017, 16:29
A subtlety here, obstruction identification surfaces are designed from 35' above the departure end of runway (DER). When the TODR ends prior to the actual DER, all the OEI climb done over the runway is "extra", over and above the required 35' (15' wet) that was planned IAW with the OEI procedures. For example on a 10,000' runway, if the TODR was 5,000' and flown as calculated the plane could be 165' + the 35' at the DER and have that extra height throughout the OEI departure.

IIRC, some operators at SXM (St Marteen) have designed OEI procedures predicated on a turn over the runway using the actual TODR rather than the TODA to plan an OEI routing due to large mountain off the end. Obviously 'tis different than the SID.

FE Hoppy
2nd Apr 2017, 17:31
You could say that "Where is the screen height? is the question we ask with every takeoff calc.

galaxy flyer
2nd Apr 2017, 23:01
But, never have other than a maximum weight allowed, no distances, no performance speeds like refusal or Vmcg!

john_tullamarine
3rd Apr 2017, 00:07
A subtlety here, obstruction identification surfaces are designed from 35' above the departure end of runway (DER)

One of the problems with Type A charts.

The sensible ops engineer then has to look at that area either side of the Type A splay to make sure whatever the relevant splays are might be observed for the shorter TODR. Usually that is not a major problem .. but, if there be significant obstacles upon which the splay is overlaid .. a different matter altogether.

The problem with using just the declared data is tracking over the DER .. not so much these days as we get better gee whizz gadgetry to help. Years ago, it was a pipe dream to argue along the lines that the pilot, say, could track visually. About the only use of visual reference was for those departures where a DER turn was required to turn away from tiger country some distance beyond and head off down a benign track, eg HBA 30 in Oz.

lion-g
21st Apr 2017, 04:36
On a side note, why only 15' for wet runway? Been hunting around the forum and it seems like it's a regulatory figure rather than anything else.

Thank you for the enlightenment in advance.

slast
21st Apr 2017, 08:57
Wet V1 is lower than dry, so more runway is used to accelerate between V1 and VR while minus one engine. As the rotation point is now nearer the end of the runway you will have less altitude at the end of the TODR. It's a tradeoff: reduce the probability of going off the end in the stop case, in exchange for a slightly higher chance of hitting an obstacle in the go case. The choice of 15ft vs 35ft was a compromise that goes back to ICAO Standing Committee on Performance, and then Airworthiness Committee meetings in the 1960s.

lion-g
21st Apr 2017, 15:27
Thank you Sir