Takeoff performance - Intersection vs full length.
 

If you calculate your takeoff performance from an intersection and ATC subsequently clear you to a holding point which will result in a full length takeoff, is it necessary to recalculate?

Which limit will come to bite You when You use an intersection with more runway length available than the one You calculated?

- TORA/TODA? No, because You have more runway ahead of You than You calculated for.

- ASDA? Neither, because You have calculated the speeds for a shorter runway and should therefore come to a halt at a distance from the far threshold equal to the additional distance You gained from the non-intersection T/O in case of a rejected takeoff.

- Climb gradient? Neither, as You rotate and lift off earlier on the runway than You calculated, thereby increasing the distance from the obstacles.

- Brake energy? No, as You are still using the same V speeds as You calculated for the more limiting intersection.

- Vmc, Tire speed? no, not really.

So: if You have calculated Your performance for a more limiting intersection than the one You actually use, You should be good to go. However, You may be missing the opportunity to use a higher derate/flex or whatever a reduced power takeoff is called on Your type.

If you go from the full length, the obstacle splay will be different, and you may now have a lateral obstacle that was not limiting for the Intersection case.

Are you sure?

I too am left scratching my head. The lateral fan starts from the end of the Take-off distance available (at least under our rules). It's fixed; it doesn't vary whether you get airborne halfway down the runway or right at the far end.

Just checking... good to see you blokes know your stuff! :ok:

However keep in mind that:

PANS OPS
2.3 BEGINNING OF THE DEPARTURE PROCEDURE
2.3.1 Aeroplanes
2.3.1.1 For aeroplanes the departure procedure begins at the departure end of the runway (DER), which is the end of the area declared suitable for take-off (i.e. the end of the runway or clearway as appropriate.)
2.3.1.2 Since the point of lift-off will vary, and in order to protect for turns prior to the DER, the protected area
begins at a point 600 m from the start of runway. This is based on the assumption that the minimum turn height of
120 m (394 ft) above the elevation of the DER could be reached 600 m from the start of runway.
Note.— The elevation of the DER is the elevation of the end of the runway or the elevation of the end of the
clearway, whichever is higher.

This said, there are SIDs that prohibit to turn before the DER. When this happens, it is depicted.

SID and EO have different flight paths but you should be covered in most of cases unless you depart with a small prop (but there is no flex, isnt it?)

I have seen performance charts for an intersection departure that gives a slightly higher RTOW than the full length with the same flex temperature and flap setting. I have queried the reason why but don't remember the answer, I think it was vague and the person answering couldn't remember either. What he did know was that it had been queried in the past, checked, and found to be correct.

I have seen one such table too, but this was rather a definition issue than anything else.

In HAJ, on the 27R, lineup is normally via taxiway N at the displaced threshold (with 3200m TORA), and this is defined as "full length". For performance reasons, lineup via M may be requested, giving an extra 300m. This is defined as "intersection" by the authorities.

This is the only runway I have ever come across where an intersection T/O gives more distance than a full length departure.

The one I've seen is for a genuine intersection. The acceleration altitude is also slightly different. I'm looking forward to the day I have to "require" the intersection departure because we don't have the performance to depart full length. ;)

This conundrum rings a bell with me. I think Lantirn may have hit on the relevant issue that in rare cases could favour the intersection departure. IIRC, it's a close-in obstacle that ceases to be an issue if the take-off run is started beyond a certain point on the runway. Could be tall trees or structures close to the side of the runway, such as a control tower. So it's a NTOFP limitation, not WAT.

Think of it as a rolling takeoff from the intersection. :O

This seems to be true... OMAA 13R . My b737 Rtow charts for various intersections have different ACC Alt . The ACC alt is higher at the Beginning of the runways then reduces and then increases again.

Chris Scott, how does Lantirn's post answer any mysteries? The rule he quoted says start of runway, not start of run.

I've seen better preformance from an intersection too. Calculated directly from A big OEM's certified performance tool, at home-base where accuracy of runway data was easily verified.

After about 3 hours (net) of scratching head it turned out to be a funny - still correct though - quirk in a game of slopes, pressure altitudes and number roundings. Only AST value was affected, not MAX PERF WT.

This apparent discrepancy may be because of the runway analysis to determine V1. On YSSY 16R an intx departure gave a lower V1 for the B747 Classic using the same weight because the runway slope was different starting from the intx.

The take off performance is optimised for nine limitations. The MTOW should not increase with reduction in TOR/TODA unless the limitation codes change. Can you give codes for full length and intersection take off?

The logic (Yorkshire style) where a shorter (same) runway gives a greater RTOW than a longer (same) runway seems to defy common sense. It could only be a number crunching game that only a computer could devise. I'd be delighted to read a full explanation because my brain does not compute that idea.
My TomTom tells me that route A is faster (not necessarily shorter) than route B. But I know that from experience that not to be the case; at least at certain times of day. Equally I know that the 'short-cut' through the housing estate with speed bumps is definitely not the quickest. It might be the shorter by 200m, but hey, it's a computer number cruncher.

It sounds to me more a confusion about flex versus derated takeoff in a Vmca limited takeoff, rather than a full versus intersection takeoff.

Which one?

Lantirn
I also think so. Derate can increase MTOW on shorter RW due to reduced VMCG but not on flex or full power.