Take-off performance calculation errors

[JammedStab]

I am currently reading a report about these types of errors. One example I don't understand is what happened in this case. How would a visual departure affect takeoff performance.

"Boeing 737: September 2007

Location: Alice Springs, NT

In preparation for takeoff, the crew calculated the take-off performance data based on a required navigation performance (RNP) departure. While taxiing, the crew were advised by air traffic control (ATC) that there would be a delay for the RNP departure due to an inbound aircraft that required priority. The crew received a revised clearance from ATC to conduct a visual departure. After takeoff, the crew realised that the takeoff data had not been checked or amended to take into account the revised departure clearance."

"A change in operational or environmental conditions was recorded in six of the 11 occurrences. This change necessitated the crew to either check, amend and/or update the take-off performance parameters previously calculated. These included:
• the ambient temperature increasing above the FLEX temperature
• a change from an RNP departure to a visual departure

• a change from a full-length runway departure to an intersection departure
• MAC TOW re-calculation using a ZFW work-around
• an unserviceable ACARS printer requiring verbal transcription
• a change in V speed/s due to a wet runway."

[mutt]

How would a visual departure affect takeoff performance. Their takeoff weight was based on the obstacles assessed in the RNP departure, just say that it went left after takeoff, but the visual departure went straight ahead and might have had a more limiting takeoff weight due to different obstacles in the takeoff path.

[JammedStab]

I am not aware of any airport that we operate to where we put the departure procedure into the calculations for takeoff V-speeds or runway limit weights. Is this example a unique situation?

[aviatorhi]

There are plenty of airports which have a special departure published in your performance data in order to meet climb segment restrictions.

That being said this sounds like typical CML flying... Fly right into the mountain because the FMC said to.

[GaryGnu]

Jammedstab,

I don't know if you work in the same jurisdiction of the operator in the example you cited, however, Aus CAO 20.7.1B - 12.1.1 (b) allows the lateral expansion of the takeoff area to be discontinued when 0.125xD intersects the RNP containment area (2xRNP).

In plain language, when doing an RNP departure the area for obstacle assessment is narrower, potentially allowing greater uplift as limiting obstacles would not be considered (as mutt alludes to above).

I would hazard a guess that the operator concerned has an RTOW chart labelled "RNP" or it's EFB performance program allows selection of a variable or runway label that accounts for the reduced takeoff area with an RNP departure.

Alternatively, the "RNP" option may only be provided at airports where there is a commercial benefit for doing so. Cairns RWY 15 (when RNP departures were used) comes to mind as an example.

[JammedStab]

I guess what happened was that the crew flew a different routing than the RNP routing and this route had higher terrain and therefore had there been and engine failure at a during a critical time period, terrain clearance would not have been assured.

Does this sound reasonable?

[Capn Bloggs]

Does this sound reasonable?
Yes! In this instance, with the benefit of local knowledge, they were probably departing off YBAS 30. I don't fly RNPs but, if their RNP did an immediate left turn to the south (no hills on that path), that they would have had XXXXXkg RTOW.

Now if ATC had said "maintain runway heading", straight towards the large mountain (ok hill) range on the extended centreline, their RTOW (they would have had to have used a different RTOW chart or different departure in their EFB) would have been less. Some departures may be so different, obstacle-wise, that you need to use a different chart/EFB input to arrive at the RTOW.

[underfire]

Enjoy!

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