That sounds like a "at the bar with a beer" explanation... (nothing negative, but that's how stories go along in this world)Matt wrote: ↑11 Jan 2023, 13:50
Payload available (fuel, passengers, cargo etc) for each type would be about
...
Johannesburg is a HOT AND HIGH airport. Even the A359 from Delta struggles to do the ATL-JNB route. It makes it barely.
Note how well the 346 and 343 are performing out of JNB. There is a reason why SAA had so many. You NEED power to get out of JNB. Also note the runway length of JNB. Over 4000 meters. Why you think that is?If you ever flew to JNB: didn't you find it weird that the take off roll is exceptionally long?
First off, payload is paying-load: pax & cargo. Fuel is not payload. Important because payload is limited with a maximum zero fuel weight.
Secondly: the reason why a twin has a disandvantage in hot & high conditions has little to do with runway length in itself. High density altitude has an aerodynamic effect (higher true airspeeds required) and (maybe) less thrust available. But twins and quads are hit equally hard by the reduced aerodynamic performance (increased true airspeeds required) and less thrust available (if you need full thrust)
Because you talk about runway length, it is actually an interesting idea to compare twins and quads. The advantage of a quad vs a twin is (always, not strictly hot & high) the engine out climb performance. A twin is hit harder in the "2nd segment" compared to the 4 engined aircraft. For this reason you will find that twins are actually higher powered than quads. And because quads need "less total thrust" to cope with engine failure regulations, you will actually find that they are quickly field limited. In other words: if you would compare similar takeoff performance of a quad and a twin, you would see that the twin would takeoff with less runway required because it uses more power. The long runway is long because of density, and because the quad doesn't accelerate like the twin.
On lower altitude airfields, a twin can be restricted by terrain and use alternative procedures to avoid terrain and lower the 2nd climb segment. At high density altitude, this is not an option anymore. You can "unbalance" the takeoff performance, but that effect is only limited as at high density you already have increased true airspeeds/groundspeeds. There is also tire speeds and brake energy in case of a high speed reject to take into account.
I'm not familiar with the A330-NEO, I know some A320 had a "thrust bump" option, I've seen it in 737-NG FCOMs as well. This is a modern world of jet engines flying around with derated engines, thrust limited by money and maintenance plans (and not by capability), the thrust bump option allows increase of thrust for a specific takeoff situation (unlike the water ingestion used by the 747 back in the day, which "boosted" the engine).
Anyway, some background to show you that if you would look at runway length and thrust, the twins are the winner... but it is hit hard in the possible engine out case due to the high density. Compare the A340-300 to the A330neo. The A340 is probably the most underpowered jet in the skies these days. It is so underpowered, ATC probably wouldn't even notice it had an engine failure
