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Originally Posted by A350XWB . It's not that these airports are not served by twins, but the payload limitation is higher in case of twins. For twins, a single engine failure means 50% loss of the total available thrust and for a quad, it is a 25% loss.
You can clearly see the single engine out scenarios and the available thrust for each type (Thrust to MTOW ratio), in such a scenario. The fall in numbers are significant for a twin and not so much, for a quad. So, the payload limitation imposed on a twin is more in case of these specific airports, than a quad, to keep a safe margin.
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Couple of comments.
It’s difficult comparing twin to quad performance as they are always very different planes. There are no planes available in both a twin and a quad configuration to my knowledge? So it’s always a bit of an apple to pear comparison. But lets look at it from a real life/practical perspective:
Obviously, when an engine fails on a twin you have lost 50% of your thrust. But you can’t just relate reduction of thrust to how much payload reduction is required. Twins are vastly over dimensioned to start with. It’s not how much thrust you have lost, but how much thrust remains and what performance remains.
The initial climb consist of four segments and quads are required to have better performance in these segments on a one engine out then twins!
https://aviation.stackexchange.com/q...gments-defined
There are twins around that really struggle on one engine, there are twins around that have unbelievable one engine out performance. On any plane, irrespective of how many engines, the performance after an engine out is heavily depended on your total pay load. If you were at half the max weight to start with, an engine out performance is substantially different compared to being close to your max weight.
As you mentioned, there are other considerations as well. If you loose an outboard engine on a quad you have to dial in considerable opposite lock on the rudder and ailerons to keep it going straight. Lots of drag!
The below charts are from my KLM AOM. It’s a bit dated, but the principles remain and these sort of charts are still in use in every cockpit and every dispatch/planning department. Although, these days an app might give you the numbers a little faster then tracing all these lines. When I did my pilot test on both the written (so called knowledge test) as well as the oral test we were using these kind of charts. No electronics allowed!
This one shows you how ambient temperature and field elevation affects the Maximum Take off weight with relation to climb ability.
For instance, if it’s 30oC at an airport with an elevation of 10.000 feet this chart will tell you your MTOW is 293 tonnes. At the same temperature but at sea level that would be 428 tonnes!
Next: Is the runway long enough, taken the weather conditions into consideration as well.
The example shown here:
On the right hand side is the runway length in feet. So here we are looking at a 3000 feet long runway, with a 1% upslope, 10 knot headwind. Go the left, at 20oC, sea level, your maximum take off weight is at the intersection of the two lines, so 373 tonnes.
Note that none of these performance charts mention engine out. It’s a non issue for the take off. You won’t commence a take off, with one engine out of course. It might happen during the take off run. Typically below the so called V1, the pilot would stop the plane. Everything after the take off is just climb performance.
Whereas you wouldn’t take off with a wonky out of service engine, it can be perfectly legal to take off with a braking system out of order. So there are similar tables that show,next to the normal brake energy limits you need to stay within, whether you can still come to a full stop, prior to V1 with fewer brake systems operationally.
When you plan a flight you have to ensure you can safely execute upon that flight. So you plan by using these sort of charts tools. Whether you have one, two, four or more engines if these charts tell you it’s ok, in essence you are ok. (A bit simplified, but you get the point.
On some airports there are restrictions on minimum vertical speeds. You would always plan that first with all engines running. In certain cases it could be relevant to check what an engine out scenario does. If the restriction is only for noise abaitment it’s not a problem. But if you’re taking off from a valley with high mountain tops on the left and the right, you might have to be able to reach a certain altitude to get over the next mountain in front of you and diverting is simply not an option.
I fly single engine planes. In essence there are no formal legal constraints on where I can fly based due to a possible engine out. As you can imagine on a one engine plane a one engine out scenario does not leave many options, usually!
Jeroen