Standard Take-Off Minimums

[24 carat]

Can anybody help me with this one:

Why is it that the standard take-off minumums for aircraft with 3 or 4 enigines is half that of aircraft with 1 or 2 engines (2400 ft RVR vs 5000 ft RVR)?

Many thanks.

 

[A Squared]

Can anybody help me with this one:

Why is it that the standard take-off minumums for aircraft with 3 or 4 enigines is half that of aircraft with 1 or 2 engines (2400 ft RVR vs 5000 ft RVR)?

Many thanks.

Because pilots of 4 engine planes do everything better than other pilots, including seeing. We just don't need the same visibility as mere mortals require.

 

[MrSuupaFly]

Maybe it has something to do with the Takeoff Alternate Requirements. A two-engine aircraft's alternate must be no more than 1 hour from the departure airport at one engine inop normal cruise speed in still air. A three-or-more-engine aircraft requires that time to be no more than 2 hours. Some companies are authorized to use lower than standard takeoff minimums, as low as 500 RVR (no matter how many engines).

 

[nosehair]

Single-engine takeoff briefing: "If engine failure occurs on take-off with runway remaining, land on remaining runway."
Two-engine airplane briefing: "If engine failure occurs on takeoff with runway remaining, close throttles and land on remaining runway."

This is kinda standard because it is kinda standard that in most singles and light twins, there may be available runway to land on, so a mile vis would give you that option on a 5000 foot runway.

But 3 and more engine airplanes have a much more limited capability to land on remaining runway and much more capability to continue the takeoff into IMC, so the vis is reduced to a more workable number.

Also, we are talking part 121, 125, 129, and 135 operators which each have their approved minimums which may be different. The real answer is in the operator's justification.

If you are operating part 91, no takeoff minimums apply. Just good sense to abide by these standards, but not regulatory.

 

[CFIse]

Single-engine takeoff briefing: "If engine failure occurs on take-off with runway remaining, land on remaining runway."
Two-engine airplane briefing: "If engine failure occurs on takeoff with runway remaining, close throttles and land on remaining runway."

I'm sorry - I fly a woefully under-powered jet and yet that is no part of the takeoff briefing. the takeoff briefing, like it is for most multi-engine jets is "before V1 we stop, after V1 we go".

So the question stands - why does having 3 engines give the pilots better eyesight than just having 2 engines?

 

[A Squared]

I'm sorry - I fly a woefully under-powered jet and yet that is no part of the takeoff briefing. the takeoff briefing, like it is for most multi-engine jets is "before V1 we stop, after V1 we go".

So the question stands - why does having 3 engines give the pilots better eyesight than just having 2 engines?

Yeah, I tend to agree with that. I'd much rather continue on into low visibility in a most jets with on one engine than try to continue a takeoff with a Trilander (part 23, 3 engine airplane) at max gross weight.

 

[nosehair]

So the question stands - why does having 3 engines give the pilots better eyesight than just having 2 engines?

Notice I used the word 'kinda', meaning this is sort of a general idea. There is no 'one-size-fits-all' rule. Your procedure may vary. Jeez !

This rule has been there since before there were jets, and especially the modern breed of 'underpowered' jets. My thinking is of 4-engine props.
And again, each specific airplane, pilot, and/or company has it's own procedure.

Again, it only applies to commercial operators, so the person to ask is your chief pilot.

If you are flying a jet part 91, it does not apply.

 

[ptarmigan]

Answer is in the amount of performance lost with an engine failure. All else equal, the climb gradient is much less after losing 50% of your power as opposed to just 25 or 33%. That means that you can clear obstacles better. A mile allows for better 'see and avoid".

Obviously, the Ops Specs take that into consideration as well as other factors, which allows twins be able to go down as low as any others.

 

[nosehair]

Answer is in the amount of performance lost with an engine failure. All else equal, the climb gradient is much less after losing 50% of your power as opposed to just 25 or 33%. That means that you can clear obstacles better. A mile allows for better 'see and avoid".

Obviously, the Ops Specs take that into consideration as well as other factors, which allows twins be able to go down as low as any others.

Exactly what I was trying to say, but in a much better way. Thank you.