Flaps 8 or 20 takeoffs in the CRJ

[DairyAir]

Flex thrust is when you do not use all of the power availiable for takeoff. Say in a 172 you generally use full power for takeoff. In the RJ and most other commercial airliners, in order to increase the life of the engines do not use full power to takeoff unless we need to. I.E. icing, tailwind, short runway or terrain considerations.

The RJ can only flex to 85.0 % N1, the lowest N1 value allowed for takeoff. With a long runway the flex setting for both flap settings are usually the same, throw some heat or high density altitude and flaps 8 will win. Flaps 8 will always be able to carry more weight than flaps 20 due to the increased performance in the second segment. flaps 20 wins when you have balanced field length or terrain concerns. Hope this answers you question, however no one has taken a stab at which one is safer.

 

[DoinTime]

The RJ can only flex to 85.0 % N1, the lowest N1 value allowed for takeoff.

The CRJ can use T/O setting much lower than 85%. At -18 degrees C and sea level a full thrust T/O will be at 85%. At the bottom of the performance envelope T/O thrust can be as low as 80.9%.

Flaps 8 will always be able to carry more weight than flaps 20 due to the increased performance in the second segment. Flaps 20 wins when you have balanced field length or terrain concerns. Hope this answers you question, however no one has taken a stab at which one is safer.

As a general rule use of flaps 20 is better when you have a field length limitation and flaps 8 is better when you have a terrain limitation. As for which is safer? Neither. The appropriate flap setting is the one that is appropriate for the particular runway and airport environment you are in. In ATL flaps 8 is probably more appropriate but at GTR flaps 20 is probably the way to go.

If you fly for NW Airlink the most appropriate setting is flaps 20 because you will have no other choice.

 

[logolight]

ACA's FSM does not allow takeoff N1 settings below 85% in any condition. Also, up to this point, flaps 20 is required on wet or contaminated runways. I say "up to this point" because ACA is switching from ASAP data to Aero Data in April and I need to review the new performance specs.



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"Rudder...CHECKED...nose wheel steering...ARMED...after start checklist complete"

 

[Hovernut]

Did my first flaps 8 takeoff at COS a couple weeks ago. CA remarked and I agreed that it felt like what the larger a/c do: rotate, hang there on the mains for a second or two, then gently lift off, rather than the flaps 20 rotate, pop...you're off the ground!