Time Delay & Engine Spool Up Time
 

Hi,

It seems that some aircraft are fitted with time delay that keeps engine spooled up on touchdown.
Is this time delay a basic item on all airliners (Boeing, Airbus…)?
What’s the purpose of this time delay?

Feedback appreciated.
Regards

Keeps Approach Idle engaged instead of Ground Idle for faster spool-up time in case of a go-around or rejected landing. It's on the 744.

Also used on the 737NG...

For faster spoolup.....for go-around or reverse, since max benefit from reverse is at higher speeds.

EW73 :)

Boeing likes the term Approach Idle, 757 is the same and I think it was a gain of 20% n2. Flight idle is another term but the minute you break ground it is alive.

Actually there wasn't a time delay on the aircraft I flew - flight idle mode was entered in the air when the gear was lowered and changed to the lower rpm of ground idle when the undercarriage weight on oleo micro switches closed.

My first jet aircraft took around 13 secs for the engines to fully spool up.

Hmmm, reminds me of the B707 straight-pipe powered types that I flew, long ago.
You could nearly go out to lunch waiting for spool-up...:*

As far as I'm aware, as it's normal system operation, the resulting landing distance is the normal when not using the reversers, see the performance tables.

Once the mains are planted, you can start the selection process for the reversers, whilst you are smoothly lowering the nose, then throttle the reversers at that time, that's very efficient use of the system.

BTW...the 737NG also has a backup system where if there is a fault preventing the EEC sensing of the engine anti-ice system or flap landing positions, the engines will go into Approach Idle at 15,000 feet AMSL on the descent.

As far as an RTO is concerned, since the weight remains on the wheels throughout, the EEC keeps the engines at minimum ground idle (normal ground idle setting).

EW73 :)

ReReading your post aerotech, sounds like what you are hearing is a commanded advancement in thrust while in reverse to slow the aircraft down.

In the 744 we ALWAYS select reverse after the landing is committed, even if we only expect to use idle reverse.

Assuming the RTO is began with thrust levers at take off position, then by the time you whip into reverse at high speed the N1 would be passing about 60 percent by the time the power had started to spool down and reverse levers actuated. In other words the situation should never arise where the engines have spooled down to idle thrust (23 percent N1) before you pull the levers into reverse thrust.