En-route speed/mach question

[tubby linton]

I have a question about the filed plight plan speed and adherence to it in a normal radar controlled environment
The fms in my aircraft never seems to fly the filed flight plan speed.It can be up to fifteen knots slower especially if I have decided to stay at a lower level for a better wind.The fms reduces speed as weight burns off and having started a longish flight at 0.78mn it can be back at 0.75mn as I approach the descent point.My question is should I be flying the filed speed and thus a constant MN or is it acceptable to let the fms do its own thing?

[Lon More]

ICAO says changes of +/- 5% of Flight Planned Speed should be advised IIRC

[Blockla]

If it's FMS speed reduction from M.079 to M.076 it's unlikely that it'll happen all at once, so no need to say anything...

Except in 'specific environments' such as oceanic, filed mach numbers/speeds will be not even noticed by the ATC... Most plans have a TAS speed anyway and very few ATCs will be accurately predict what that means relating to Mach.

However, most ATCs are very good at 'predicting' what speed a particular type will be flying... So unless you want to fly outside a 'normal' profile such as slowing up for a slot time/curfew time etc then we probably won't notice/care. Where we need a specific speed we'll let you know... As a rough guide if you are changing M.03 (in one go) or more let us know ASAP...

[tubby linton]

Thank you for the info.Is there any reference for the +/- 5%?

[5milesbaby]

Except in 'specific environments' such as oceanic, filed mach numbers/speeds will be not even noticed by the ATC...
There are some area sectors where filed speeds are used to decide streaming order too.

If a controller thinks that aircraft may be a problem longitudinally then they will apply direct speed control. However, an Mach number issued may mean a request to increase back to what you have filed although the slower speed would have worked fine - but this happens as we don't know what you are actually flying (and don't always have time to ask).

A good rule of thumb could be that if you think we may need to know that you are slower than filed/usual then that should be a trigger to inform the controller regardless.