Speedbrales

[PTK]

I have often used the speedbrakes to descend quickly to pattern alt and would see better than 1000 fpm descents.  Recently I was coming home and was at 6000 feet. About 15 miles out I deployed the boards and something happened that I had not seen before.  I hardly got any descent!!  I cycled the speedbrakes they operated normally but hardly any descent.  It was very cold 30 degrees.  Has anyone experienced this?  I'm thinking trim has something to do with this.  Or maybe the cold air.  I had the autopilot engaged on alt hokd prior to this.  I disengaged it before I activated the speedbrakes.  Any input is appreciated. Thanks.

[randypugh]

I've never seen this before. Did the airplane shutter with the brakes out? I always get a good vibration (shutter) when the speedbrakes are out so you know without looking what's happening, then it just starts down and slows down.

[Parker_Woodruff]

I don't think it's going to be a function of cold air, but a function of IAS.  Temperature means nothing in terms of IAS except for its effect on TAS.

[PTK]

Thank you for the reply. No shutter. I know what you mean. This is why this seems odd to me. Usually there is a shutter that gets your attention.  In this instance nothing!! I'm thinking  the autopilot had the airplane trimmed to hold altitude and it was still at this trim attitude when I deployed the speedbrakes. Does this make any sense?? 

[Parker_Woodruff]

Quote: allsmiles

Thank you for the reply. No shutter. I know what you mean. This is why this seems odd to me. Usually there is a shutter that gets your attention.  In this instance nothing!! I'm thinking  the autopilot had the airplane trimmed to hold altitude and it was still at this trim attitude when I deployed the speedbrakes. Does this make any sense?? 

[PTK]

I was maybe 135 knots.  So the slower the spees the less the descent rate?  

[randypugh]

Yes, the slower you are the less effect they have. That's why they are approved in all phases of flight.

[Parker_Woodruff]

Quote: allsmiles

I was maybe 135 knots.  So the slower the spees the less the descent rate?  

[jlunseth]

It sounds like you had the autopilot ALT hold on.  Was that the case?