Aircraft shaking as aircraft prepared for take off roll
 

Just returned on a flight from ORD and on a A330 and I had an unusual experience and was wondering iof anybody can shed any light on it?

Weather as Chicago was very cold and very foggy. There was snow but it wasn’t actually snowing and no need for de-icing. As we lined up for take off the engines began to rev in preparation for the take off roll but the Aircraft didn’t move and instead began to shake quite violently for about 7 or 8 seconds whilst the engine revs increased to a level where you would certainly expect to be moving! Eventually the Aircraft did began to move and the take off and flight was as regular as can be. I've flew on this type of A/C many times before and never experience this before.

Any suggestions?

They were performing a de-icing run on the engines. Normal procedure to shed ice from the back of the fan blades that can accumulate in the conditions you describe (ie. freezing fog). On the 330 it's 50% N1, which is quite a bit more than normal ground power settings, for 10 seconds.

Many Thanks for the reply

Tim

Core de-ice procedure in Freezing Fog. Probably find the A330 was RR Trent 700 engined.

http://www.pprune.org/tech-log/20694...e-removal.html

DTG

For ground operation there are three possible sources of ice to be shed. One is ice adhering to the back side of the fan blades (most common to freezing fog) The other is ice adhering to the nose spinner on the very outside front of the engine (where the black and white appearance keeps changing as you taxi at different power settings) and the third is the core inlet vanes behind the fan blades that can only be seen by a few passengers on one side looking through the fan blades from their window line.

The idea behind shedding the ice on the rotating components (the first two) is to get it up to a high enough centrifugal load to shed all the ice. Since the ice never sheds evenly some vibration may be experienced as you spool up.

For the later source of ice (on the non rotating inlet vanes to the core behind the fan) you need windage loads to break the ice loose (there is just not enough heat to melt it loose). most manufacturers recommend 60% N1 to provide this windage (read the manual and check). Again the idea is to shed this ice before it becomes thick enough to cause downstream compressor damage. If you don't perform this operation often enough then its sure to shed at takeoff power and result in downsteam damage to the blades. The greatest problem is with internal blade downstream damage causing the engine to surge shortly after rotation so be prepared and follow your manuals faithfully.

Maybe the plane was ffffreezing:}

In addition to the replies already received, some types require these higher thrust settings prior to brake release to ensure that the engine inlet is ice free.

The venturi/rapid cooling effect at slow speeds if not done as described can, on some types, lead to rapid inlet ice accumulation....which tends to break off just at rotation.
Not good.