View Full Version : 767-3 PLI's and speedtape anomaly
blue up
23rd Aug 2009, 08:38
Just a quick one. Heavyweight take-off at about 160+ tons. Left side shows PLIs even after the flaps had been retracted but the right PLIs removed themselves. The low speed yellow and red arcs were much longer on the left side, also.
Any reason for this? Not much on our laptop tech manuals. Could it bee a stallwarning computer mod that has only been done on one side? UK CAA airframe.
Cheers
oceancrosser
23rd Aug 2009, 15:15
I am sure someone will come up with the exact answer, but as I scratch my head, something to do with Air Data Computer comes to mind...
But I stand humbly to be corrected.
blue up
24th Aug 2009, 08:16
Could you elaborate on this? If the flaps are 'in' then the PLIs should be blanked, shouldn't they? Vref30 plus about 100kts and one set of PLIs are still out.
What do you mean by "longer" red and yellow arcs?
The 767 Stall Warning Cards get flap position from the Right and Centre FSPM's. The slat "position" and "in motion" information comes from the PSEU and FSEU respectively. The Stall Warning system also knows if the slats are skewed. The Centre FSPM feeds the Left Stall Warning Card, so possibly there is a problem with this (or the Left Stall Warning Card).
The 747-400 sometimes has an error where the barber pole is higher on one PFD than the other, but the yellow manoeuvre margins are at the same airspeed (but too high, often greater than the min flap speed). The cause can be a faulty AOA on the side with the longer barber pole. 1 degree of AOA error can result in 5 knots airspeed error in the computation of stall speed. Your problem seems more flap oriented, though.
Rgds.
NSEU
blue up
24th Aug 2009, 16:44
NSEU. The vertical speedtape display on our 767s has a "Bank40 margin to stickshake" arc in yellow which runs from the bottom of the tape to the appropriate position and then a red section from the bottom up to bank0 degrees speed. The display on the left side showed the speeds as being faster than the right display by a few knots, suggesting that the left AoA was steeper or that the left ADC thought we were heavier than we actually were. This doesn't explain why the PLIs stayed out for longer, though. The eng wonders if it is a software error but there is no suggestion of it being hazardous. They're going to download the data when it next lands back in the UK and have a look.
He will report back to me when he has an answer.
dc873
24th Aug 2009, 19:50
There are different dash no. of the SWC,s available. The -21 and higher shows the PLI even if the Flaps are up, the older one not. Intermix ops is allowed.
stilton
25th Aug 2009, 01:17
'Sticky' Aoa vane ?
blue up
25th Aug 2009, 07:56
DC973. Sounds plausible. Strange that the eng's didn't know about that one. I guess that a heavyweight departure (Vref30 about 180?) will have the PLI displaying quite a long time out of Manchester with the 290 speed control up to FL200.
Stilton. The PLIs and speedtape bands were moving. I'm not discounting it as a possibility (computermabobs do funny things to signals) and expect to have something back from 'Eng' this afternoon.
LeadSled
25th Aug 2009, 10:00
Folks,
Based on my personal experience of this (both on 767 and 744) start with a sticky/reversed (as in strong tailwind on the ground) angle of attack vane.
Tootle pip!!
The vertical speedtape display on our 767s has a "Bank40 margin to stickshake" arc in yellow which runs from the bottom of the tape to the appropriate position and then a red section from the bottom up to bank0 degrees speed
Thanks Blue Up.. I wasn't sure what you meant by "arc" (I was thinking of something circular) ;)
The PLIs and speedtape bands were moving.
AOA's are normally damped. However, I'm sure that damping can fail from time to time, turning to intermittent stickiness. Could there have been icing conditions? Probe heat failure should generate a message, but I would still make sure there was heat present at the AOA vanes.
I've seen physically bent vanes, too, which can cause errors.
Strange that the eng's didn't know about that one.
The guys on the floor are usually the last ones to know about changes made to the aircraft they work on. Maintenance manuals seem to be increasingly dumbed down or sanitised. There is not an adequate description of the logic behind the red and yellow margins in the maintenance manuals. Often it takes weeks or even months for information to filter down from manufacturers.
Rgds.
NSEU