View Full Version : VSI's & DFDR questions

28th Dec 2001, 00:04
Recent Instruments question. If a VSI has a partially blocked orifice then:
a. The instrument will under read
b. The instrument will over read
c. There will be no effect on the reading.
I put (a) down, going on the theory that there will be a time lag induced, until the pressures inside the VSI equalise. Anyone think it's different.
Also, when does a helicopter's DFDR become active? Is it:
a. On engine start
b. When the battery is turned on.
c. When the machine lifts.
Any help much appreciated. Season's greetings to everyone.

28th Dec 2001, 23:53
I'd go for answer b.),it will OVER read,the instrument will be calibrated for a certain orifice-diameter.If the orifice is (partially) restricted this should result in a greater deflection.
Can anybody prove me wrong?I've had 2 beers..

2nd Jan 2002, 04:33
Agreed B2N2, answer B is correct for the reasons stated. Try more beer.

4th Jan 2002, 03:26
And while the second question is stupid IMO since that is up to the manufacturer to decide, the DFDRs I'm familiar with are turned on when you turn an engine on since you're not too interested in recording half an hour of setting the switches right. If there's a complex hyd system with electrical backup pumps I guess it could be of interest to have a bit of recording going already on power up, for troubleshooting if nothing else.


4th Jan 2002, 14:36
A standard pressure operated VSI admits static pressure into a belows, with a capilliary tube "leaking" that pressure into an airtight instrument case.

If the static vent were partially blocked, then the instrument would underread, if the orifice on the capilliary tube were partially blocked, then the instrument would overread.

The Flight Data Recorde on the 737 at least is powered from the No. 1 Transfer Bus and the Battery Bus. On the ground it starts to record when either engine oil pressure switch exceeds 35 psi - in the air it records regardless. I would gess that the DFDR would start recording on engine start.

4th Jan 2002, 19:11
static vent partially blocked:


(D)FDR become's active during an engine start (usually oil pressure switch).


6th Jan 2002, 02:16
If the static vent is blocked the air would move through the static vent at a higher velocity and lower pressure. Wouldn't the VSI underread then?

9th Jan 2002, 10:59
Can't help on mechanical VSIs - all our aircraft use DADCs.
The requirement per ED-55 is to record data from engine start (applies to fixed wing & rotorcraft). A lot of Boeings (e.g. 767) and other aircraft like the CRJ use the Beacon or Strobe Light Switch(es) as the SOP is lights on prior to start. The back-up is WoffW to ensure FDR is recording in air even if lights are forgotten or switch failed.