ATPL AIR LAW - Instruments required

[jenaby36]

I'm rather confused as to what the following paragraph is getting at, maybe someone out there can make sense of it:

CAO 20.18 Appendix II subparagraph 2 (a)

"For aeroplanes above 5 700kg MTOW, the instruments used by the PIC and which are specified (ASI, 2 x Altimeter, VSI, Machmeter) shall be capable of being connected either to a normal or alternate static source but not both sources simultaneously. Alternatively, the aeroplane may be fitted with 2 independent static sources each consisting of a balanced pair of flush static ports of which 1 is used for the instruments specified above. Instruments and equipment other than flight instruments provided for use by the PIC, shall not be connected to the normal static system that operates the instruments of the PIC."

[AerocatS2A]

It means if you are flying a medium to big aeroplane then the air speed indicator, primary and secondary altimeters, vertical speed indicator and machmeter fitted to the captain's side must be able to be connected to two independent static sources but not at the same time.

This might be accomplished by a lever somewhere marked "STATIC SOURCE", "NORMAL", "ALTERNATE". Normally the lever would be in the NORMAL position but if you had instrument discrepancies you can put it in the ALTERNATE position, but you can't have it in both positions at once because it's a single lever.

They then give you another option. You can have two normal static systems each consisting of two static ports, one on each side of the aircraft in the same position (a balanced pair.) One of these static systems is to provide static pressure to the captain's instruments described in the first paragraph and any other instruments (i.e., the co pilot's instruments) must be supplied by the other static system. That way if one system fails you can use the instruments on the other side.

As an example, the Dash 8 covers both options by having three static systems, two normal and one alternate. The two normal systems supply static pressure to the captain's and copilot's instruments separately and both systems can be linked to the alternate system via a lever in the cockpit.

Of course these three static sources are all very close to each other so it's still possible to cover them all up by leaving covers in place or leaving masking tape on after painting.

Edit: In case you meant it to be a more fundamental question, the static source is a small hole that transmits ambient air pressure via a tube to certain instruments that rely on air pressure, the ASI, altimeter, VSI, and machmeter.

[jenaby36]

Ok great, that clears things up, especially when you use an example such as the Dash 8.
The question I received was asking about the way in which the captains vs co-pilots instruments shall be connected and the two possible answers were:

a) the co-pilots static pressure instruments must be connected independently to the captain’s
b) the co-pilots static pressure instruments must be connected independently to the captain’s and must be capable of switching to an alternate static source

I went B but was incorrect. According to that paragraph in the CAO, is it true to say that the instruments do NOT require the capability of switching to an alternate static source? and hence A would be the correct answer.

[AerocatS2A]

That's correct, the copilot and captain instruments are only required to be supplied independently or able to connected to an alternate source. So answer B is wrong because it says they must be independent and have an alternate source.

As you can see from the Dash 8 example, aircraft sometimes do cover both options but it is not a requirement to.

[jenaby36]

Cool, thanks for that.
While we are on the topic of instruments,

CHTR operations >5700kg MTOW, what are the minimum instrument requirements in regards to the compass and heading indicator?

My interpretation of CAO 20.18 Appendix II subparagraph (5) is that if the aircraft has a remote indicating compass and a standby direct reading magnetic compass with duplicated power supply, then the heading indicator is not required. Do you agree with this?

[AerocatS2A]

I don't know, will have to have a read of the CAO (waiting in an airport at the moment to be flown home.)

[jenaby36]

The relevant regs are:

(5) A gyro-magnetic type of remote indicating compass installed to meet the requirements of subpara 1 (c) (ii) of this appendix may also be considered to meet the requirement for a heading indicator specified in subpara 1 (h) of this appendix, provided that it has a duplicated power supply.

(1) (c) (i) a direct reading magnetic compass OR,
(ii) a remote indicating compass and a standby direct reading magnetic compass

(h) a heading indicator