Ian, question about the 67's VVI: In PIC it's apparently integrated with the IRS systems (is not functional unless there is full alignment). I would have thought that the VVI would have been part of the pitot/static system as in other aircraft but obviously there is some advantage of using the IRS. Just wondering, what is the deal? Thanks very much, I always enjoy your insight on this stuff.

Light aircraft have a VSI (vertical speed indicator) which runs off the static system. VSI's always lag slightly and can not be relied on for accuracy.Jets have an IVSI (instantaneous vertical speed indicator) which is always 100% accurate, NO LAG at all.To achieve this the IVSI runs off the IRS system, which is able to provide instantaneous acceleration information to the instrument.

From my 757/67 pilot's reference manual, An electrically driven VSI is installed on each pilots panel. The captain's VSI is connected to the left IRS and the F/O's VSI is connected to the right IRS. The center IRS can provide alternate VS data for either crew member when ALTN is selected on the respective IRS switch. The VSI is inoperative until the associated IRS has completed alignment and entered the navigation mode. Jim

HiThis is true, the V/S information is taken from the IRS in modern jets.This is however not the only IVSI, the more simple IVSI is a static rate instrument, with a double piston arrangement, whereby the pistons move in realtion to the vertical acceleration, the pistons create a drop in static pressure (climb) allowing the instument to read before the actual static pressure has caught up as it were.Thease instruments are used on older aircraft and possibly on twin GA aircraft im not sure.Stumar.

H, Jonas.The IVSI's do indeed use the IRS's, but, they also use the old fashioned Air Data system. The IRS's provide short-term /instantaneous vertical speed and the Air Data system's provide long-term vertical speed data. This is because the IRS's slowly build up errors over time: As I understand it, as the aircraft flies along, the IRS's are constantly measuring the "ups and downs" of the aircraft with accelerometers. As you can imagine, even slight turbulence will make the aircraft go up and down hundreds of times during a flight. Over a long period, the constant addition of tiny positive and negative accelerations will eventually result in a small non-zero value for vertical speed (even when the aircraft is flying straight and level in cruise). The pitot-static (Air Data) System corrects this (it's better at long-term assessment of vertical speed than the IRS's).As I recall it, the IVSI will also become active in IRS ATT mode (but don't quote me on that... I may have to double check this at work ;-)). Cheers.Ian.

You learn something every day!;-)

>You learn something every day! >;-)....and it may be important to note here that either the loss of the IRS data or Air Data to the IVSI will result in the failure of the instrument (although, I read, you can use instrument switching to lessen the damage). ;-)BTW, I think I learnt about 20 things yesterday, but I still feel like I'm losing the race with technology *:-*Anyway, gotta go... More things to learn :-hahCheers.Ian.

Thanks for the insight guys, very interesting stuff. Dang you people are smart.