INS Alignment

[deltafan 1]

Hey guys, I'm doing some flights with an Alaska 737-200 using the CIVA-INS 1.40 around some of the small airports, commercial bush flying. I was wondering that while on the ground at these airports, before flying to the next one, is it better to find a VOR near the airport (if there is one) to perform a DME update for the INS, or is it better to shut it down and realign it if I have time? I am following published Alaska flight schedules, so I usually have time, about 30 minutes on the ground at a time.Also, if I shouldn't DME update, I can assume DME update is only good for when I am in the air? And is DME updating efficient or necessary on short legs?Thanks in advance.

[JP_Visser 3]

iirc you are only supposed to perform DME updates when airborne.also,it might be even better,seeing as you have time,to shut the unit down and then align again,using the proper coordinates.i myself just punch in a new set of numbers,and then carry on,as i can always perform a dme update while airborne.once the unit is aligned it'll always know where it is,so i finf it cumbersome to re-align every time i land atsome small gravel strip only to wait ten minutes for it to align again.i usually fly using vor/ndb nav though,as that seems the way most 732's flew.cheersJP.

[Bob Scott 7,721]

It's generally better, in the real world, to dump the platform and realign during any ground stop. The reason is that the rate of drift increases the longer the platform is in NAV mode...basically at an exponential rate. All you are doing with any update is providing a known-error offset based on the difference between the known position (derived by DME or a manually-entered position) and the raw gyro position. The gyro's raw position is never changed once it goes into NAV mode...all that happens is an error-correction routine in the INS uses an offset based on the last update to apply a correction to the gyro's raw position, and that corrected position value is then used for all navigational computations in the INU.But the longer the platform is in NAV mode, the larger the drift rate becomes. So even though you update the position, it will drift off of that updated position faster and faster still the longer it has been in NAV mode.So the ideal is to realign and enter nav mode close to engine start and have a freshly initialized interial platform at the start of each leg. There aren't many ground times less than the 10-min alignment period, anyway.RegardsBob ScottATP IMEL Gulfstream II-III-IV-VSantiago de Chile