Gen Nav -conflicting feedback

[going round]

I have come across the same feedback question from Bristol and Oxford that appear to give different answers. Can anyone give the diffinitive answer (thats to say the one that the CAA says is correct!)?

The Oxford question states:
If an INS is switched off and then on again in flight, which of the following happens?
a) It is unusable for the rest of the flight
b) Carry on as before
c) It is usable after a position update
d) It is usable in ATT REF only

The correct answer is given as (d)

The Bristol question states:
A pilot turns off the power to his (or her please!) IRS in flight. If S/He switches it back on after just a few seconds will the effect be
a) The IRS can be used providing the position is checked
b) There will be no effect and the IRS can continue to be used
c) The IRS cannot be used and should be shut down
d) IRS cannot be used for NAV but can be used to give attitude information

The correct answer is given as (c)

I note that one question states INS and the other states IRS. Is this significant?

On a similar matter I have found some contradiction in the text notes of 2 schools regarding whether for practical navigation purposes the earth is considered a perfect sphere. I would say it is but has found one publication that says not.

Anyone?

[Thunder Child]

Seen this question before.

Would definitely go for Oxford's answer - D

I think Bristol are trying to screw with your brain here. Think about, are you actually going to get attitude information from the unit that you've momentarily shut down? After all, it doesn't come with a pictorial view to display attitude - you look at the AI for that.

Both answers are correct, as the INS in the Oxford wording can still be used in Att Ref mode (when used in conjuntion with an AI).
Bristol's answer C is more correct than D, as they are specifically referring to IRS against using the unit for attitude information.

Oxford's wording is much more like what you'd come across in the writtens, so I'd focus more on that syle than Bristols'.

Difference between IRS / INS - you should know this if you're doing ATPL's!!

IRS - Utilises Ring Laser Gyro
INS - doesn't! Old version

[Grivation]

Ref the shape of the earth question - the practical application of navigation considers the earth a perfect sphere for journeys up to 400' and an oblate spheriod for distances beyond 400'.

In fact up to 400' you can consider the earth as flat (plane sailing). After that distance we get into the relm of mercator sailing and (god forbid) the PZX triangle to calculate great circle waypoints using spherical trig.

Thank god for the FMS!!

[Alex Whittingham]

I think we are wrong here and Oxford are right, my apologies.

I've researched the checklists for both the stable platform INS and the IRS systems and in each case there is a procedure for using the ATT mode after a failure of the power supply for one reason or another.

In the INS case you need to fly level for a period without accelerating to allow the gyros to align, the checklist says 'until the attitude failure flags on the ADIs are withdrawn'. For the IRS you fly level for 30 seconds, it doesn't say this but it would be to allow the IRS computer to detect the gravity vector, then you must input mag heading on the POS INIT page of the FMS or on the IRS control unit.

When operating in ATT both systems provide pitch and roll attitude and heading outputs.

Thunder Child, too complex and too paranoid! The JAA exams are generally much more straightforward than the old CAA exams. Most questions that appear to rely on dubious logic just look like that because of imprecise feedback or translation errors.

Concerning the 'shape of the earth' question it depends which nav systems you are talking about. The great circle and rhumb line questions and the concept of departure all assume a perfect sphere. Some older self-contained nav systems, particularly the Doppler based ones on V bombers, also assumed a perfect sphere. INS and Omega systems assume(d) an oblate spheroid whereas GPS and modern FMS nav systems assume the WGS84 spheroid. The statement you are talking about is usually made when you are teaching the very beginnings of navigation theory and is 'applicable until told otherwise'!

[peter knapp]

If you switch the INS or IRS off in flight the gyros have started to wind down. The system is completely unserviceable - I tried this when I was flying Hercs it did not work. Remember that for INS (gyros) you cannot re-align in the air but IRS can be re-aligned but is then only usable in the ATT mode.

[Alex Whittingham]

Well I've never tried it, Peter, so I'll bow to your experience.

For interest, I took my reference from the INS troubleshooting table in the L1011-500 operating manual under 'abnormal procedures'.

A drill is given for a situation where the ADI has tumbled, and flags are in view on the ADI, HSI and RMI. The INS CDU warning light is ON showing action code 1, a platform problem, and malfunction codes 37, 38 and 39.

The diagnosis is a system overheat with the MSU in NAV.

The drill is to select the MSU to OFF, flight or ground. If a single INS has failed it is left OFF and referred to the engineers after landing. If both INS in a two INS system are affected, check the equipment cooling. If the cooling was off and is sucessfully restored wait two minutes and, in flight, select ATT. Fly level until ADI flages retract and ADI is level. If the CDU warning light comes on again turn MSU to OFF and leave it off.

The existence of this drill suggests to me that the INS is capable of detecting an approximate gravity vector in level unaccelerating flight.

The drill I found in the B737-400 Flying Manual is 'IRS attitude failure after generator failure'

'If neither IRS attitude display recovers after a generator bus is restored:

IRS Mode Selector Switches...................................ATT
Maintain wings level, constant IAS for 30 secs
Magnetic Heading..................................................ENT ER
Enter heading on either FMC POS INIT page or on overhead IRS display by selecting HDG/STS.'

The 737-400 Technical Manual also says that ATT can be selected and the system can be coarsely re-aligned in the air following loss of both DC and AC power.

[oxford blue]

If an INS or IRS is switched OFF in flight, either inadvertently or in response to a checklist action, then the alignment is lost. If it subsequently becomes safe to switch it back on, you cannot get a full gyro-compassing in flight because the lateral accelerometer of the East gyro will not be able to distinguish between Earth rate and accelerations caused by movement of the aircraft. This is why the aircraft has to be stationary during ground alignment.

However, if you select ATT REF (on a typical IN) or ATT on the B737-400 IRS, either after switching OFF, or directly from NAV (following a computing, but not gyro, failure), the gyros now work as a combination of a superior form of DGI and ADI. If you have a period of steady unaccelerated flight, the gyros will erect to the vertical by sensing gravity from the accelerometers and the heading can then be set by the pilot to be the same as the heading from some other source (such as a slaved gyro compass).

Attitude information will stay good for the rest of flight, but the azimuth information is not corrected for Earth rate or Transport Wander, so whilst it will have the very low real drift rate associated with IN-quality gyros (in the order or 1/100 degree per hour), it still needs to be corrected every 20 minutes or so from another source because of the ER and TW drifts.

However, all nav computing facilities (readouts of drift, track, waypoint steering, distance to go, etc) are no longer available.

[Alex Whittingham]

Thank you Oxford Blue, we agree then.

[going round]

thanks guys, I'll print this off and take it into the exam with me. If the question crops up I'll just pin the print out onto the exam sheet.

Seriously, thanks again.

[Thunder Child]

Alex Whittingham, too smug and too patronising!

Having sat the JAA ATPL writtens, I don't ever recall seeing questions referred to in vaguely self-humoring male/female script.
But hey, I must be wrong as you quite clearly know what you're talking about......I stand, in fact, kneel corrected.

I wish you taught me Gen Nav......(sigh).

[Ray Ban]

Thunder Child,

No need to be like that to Alex. I have never met him but he was kind enough to help me out when I was doing my ATPLs and I wasn't even a Bristol student! He is a very valuable resource for wannabes and we need his ilk. Thanks Alex! :)

[5150]

So what? TC has nothing of value to offer because he might not be a ground instructor...(or maybe using this thread as a plug for business also - did I say that, surely not?!)

Jeez - I think if you read Whittingham's response to TC's (also helpful) response, TC is just taking the ****.

Heaven forbid, but I have known ground instructors to be wrong on occasions (as well as less patronising).

[Alex Whittingham]

Thunder Child, the male/female thing was not in our feedback maybe you are looking at going around's version of it. You think all pilots are male?

Going around, I agree its frustrating when there is no clear answer to a question but some questions are easier to ask than answer. The closer you look at things the more you realise 'truth' is not always absolute. This question is easier to answer if it refers to the IRS than INS. If it comes up again in the exams perhaps we can establish that it does.

Maybe we should keep these discussions on the Tech Log forum.

[Send Clowns]

Thunder Child, your posts seem to be unnecessarily confrontational and petulant.

Even though Alex is a competitor (directly, as I also teach General Navigation) I would say he is extremely competent, knowledgeable, approachable and helpful . Your comments about him are not justified by his posts in any way.

Alex's replies here have been reasonable, and he has corrected the answer to his question. We all end up with errors in our teaching material. You ask a professional editor about this sort of work, you will find that however much proof reading is used errors remain (I did talk about this with a professional editor).

As you can see from this thread the INS questions can have complex answers that are not always clear-cut. I generally don't teach INS (though I have), our instrument specialist does. His other job is as a 757/767 training First Officer. He still comes across questions to which he does not know a definitive answer as they are type-specific. That seems to be part of the confusion over this question, and is a common complaint about JAA questions. We have to second guess what the examiner means!