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APenNameAndThatA

Accuracy of the "1 in 60" rule for navigation.

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I like to find Lord Howe sitting in the back of a Dash8-200, I assume the pilots are finding it with a couple of TSO GPS's. It is a very beautiful place and the walk up Mt Gower is truly unforgettable.    

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9 hours ago, turboplanner said:

Have you read "We, the Navigators?"

It's a great story, and they did amazing things with the most basic tools, but they had the luxury of time - they were doing ~7kts, and had weeks to do it, and all nice visual clues that we use too, some of anyway, not all of which are terribly useful in aviation.  They had multi-crew to manage the workload. If they got a bit lost they had time to fix it.  You don't hear from the ones that didn't make it though ... Survivor bias again.

 

Harold Gatty has an amazing story too, but .. they were _lucky_ and a lot didn't make it.  https://en.wikipedia.org/wiki/Harold_Gatty

 

Prince of Navigators is a good telling of his tale.

 

Seriously, while we learn DR and 1:60 and all that stuff, and look at the charts (electronic, not paper), with a charter pilot hat on, instead of a XC instructor hat, I'm using AvPlan and a Garmin 430.  When we get a "you're going to XXXX in YY minutes", you have time to fuel up and bang the plan into AvPlan*, print out the flight log and relevant DAP/ERSA/Country guide pages, look at the satellite pics of the aerodrome if it's unfamiliar, and go.  In flight I am managing passengers, fuel and weather, and keeping my iPad cool 🙂

 

* - NOTAMs, weather etc from AvPlan, brilliant. I have a backup iPad in the plane as per our ops manual, and a phone with it on it too.

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I guess the question is however many have died from over reliance on GPS, compared to some use is DR.

 

I have been using GPS since they were commercially available, spent the first decade decrying their use and forcing the use of map and compass as primary, and the last decade embracing the convenience and accuracy!

 

I'm sure the use of VOR etc was considered cheating once!

 

Mike

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I have never heard it referred to in that context.. CASA's equivalent at the time spent a lot of time  checking the accuracy/calibration of all approved radio aids at their particular sites with airborne testing facilities.. Most approaches use multiple aids ie DME & marker beacons as a distance height check and often VAR, NDB  VOR are co sited. LOC is over a NDB final approach fix where transit and G/S co incide. Generally there are checks  and you aren't relying on ONE procedure or  Aid or process. Nev

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Most NDB's are being decommissioned now. Never found them to be reliable, mostly because of old rubbish radios. Did have a King KR87 (To get it NVFR) in a little machine, it was mainly good for listening to the radio. Pretty amazing CASA still has reliance on 1920's technology, indicates how progressive they are.

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 They probably get some pressure from people who don't have new equipment. NDB's always had problems Coastal refraction and influenced by electrical storms as a couple and the line of sight aspect they shared with others. . Still OK as an overfly position marker if no DME. GNSS is the go but not without coping with and acknowledging it's problems also.  Nev

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On 13/01/2020 at 5:16 PM, Mike Borgelt said:

In 1991 the satellite constellation was not complete. There were outages when you couldn't get a fix. The 1991 Gulf War had plenty of times when GPS wasn't available when the coalition forces were trying to locate for later reference, Iraqi graves in the featureless desert.

 

So tell me guys.  Suppose I'm going to fly my BD-4 to New Zealand via Lord Howe and Norfolk Islands. It is well with the capability of the aircraft. How do I find the islands? Dead reckoning or multiple GPS's?

In late 1990 early 1991, as I recall it, 0730/1930 local was the peak time to get a NAVSTAR fix in Arabia, with the visible constellation reducing either side of those times.  I think there were at least six hours a day with nothing in sight, longer where you would have trouble getting a solution.

 

If you are serious about that flight and having a back up, I'm working on validating the performance of an Italian integrated INS/GNSS solution based on Spanish UAV sensors intended for GNSS denied areas.  If it works as advertised it should give you everything you need.  www.aircraftspruce.com/catalog/avpages/avmap-ultra-efis-2.php   That's to solve the "Find Honolulu problem" for a C1a MTOW<500 kg circumnavigation if GNSS is tits-up.  INS drift over 2,000 nm should be in the order of 30nm - presumably, GNSS failure before PNR would have a diversion, so the INS drift would be less as it would have a more recent position.   Put a small camera that talks to an ipad underneath the aeroplane.  As well as being helpful for pilotage, with a transparent overlay of a protractor, you now have a drift meter to supplement your DR.  Of course the whitecaps are moving, but it still gives you insight into very different winds aloft than forecast, if your GNSS solutions fail.  For curiosity's sake - the regs still make provision for you to drop drift bombs for this purpose.  These are wooden bombs full of golden glitter, or smoke pots and flares.  The idea is that wave movement is faster than currents, so you get a better drift reading with something on the surface, rather than a white cap.  I don't see anyone being prepared to bother with such a thing nowadays, other than for having a legal excuse to drop bombs from an aeroplane!  But a basic drift meter is cheap, simple, light and useful for 'looking' through the floor of the aeroplane.

 

Part of the back up plan should include a late model ipad and/or high end android device - the consumer GNSS has access to many more constellations than most of the TSO equipment.  Garmin only work with NAVSTAR and GLONASS, for instance.  Some of these phone GNSS even have access to the NAVSTAR civilian L2 frequency.  It would seem entirely reasonable to me if that it's all you used.

 

Full on celestial navigation is probably impractical, but an astro compass could be helpful.  I did some airborne testing using precomputed sight reductions using an iphone "sextant" app, the best I could do was about 100 nm, 130nm more like it - however, I never got around to calculating index error, which presumably would tighten that.   100nm won't get you there.  If you look at what Chichester did, going the other way across, it could be a useful last ditch defence.  You make sure, in effect, that you will definitely be left or right of the destination at a precomputed line of position, using something simple like a sun sighting, you then know to turn right or left to find the destination.  Which all sounds silly given how ubiquitous positioning information is for us today.   And there would be a lot of learning to use them.

 

Given that these are fairly long legs, if you accept that multiple GNSS alone is sufficiently reliable (which is entirely reasonable), you could simply use that to track your position as you follow a constant heading course using the bellamy drift equations.  You use the barometric pressure predictions corresponding to your cruise altitude and the mean latitude between origin and destination to calculate the net drift over the leg.  Instead of following a track on the ground, you take the shortest path in the air.  This is of greatest importance on long legs where reversals in drift direction occur.  By staying fixated with a great circle on the surface, your drift corrections to stay there mean that you use more air miles to get between A and B.  Using the constant heading, you may be left or right of the great circle route on the ground, but you aren't wasting effort to go first left then right in the parcel of air.  Savings in the order of 30% of air time/miles were found using the Bellamy drift method.   But you do need to be able to fix your position when going towards a little island rather than aiming at a continent.  As you get close, using your fixes, you make up for any errors in the computed Bellamy drift and arrive at your little island earlier with more fuel.  It does mean that if something goes awry, you won't definitely be able to tell if you need to turn left or right to search for the island if its LOP comes up and you can't find it.

 

Drift nm = 635*(|p2-p1|)/(sin(mean latitude)*TAS   where p2-p1= difference in hectopascals of the pressure at cruise altitude between origin and destination.  Looking at the forecast weather for 17Jan, it's probably worth about 10% to Lord Howe and 15% to Norfolk.  For the 18th, it could make a 30% difference Bankstown - New Plymouth.

 

Also handy is that if the pressure is the same at both points, you head straight for it, ignoring winds aloft in between.  For the southern hemisphere, if the destination pressure is higher, the drift is right and the wind correction heading left, if the dest. pressure is lower, the drift is left and the wind correction heading is right.  Vice versa for northern hemisphere.

 

Whilst I don't recommend this, with a ranging altimer (radar/lidar) you can also do pressure navigation.  As you sit on a constant pressure altitude, you are actually going up and down in relation to the earth, which the radar altimeter can measure.  By using the radar altitude over the sea, you can get a pressure line of position against the synoptic.  There are some home made radar/lidar altimeters that are reasonably light, cheap and simple.  As a last ditch navigational defence , a pair of daylight visible lasers could be aligned to give you some reasonable altitude above the sea, say 200 feet, the pressure at this height could then be used to give you a pressure line of position to follow to your island.  If you have comms, you can find out the current pressure at destination and follow it in.  I won't go into the details of pressure navigation, as we have GNSS, but it is surprisingly powerful.  The part that remains useful in the GNSS age is the Bellamy drift for constant heading navigation.

 

New Zealand is a much bigger target, maybe a good ferry tank and direct routing might be better?  😉 It's a long way to the islands without an alternate, maybe a small ferry tank to allow reaching Norfolk, the mainland or NZ would be prudent if island hopping?  You should have no difficulty in configuring the BD4 for 1,600 nm with reserves, you don't even have need an overweight take-off with 200 kg load.  That'll leave you with a range from Bankstown to divert anywhere in NZ if you have to.  It's a good deal shorter than the island route.

 

I hope you give it a go.

 

regards,

 

Paul

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