PDA

View Full Version : Are sextants still required to be carried aboard as backup...?


airship
26th Jan 2014, 12:31
If not, why not?

JeroenC
26th Jan 2014, 12:44
Through which windows would we use them? They're gone!

Fox3WheresMyBanana
26th Jan 2014, 12:49
Define 'required', and aboard what?

I doubt very much it's an insurance requirement for non-commercial yachts, as almost all of the insured skippers I met before crossing the Atlantic in November/December 2008 neither had nor knew how to use one.

Judging by the fact that most of the commercial boats I spoke to on the radio either weren't looking at or didn't know how to use their radar properly, I doubt very much they knew how to use a sextant either.

I carried a gps receiver linked to my laptop, a separate handheld gps+batteries, and a cheap plastic sextant plus tables (full table on disc, printed tables(waterproofed) sufficient for sun-run-sun's in my area/time)in my grab bag.


Friends of mine were hit by lightning mid-Atlantic. Their main gps was fried, the back-up survived in the oven (faraday cage).

vee-tail-1
26th Jan 2014, 16:15
The periscopic sextant hole was still there in 1993 on the Classic 747, but we did not carry a sextant and/or Sight Reduction Tables.
We had by then incorporated the sextant flap in the flight deck smoke removal drill, which worked well but the sound was awesome.
Last time I remember seeing a sextant used for real was on the 707 route LHR to Bermuda. With the best efforts of the Nav it took around 15 mins to work out a sight, by which time we were 300 or more miles from the fix.
Crews who tried to reach Bermuda by turning onto a direct track from Lands End often arrived at Bermuda's latitude without seeing anything. The problem then was whether to turn right or left to find the island! Usually resulted in an increasingly frantic 'square search' as fuel ran low.
My crew made things easy by heading more south from Lands End thus arriving at Bermuda's latitude with the island safely to the west. We only needed to turn right and follow the latitude with the sextant (easier than a full blown position fix) until reaching Bermuda. Those were the days ... much less heroic now. :cool:

Pom Pax
26th Jan 2014, 16:41
vee-tail-1, he should have precomputed. It might take 15 minutes before hand but the results come a lot quicker.

vee-tail-1
26th Jan 2014, 17:26
Pom Pax
Yes I understand there were ways to make things easier, but sometimes the two first officers tossed a coin to see who would be nav!
Not necessarily the best nav won.

Have you seen Robert Redford's new film "All is lost?
Really nostalgic stuff for me, including a bit of astro nav and various survival tricks.

galaxy flyer
26th Jan 2014, 17:30
vee-tail,

Your airline didn't carry dedicated navs?

Sextant ports do, with proper kit, make great cockpit vacuum cleaners to tidy things up. Then again, enough dirt and the rudder paint gets chipped.

GF

vee-tail-1
26th Jan 2014, 17:52
galaxy flyer

When I joined BOAC in 1970 the specialist Navs had just gone.
Some took early retirement, and some were offered pilot training with enhanced senority. (Not a good idea in hindsight!)

First Officers were given astro training and could swop seats where required.

Some F/Os were good at astro, and some Navs became good pilots.

However the inverse also applied with more than one tragic result. :(

Checkboard
26th Jan 2014, 18:33
With the best efforts of the Nav it took around 15 mins to work out a sight, by which time we were 300 or more miles from the fix.
Gosh those old 74s where quick! :eek:

ExSp33db1rd
26th Jan 2014, 19:10
If not, why not?

Because no one would know how to use them, that's why.

Not too long ago I flew a microlight type through Auckand air space (Shock ! Horror ! ) and to do so ATC required some navigation info. but rather then re-programme the panel mounted GPS that I was using to back up the evidence of my two eyes, I pulled a handheld from my pocket to source the required detail.

Had to laugh, 50 years ago I was using a sextant on the 707, now TWO GPS's - on a microlight ?

"Because no one would know how to use them, that's why" - I've read that someone wanted to re-fly Lindberghs' Atlantic crossing in a replica of his aircraft, but as this was to be "original" there was no modern nav. equipment built into it, he was going to use a sextant, a-la Lindbergh, but the FAA then stated that he would therefore have to hold a Flight Navigators Licence and -- there was no one still around to examine him for the issue of that licence - permission to depart denied !! ( I have no proof of the veracity of this, but can believe it )

When my friends and I were learning the black art of Navigation, on BOAC Stratocruisers and Britannias - with sextants - one of my colleagues set off from Algiers to Khartoum - and was way off course approaching Khartoum, having a "cocked hat", as a result of his Astro endeavours, about the size of Africa on his chart !

At the subsequent enquiry, the Fleet Nav. told him - had the crew just steered the flight plan headings, the winds that night were such that they would only have been about 10nm off track when they reached Khartoum - he was no help, he was a liability !!

I never wanted to be a Nav. I was a pilot for Chissakes, but now I'm glad I had the Astro experience. Crossing the Atlantic as an early Twenties Youf, I got satisfaction from knowing that for the last 5 or 6 hours I had been the only b*gg*r on the flight deck who had truly known where we were !!

The sextant "hole" was built into the first 747's, ;cos when it was originally designed, INS was still but a gleam in someones' eye, so it was assumed that Astro would be the primary source of Oceanic naviagtion. It would have cost more to re-engineer without the hole than just keep it as designed, hence it was renamed The Smoke Removal Port. Cheaper.

Happy Days.

vee-tail-1
26th Jan 2014, 19:34
Gosh those old 74s where quick!

Oops! even with a GS of 600 knts that was a bad typo more than 100 miles was what I meant :O

Captain Dart
26th Jan 2014, 20:19
The Lockheed P3C had a sextant valve in the flight deck roof and it was designed to have a vacuum hose attached. Some nav's could do 'astro' and it was surprisingly accurate.

It was an AEO (Airborne Eating Officer's) job to vacuum the flight station.

Fareastdriver
26th Jan 2014, 21:03
As a co-pilot on the V force when I had a go on astro fixs they would use my cocked hats as next week's navexs.

You could always find Jupiter. It flashed "J""T".

Fox3WheresMyBanana
26th Jan 2014, 21:05
they would use my cocked hats as next week's navexs

:D:D:D:D:D

ExSp33db1rd
27th Jan 2014, 04:08
vee-tail-1, he should have precomputed. It might take 15 minutes before hand but the results come a lot quicker.
Yes, but with a bit of practice one could get an Astro fix every half-hour, and read a paperback novel purchased at JFK before departure in between !

Having done the pre-comp. work it was then vital to "shoot" the stars for the requisite 2 minutes per star on schedule, not helped if a pre-primed stewardess ( usually primed by the Flt. Eng. )quietly entered the cockpit when one started the sextant timer, loosened ones' belt, lowered ones trouser and ........... well you know, dedication to completing the fix was paramount !!

Happy Days.

John Hill
27th Jan 2014, 04:20
So how was the sight reduction calculated? The easy way with 249 tables (or equivalent), Bowditch's method or sharpen the pencil and reach for the four figure mathematical haversine tables?

ExSp33db1rd
27th Jan 2014, 08:22
So how was the sight reduction calculated?

In my case the H.O. 249 tables.

Whiskey Kilo Wanderer
27th Jan 2014, 08:29
In the dim and distant past when I earned a crust as a Surveyor / Party Chief / Client Rep around the oilfields of the world I sometimes had a go at evening stars from the bridge wing of our survey vessel. I was never up early enough to do morning stars.

Based initially on the Air Sight Tables, I eventually worked out an Excel spreadsheet to do the calcs. Needless to say the sheet was quite complex, particularly for my limited capabilities and usually on subsequent trips Id forgotten how it all worked so had to re-invent it from scratch each trip. Having the unfair advantage of dGPS to generate an initial position and give very accurate GMT may seem to be cheating, but the numbers usually came out to within a mile. The fascination of setting the sextant up, pointing it in a computed direction and seeing a dot of light on the horizon never faded.

According to a Comment embedded in the spreadsheet, the calculations were:

Calculations used below:
S = Sin (Dec) C = Cos (Dec) * Cos( LHA)
Elevation = Hc = Sin-1 (S * Sin (Lat) C * Sin (Lat))

X = (S * Cos ( Lat) C * Sin (Lat)) / Cos (Hc)
If X > +1 then X = 1 If X < -1 then X = -1

Azimuth = Cos-1 (X) If LHA > 180 Z = Azimuth If LHA <= 180 Z = 360 - Azimuth

Tankertrashnav
27th Jan 2014, 08:49
Before inertial nav, aviation used a periscopic bubble sextant.



I learned astro using bubble sextants at nav school c 1969-70, but on the V Force we used the later type which used a little device rather like the old London Transport "bus stop" logo to indicate that the instrument was level. This avoided messing about with the little screw to adjust the size of the bubble.

Crossing the pond in the mid 70s, without INS or Loran and with our H2S radar of no use as we were nowhere near any land, it was astro or nothing. Always managed to find Goose Bay though!

ShyTorque
27th Jan 2014, 09:58
Were sextants ever on the minimum equipment list for aircraft?

vee-tail-1
27th Jan 2014, 10:16
H.O. 249 tables.

If lost you could always set light to the tables :eek: See Robert Redford's new film "All is Lost"

Were sextants ever on the minimum equipment list for aircraft? We would never have found Bermuda on our 707s without one :ooh:

Limeygal
27th Jan 2014, 10:46
We used to carry a navigator when I worked for Dan Dare. Trips over the Indian Ocean, where there was a navigational "black spot," required a navigator on board. He used to "shoot the stars," very interesting it was. This was on the 707.

goudie
27th Jan 2014, 12:35
not helped if a pre-primed stewardess ( usually primed by the Flt. Eng. )quietly entered the cockpit when one started the sextant timer, loosened ones' belt, lowered ones trouser and ........... well you know, dedication to completing the fix was paramount !!

With such dedication are you sure it was a stewardess?

Fareastdriver
27th Jan 2014, 14:43
When I used to sail backwards and forwards between Southhampton and Cape Town on Union Castle the daily entertainment around midday was listening to a row of deck officers and cadets on the bridge rattling off readings every fifteen seconds to get the local midday and lat/long.
After a couple of days good weather you could have a fairly accurate guess as to where the ship would be the next midday so you had a good chance, and I did, three or four times, of scooping the ship's forcast position pool.

airship
27th Jan 2014, 15:12
I suspected that the somewhat ambiguous thread-title might draw-out some of the "old-timers" (said with RESPECT...?! :) ) here.

My main motive was to demonstrate the great wealth of knowledge lurking here in JB, just waiting to be called upon.

An ulterior motive may have been that I'm considering taking up writing novels. Perhaps with some help from Wilbur Smith, and in conjunction with Loose rivets "one of our own" (though he's not aware of this new opportunity yet):

The storyline commences with what might look like a massive EMP event which basically wipes out most satellite-based navigation systems etc. etc. Noone knows what really causes this initially (most communications are down), so could be the initial results of a surprise 1st nuclear strike followed by World War III; maybe due to extremely severe solar activity within our own solar system, or external to it. Or perhaps simply a massive virus-attack launched by terrorists...

...AirForce 1 is currently flying mid-Atlantic, carrying the US President and the 1st Lady; coincidentally, the USS Barack Obama CVN-79 is under-going final sea-trials 1,000 miles off the coast of west Virginia...?! :O

What about LORAN-C? The USA and Canada both ostensibly ceased transmitting signals in 2010. Does the infrastructure still exist though? And how many hours would it take to "re-activate"...?! Or could the "friendly" nuclear subs, perhaps hitherto-protected from all of the direct effects "come to the rescue"...?!

Want to read more?! Hmmm, forget Wilbur Smith. Let's make this a JB collaborative effort. Loose rivets could be responsable for editing and somehow making a novel out of it. All royalties would be shared out equally after both airship and Loose rivets had each taken their initial 10% share each. :ok:

PS. Tom Clancy, "shake in yer grave 6ft under" - "Hunt For Red October" will look like a childrens' story once we're finished... :D

handsfree
27th Jan 2014, 15:19
I've just been looking up aircraft sextants in wiki and it claims that aircraft sextants are no longer in production. If true I don't suppose it's all that surprising really

Keef
27th Jan 2014, 15:25
I bought a sextant on Fleabay last year. It looks the biz, but has a fundamental design flaw. If I didn't know a little bit about how sextants work I'd have been very lost indeed!

John Hill
27th Jan 2014, 17:10
Whiskey Kilo Wanderer

Calculations used below:
S = Sin (Dec) C = Cos (Dec) * Cos( LHA)
Elevation = Hc = Sin-1 (S * Sin (Lat) C * Sin (Lat))

X = (S * Cos ( Lat) C * Sin (Lat)) / Cos (Hc)
If X > +1 then X = 1 If X < -1 then X = -1

Azimuth = Cos-1 (X) If LHA > 180 Z = Azimuth If LHA <= 180 Z = 360 - Azimuth


If I recall correctly it was Bowditch who devised a sequence of doing this which involved nothing more than addition although there are several steps. The process involves tables of haversines (half the inverse of the sine(?)).

We managed to find several fly spec islands in the Pacific using this.

Whiskey Kilo Wanderer
27th Jan 2014, 18:19
Hi John

I cant remember where I got the equation from; it may have been in the back of the nautical almanac.

It was basically intercept method, choosing selected stars from the almanac that gave elevations between thirty and sixty degrees. Having the ships gyro repeaters on each bridge wing made it easy to point the sextant in the correct direction.

I did once see an aeronautical sextant at an aero / auto-jumble at Popham Airfield, but it was a bit expensive. In hindsight I now wish Id bought it. Ive still got my marine and surveying sextants. The surveying one is just a marine one without the sun and horizontal filters.

ExSp33db1rd
27th Jan 2014, 19:38
Always managed to find Goose Bay though!

Once tracked North of Goose Bay into the Hinterland, and Astro was the only Navaid, but with Sun and Venus the only heavenly bodies visible, accuracy was not the greatest, and eventually admitted to the skipper that I hadn't had a decent fix since crossing the coast, and was Temporarily Unsure Of My Position.

Not to worry, he said, if you're right we're not too far from the DEW LIne (Defence Early Warning ) line stretching across North America to warn of the dastardly Soviets ( or anyone else) approaching, and he duly admitted to some operator that we would appreciate some assistance. They couldn't see us !!

Don't Panic ! Don't Panic ! Eventually another commercial came up and asked where I thought we were, then asked if I could see a Spot Height printed on the chart about where my Air Plot showed us ? Yes, well he said, it's a misprint it's actually a local commercial broadcasting station, so your ADF should give you bearing ? Phew ! Actually we weren't that far off track, but it's the uncertainty - travelling at 400+ kts that gnaws away at one.

Never did discover why the mighty US Defence System couldn't pick us up !!

One Nav. instructor said I'd never make a navigator until I'd been over Berlin with the shells coming through the cockpit as I tried to get a 3-wind drift sight calculation !

I never had to.

Loose rivets
27th Jan 2014, 20:07
I took an interest in the Flight Nave course at Cass, but never went on to that ultimate goal, it looked rather intimidating. I did however clean the navigator's ground tracking sight on the DC3 and peered into that when boredom became overwhelming. I continue to live in awe of the 1940s navigators.

airship. You're kidding, right? You know how jealous I am of Lights Out author, David Crawford. That's his yarn in a nutshell - though Air force One falls out of the sky due to the loss of its electronics.

My 'The Perfect Code' took years to get finished, and now I'm faced with pulling it and a major rewrite to get rid of the over-technical boring bits. It's a hard, hard slog and I need to be working on the sequel. A book, with pictures, of puddycats would be a far better seller. ;)

John Hill
27th Jan 2014, 22:01
WKW, I was never a 'real' navigator but two of us did cross the Pacific (eastwards) in a 32' sloop. Sights were taken standing in the cockpit with knees at about sea level so a very close horizon and timekeeping was those very handy signals from WWVH on 15 Mhz. We used a plastic sextant which I think cost 12 quid and we had a few aeronautical plotting charts plus a plentiful supply of sharp pencils! Land never failed to show up at about the time expected.

Tankertrashnav
27th Jan 2014, 22:14
I did however clean the navigator's ground tracking sight on the DC3 and peered into that when boredom became overwhelming.


I think you are referring to the drift sight? Very simple instrument and very useful for giving you an accurate drift reading.

Not much use if you are above an unbroken stratus layer, though!

BenThere
27th Jan 2014, 22:15
When I first crossed the Atlantic in 1978 on a KC-135 (B707), our only over water navigation was a navigator calculating sexton shots. The leg was from Pease AFB at Portsmouth, New Hampshire to Lajes AB, in the Azores.

After hours over the endless nothing, the nav said, "You should see the island at your 1200 shortly."

We looked out into the dawn of the rising sun and there it was, 1200. Interestingly, I remember seeing the lights of the island before the TACAN locked on.

Such navigation by the stars is now, I think, an obsolete and lost art.

galaxy flyer
27th Jan 2014, 23:40
BenThere,

Funny, you mention that! About the same time, I was in the back seat of a Hun, enroute to deliver it to the Turks. The guy in the front, Bill, was an old navigator on KC-97s, very good apparently. Bill was initial pilot cadre in USAF Phantoms after UPT, also.

Bill, "by the CONSOLAN, we're about 100 nm north of track, maybe 80"

Me, concerned, "Really", start to sweat.

Bill to Tanker, " I show us about 80 north of the flight plan"

Tanker to Bill, "right on track, Sir"

Thirty minutes later, the STG TACAN locks on......30 degrees to the RIGHT!

NAV bought beers at LETO later.

GF

ExSp33db1rd
28th Jan 2014, 08:27
Not much use if you are above an unbroken stratus layer, though!
Or over Berlin with the shells coming through the cockpit. ( see post #31)

Whiskey Kilo Wanderer
28th Jan 2014, 08:41
JH Id say crossing the Pacific using a plastic sextant qualifies you as a real navigator. When I was playing with my astro observations, the bridge crew often watched with interest as they hadnt done sights since nautical college.

Ive done the odd North Sea Race in the past, but astro isnt much use as quite often you wouldnt see the sun or stars for the whole trip. We used the Pilot Pal radio for RDF and Consol as the main navigation tools.

I was intrigued by the concept of latitude sailing. The Vikings and others crossed the Atlantic using the method before the lodestone had been discovered.

tony draper
28th Jan 2014, 09:03
Here's a tale of good navigation.
http://www.pprune.org/jet-blast/379210-ships-favourites-6.html#post5025150