ETA

[mountainlad]

Hi.

Apologies as i am a non pilot but need advice of pilots to settle an arguement!!

I use OS maps to climb mountains and i can calculate my ETA using distance (in km) from the map and my walking speed in km/hr.

So, how do pilots calculate their ETA? i understand that airspeed is measured in knots, but surely they have to use a distance in km or miles from a map (assuming you dont have GPS). so if this is the case, do you have to use an estimated ground speed in km/hr to calculate your trip time?

cheers for any response.

[BOAC]

Speed in knots = n mls per hour. Maps etc scaled in n mls. Ground speed either found from on-board equipment or 'deduced' using forecast winds and air speed.

[Groundloop]

So, how do pilots calculate their ETA?
Look at the page it's displayed on the flight management computer!!;)

[the dean]

mountainlad...

i'm amused to hear you say ' estimated ground speed..'..:}

that was when i started flying....

now and again people will do it just for the hell of it...and of course students must still calculate estimated groundspeed...since, on flight tests, the use of DME ( distance measuring equipment ) which will also give a groundspeed readout or GPS, is not permitted ( except in the portion of the cross country devoted to a simulated diversion )...

otherwise..you do'nt have to calculate groundspeed any more since most people and/or aircraft now have GPS and get it accurately from that including ETA...( mores the pity maybe some would say)...but what a wonderful piece of kit GPS is...:ok:

gear up,

the dean.

[BOAC]

Remember the question was how to 'calculate', not read the ETA from a screen!

[the dean]

thats true BOAC..

ok mountainlad...

here is ( if memory serves at this stage ...subject to correction..)...a formula..

DISTANCE DEVIDED BY GROUNDSPEED MULTIPLIED BY 60 = TIME IN MINUTEs...hence giving..ETA. we would calculate that either the hard way by using a whizz wheel flight computer...of a pocket size electronic flight calculator..but for rough calculations ( especially in no wind conditions when the airspeed is the same as the ground speed or using an estimated ground speed)..it works fine.

as far as working out the estimated ground speed is concerned again this is worked out using whizz wheel or electronic calculator, based on forecast wind for the route ( since the wind angle relative to the aircraft will have an effect on its forward speed )...and then checked periodically throughout the trip to see if the estimate is correct or needs revision to amend the ETA accordingly..

hope thats of some help..and may the wind be always at your back...:ok:

the dean.

[perkin]

Might be a stupid question, but whats a whizz wheel? Is it in a similar family to the good old slide rule?

[mountainlad]

thanks for your replies.

can one of you just clarify what ground speed is measured in? knots or 'road' mph.

BOAC mentioned that maps are scaled in n mls. i think this is where my confusion stems. the maps i use are in road miles giving me mph.

I assume flight GPS uses the same software as car GPS using road miles and mph?

sorry about this..:ugh:

[the dean]

sorry perkin...maybe should have explained... ment the regular mechanical computer all early pilots use and maybe the one you are referring to...the old circular slide rule..CRP..or E6B..or other...

dean.

[BOAC]

GS (in Europe) is Knots. We work everything in Knots.

[flyinthesky]

Mountainlad

To summarise;

On an aircraft, everything is measured in nautical miles. Hence our groundspeed (which remember takes into account the effect of wind) is given in nautical miles per hour.

On a GPS equipped aircraft, the equipment gives a readout in nm/h NOT statute mph.

We do not really use maps for navigation anymore. We do have charts that will provide track and distance.

As we progress from one waypoint to another, the aircraft calculates the great circle distance (since this is the shortest distance over the curved earths surface) between the lat/long of each waypoint. The GPS feeds in the current a/c lat/long into the FMGS and therefore the a/c knows it is on track (this is a very simplisitic way of explaining this) The GPS feed also allows the a/c to deduce groundspeed. BUT we also have 3 inertial navigation systems backing up the GPS which do the same job, just slightly less accurately.

Hope this helps.:O

[Groundloop]

Remember the question was how to 'calculate', not read the ETA from a screen!

Yeah, but how many airline pilots ever "calculate" it these days?:ok:

[flyinthesky]

Nah,

it would come between me and the sudoku/coffee/hostie (delete as applicable)

Calculations are just plain time consuming!!!!!:E:E:E

Flyinthesky.

[perkin]

Cheers dean, thought it would be something like that, but wasnt 100% :)

[Rainboe]

What are these 'kilometer' things? Why would the world want to work on anything that wasn't Nautical Miles. They're lovely. They have such meaning. If Harrods sold distances and lengths, they would be Nautical Miles. These kilometer thingys are an invention of Bonny to take over Europe. Ignore them- they will go away.