Miles & NM

[caucatc]

When I was working in final approach area , I would like to say: "XXX maintain 180 KTS until 7 NM from touchdown ." Nearly all the pilots prefer to use "maintain 180KTS until7 MILES from touchdown ."When it was my first time to meet this situation , I repeat the NM again and pilots said "affirm 7 MILES "
I know there is a small difference between miles and NM ,but once the plane could not stop on the runway ,that is the accident.What do you usually say ? Shall I use KM instead of NM or mile ?

[HEATHROW DIRECTOR]

It depends on what you are trained to do.. In the UK we use nautical miles in such cases,

[GunkyTom]

Just use miles, the difference is so little as to not matter. I can't imagine how it would be a contributing factor in an accident

[Lurking123]

I think most of us have more to worry about.

[vespasia]

Since all the aircraft systems are working to nautical miles, you can say "miles" and it will mean "nautical miles". Statute Miles are not, as far as I know, a recognised unit of distance for aviation purposes.:bored:

[timelapse]

Don't american METARS have vis in statute miles?

[Roffa]

If it's an ILS/DME approach why not just say "until x DME"?

No ambiguity there.

[TinPusher]

:confused: Correct me if I'm wrong.....

ICAO standards are NM for tracking distances, km for reporting vis and runway lengths, feet for altitude.....

If you are party to ICAO recommended practices then there's no ambiguity.

[Jerricho]

Don't american METARS have vis in statute miles?

Correct. As do Canadian. Suffixed with "SM" on the Metar/TAF.

[RustyNail]

Statute miles are also used in Canada on approach plates, just for something differnt :confused:

[Check Airman]

As far as I'm aware, all countries use nm for navigation. I doubt there's much chance of confusion, since the FMS,GPS,IRS,INS...all use nm.

As for METARS, I've long wondered why they don't give visibility in nm instead of km/sm. Of course, this would be reduced to ft/m as necessary:cool:

[Miraz]

I understood that a nautical mile always indicated a horizontal distance across the earth's surface at sea level, in theory one minute of latitude - great for navigation.

Other units of distance - km, miles, feet etc can be used for point to point distances in any direction - if you stick with the icao standards it is difficult to go wrong.

The only time DME miles = nautical miles is at sea level, although the differences for an ILS approach altitudes are minimal.

[vespasia]

Not quite true Miraz. 1 NM is the "distance bisected by 1 minute of arc at the earth's surface", so the NM is exactly the same size whatever height you're at. A NM is a NM is a NM, which is why it's ideal for navigation. A DME mile is equal to a NM all the time, but if you're at FL350 you still have to take slant range into account. If you're flying an ILS approach, slant range doesn't really matter because the DME is where you want to be i.e. on the ground!

[Spitoon]

ICAO Annex 5 - Units of Measurement to be used in Air and Ground Operations makes interesting reading in the context of this thread!!

[pocpicadoor]

1 NM is the distance described on the Earth's surface by 1 minute of arc from the earth's centre at the equator.

Our home planet (well, for many of you!!) is an oblate spheroid; so, 1 minute of arc on any Great Circle (except the equator) COULD be different in actual length.

NMs SMs KMs.... all Dutch to me: just don't let 'em bump into each other!!!!!

Ho-hum: off to the sheep pasture

POC

[vespasia]

Fortunately, the international standards people have made all nautical miles the same at 1852m (6076 feet)!

[pocpicadoor]

But what's a "mile"? NM or SM.

Pecanticity will out!

"At 75DME descend to ...... ": solve the problem?? Probably not for the kilometricos!!??

"At 75 DME NMs or 77.67 DME SMs or 117.4 DME KMs, descend to ....."
There must be a table somewhere to make it this easy!

Over and out!:D

[TitanMaster]

As far as i'm aware, all flying distances in aviation relate directly to Nautical Miles. That is the international aviation measurement, the NM.

[vespasia]

But what's a "mile"? NM or SM.

Pecanticity will out!

"At 75DME descend to ...... ": solve the problem?? Probably not for the kilometricos!!??

For what it's worth I agree. 75DME means 75 nautical miles DME since DME receivers only work on nautical miles, but saying 160 knots to 4 miles means to 4 Nautical miles, and since on an ILS approach the ILS is paired to a DME there is no need to specify that the miles you mean are nautical!

My head hurts;)

[Spitoon]

75DME means 75 nautical miles DME since DME receivers only work on nautical milesDME receivers work on time - it is technically possible for any unit to be displayed. Are you sure they all/only work on NM?

but saying 160 knots to 4 miles means to 4 Nautical milesOK, if you say so - but....since on an ILS approach the ILS is paired to a DME there is no need to specify that the miles you mean are nauticalNot all ILSs have a paired DME. And many modern aircraft do not routinely display raw data - what does the PFD use?

Assumptions can be dangerous things.

I'm catching the headache!


PS - Titan, take a look at Annex 5.

[vespasia]

DME receivers work on time - it is technically possible for any unit to be displayed. Are you sure they all/only work on NM?


Yes, since Nautical miles are the standard unit of distance for navigation purposes ( ICAO ) although you're quite right, there's no reason why they couldn't be configured to show KM, statute miles or eighths of an inch if that's what you wanted! From aircrew colleagues though, I'm told that DMEs display NM and that's it.

I assume (and it may be a dangerous one) that PFDs also display NM because that's the standard ICAO unit for navigation.

You're right about the ILS/DME pairing issue also, but I'd hope that an ATCO wouldn't give a speed restriction to a specified distance if there was no way of measuring that distance:ooh:

That's me done!:O

[Spitoon]

OK, I've mentioned Annex 5 a couple of times in this thread but no-one has challenged why this is relevant. Annex 5 - Units of Measurement to be used in Air and Ground Operations sets out the standard for measuring distance and it says kilometres shall be used for long distance and metres for short distance. Nautical miles is a recognised alternative for long distances (as is feet for short distances) and the stated intent is that the use of these alternatives shall be terminated at some point in the future. For the benefit of any North American cousins (and I really do have some, as it happens) the use of statute miles is not recognised by ICAO.

Now, I'm guessing that everyone is quite correct that all aircraft display raw DME data in NM - but I don't know for sure. I have no idea whether a PFD can be set up to show something other than NM.

The original question from caucatc is about operations in China. I have no detailed knowledge of opeartions in that part of the world but I have been on the flight deck as an aircraft transits from our 'normal' European flight levels to metric-based levels and I've seen how it has to be handled. I'm unwilling to make assumptions based on my knowledge of our little part of Europe and say that things in China will also happen in exactly the same way (despite the best efforts of ICAO).

In response to caucatc's post I would be inclined not to worry about the readback and differences between NM and any other miles at the ranges involved in an approach but your question about the controller's culpability if an aircraft overruns would no doubt be raised by legal experts and is a valid concern. If there is a way to do so I would suggest that you try to get a statement put into your AIP that the word 'miles' when used in ATC phraseology shall be understood top mean nautical miles.