Speed question


I am watching a friends flight from LAX to Sidney http://flightaware.com/live/flight/QFA150 and the proposed speed is Mach .84. That seems to be an odd way of lisiting the proposed speed or is that normal.




There’s several ways of determining speed. Get a copy of the May 2008 Airways Magazine. It has an excellent article on aircraft speeds.

At higher altitudes (i.e. above 30,000 feet), indicated air speed is not a very good indicator of speed so aircraft use Mach numbers. Mach, the ratio of the velocity of an aircraft to the velocity of sound, is used to maintain separation of aircraft.


It’s not really common, but it’s not too rare either. You can mouse over ground speeds or mach numbers on flight pages to get an approximate conversion to the other system of measurement.


. Not really, indicated air speed is easily converted to true air speed (since both altitude and temperature, the two key conversion variables, are required instruments) and is displayed in even low end GA planes.



*. Thanks. Corrected error.


Source please on what I put in blue above???

Everything I have seen on TRACON screenshots on Google **does not **show mach. It shows exactly what Flight Aware displays on their airport maps which is airspeed in knots.

EVERYTHING I understand on ATC airspace seperation is based on GROUND speed for maintaining seperation.

Or is the above based on opinion???



For flights over water in a non-radar environment this is normal. The proposed speed listed is the airspeed, ie; mach number/speed that will and must be maintained in order for ATC, and its route traffic planning, to have proper in-trail spacing between aircraft in the non-radar environment for that route and altitude.



Here we go again.

It is not an opinion. Had you read my posting, you would have seen the source.

There’s several ways of determining speed. Get a copy of the May 2008 Airways Magazine. It has an excellent article on aircraft speeds.

And, as azav8r said, and as the magazine article by an airline (not private) pilot with extensive JET experience wrote, Mach speed is used for separation.


Please reread azav8r’s response.

You are aware that planning and maintaining seperation are different???

Your response said nothing about planning, only that ATC uses Mach **to maintain **separation. Please note the words I bolded.

Everything I have seen sez differently (You did read my response and note what is on ATC TRACON screens???).

What you said at minimum is marginal, and if I am correct in ATC using ground speed on their scopes just like what is displayed in Flight Aware in maintaining separation, then you are flat out wrong. Again, planning and maintaining seperation are two different entities.

Lets go this route, I fly .83 mach with a 100 knot tailwind. What you will use for seperation??? .83 or .83 + 100 knots?

I’d not want to be the plane in the front if you select .83 without wind considerations.

BTW, part of my learning how to fly an airplane includes the needed knowledge about the various airspeeds, which apparently you are not aware of?




Please note that I used the word “and” in my original post:

mach number/speed that will and must be maintained in order for ATC, and its route traffic planning, to have proper in-trail spacing between aircraft in the non-radar environment for that route and altitude.

Meaning for planning and maintaining in-trail spacing.

Maintaining a certain speed is the constant in maintaining spacing. For example, two aircraft are flying from LAX to JFK on the same route at FL410 with a 20 nm separation. If they maintain the same speed throughout the flight they will generally remain 20 nm apart regardless of the wind conditions, because they will pass through the same wind conditions within minutes of each other.

In a radar environment a controller can make fine adjustments by asking for a certain Mach number which is used for turbojet aircraft at FL290 and above.

In a non-radar environment a controller doesn’t know the ground speed of an aircraft in the Flight Levels, so he/she must rely on the crew to adhere to their proposed speed for separation and planning. When an aircraft makes a postion report at a required reporting point, the controller makes a distance calculation based on the time of the report of the preceding aircraft. If a separation adjustment is required then the controller will ask for an appropriate speed adjustment.

Speed adjustment procedures for controllers are found HERE


Duh, good point, I was more along thinking in the sense of timing at the “end of the line” for arrivals, which like you said above, for separation purposes, while en route the spacing will be the same based on wind conditions at a given altitude.

Again, while I was Googling for TRACON screens, the flights were displayed in knots, not in Mach numbers which made me think that ATC uses knots in determining spacing since that is what aircraft performance is based on, not Mach. He said spacing is based on Mach, which I STILL cannot find anything to support his position.

ATC may ask or give instructions in Mach OR knots, but everything I see on Google sez that knots is used for seperation. I would guess this is because aircraft performance numbers would be on knots (I.E. minimum approach speed, or any instructions that would encompass the operating limits of the aircraft).

Fully understand the need for the crew to adhere to speeds in non radar environment, but now-a-days is this a common situation at 30+ thousand feet? This happens for me at the lower altitudes because I am “below” radar coverage, but in the upper altitudes are there still sections of the country that still don’t have radar coverage at the jet levels?

So, I do take it correctly that under radar environment, that ground speed would be used for planning and separation???

THANK YOU for this reference. Why Dami couldn’t or woudn’t do the same as his reference isn’t from the source that we fly by is beyond me.

Thanks again for clarifying where I was flawed in my way of thinking regarding seperation of traffic at jet altitudes.



These days the only non-radar covered areas are oceanic and some remote international locations. Some of our more well traveled intl pilot members like porterjet and G4Driver would have better knowledge. While I have international experience, I haven’t flown the world extensively.


I edited my post after you posted, so I think you have responded without the chance to see my new question…

In my prior post, I added.

Would ground speed be used for traffic separation under radar environment?

I added this question because timing from various waypoints would be different for planning purposes with ground speed factored in which would negate the use of Mach speed alone.

As you know, from day to day, the wind conditions will vary greatly especially this time of the year, so planning just on mach speed alone wouldn’t work for planning way point to way point.

On head wind situations, one couldn’t get in as many planes in a certain period of time in one segment of a flight where as the inverse, tail winds, you could push more planes through that segment in the same period of time.

I do realize the separation remains the same no matter how many planes you have in that segment of flight, but I am talking now the planning stage for ATC.

If they only have room for 10 flights in an hours worth of time, and they need to push 10 through, but can’t based on wind conditions (only get eight through), somebody will get that amended clearance no matter what airspeed you indicate whether it be in knots or mach? This of course becomes a “separation issue” since ATC cannot accommodate the volume of planes.

Thus the importance of accurately reporting your TAS on your flight plan which is I think generally filed in knots???.



I understand where your going and there several things that can come into play… As you mentioned, when filing a flightplan you report your cruise flight TAS. The controllers use that, along with your A/C type on their “list” (7110.65 Appendix A) to determine your performance category/capability. So, enroute spacing along the same route at the same speed is generally not a problem (our discussion above)…

Flow into the terminal area becomes a different game… The NAS Command Center just down the road from IAD, bases traffic management on airline schedules and historical statistics. All kinds of factors can affect this though, such as weather, wind, special events (Super Bowl, Nascar races, etc…) Dependant on traffic levels flow programs can be implemented. During a flow program speed limits and/or vectors are issued for spacing.

Now…in regards to your question, in the radar environment the controller bases spacing on the groundspeed the computer displays to him/her. But pilots fly by IAS or Mach. The controller will ask “N12345 say speed” (they will specify IAS or Mach) if necessary then a restriction is issued…knots are knots or a Mach number based on the conversion they have. By speeding up one and slowing another several times over as they enter the airspace, the “conga line” is created. Sometimes restrictions are issued 1000 nm out from destination…other times not until the terminal area based on saturation. I’ve been given a speed assignment going into the NY area over KS… :open_mouth:


I wasn’t going for a PhD in aircraft separation. He asked a simple question and I gave an answer.

Just once allen, I’d like for you to admit that what I said was not my opinion. In fact I would love to have an apology from you saying I was wrong (or at least implying it) in this particular case but that will never happen.


:laughing: Well…hopefully now it’s been thoroughly covered for OP Mike… :wink:


You still think you are right after Azav8r latest response to me???

I don’t think so.

You gave nothing to support your position about ATC spacing from the ATC pubs like Azav8r gave me or any FARS references which are the rules we fly by. Just a reference that I can’t even read online so I can only assume it’s based on your opinion. I have to wait til May???

Your reference by the way, based on the way I understand you talk, only talks about airspeeds of aircraft (you didn’t provide an online reference again for me or anybody else to look at) not ATC spacing rules. Let me remind you what you said…

I say ground speed. Would you like to try again with a correct answer???

Your “simple answer” above does not give distinction between radar and non radar environment and sorry, with a blanket answer like that, it’s flat out wrong especially if you don’t know the parameters or environment of the flight (radar vs non radar)

Now to give you the benefit of the doubt, which I doubt you even considered, it is an international flight in question and just as I learned tonight myself, it is possible the flight is going into non radar coverage and speed is based on what is put on the flight plan (TAS) and must be adhered to no matter if it’s communicated to ATC in knots or mach for spacing of traffic.

If it’s being conducted under a radar environment, your answer is flat out wrong.

I’d suspect you didn’t consider that or you would have put it in your original response, the knowledgeable person you are.

And I will keep repeating 1/2 right answers you provide are worse then NO answer.

Customer service anyone???



The usual married couple-like bickering probably scared him away…


Over water, non-radar separation is done with the Mach number.
TAS from the flight plan is used by the controllers long before the airplane arrives at the coast out point for their planning. Aircraft with autothrottles at high altitude select Mach hold instead of IAS hold while climbing, cruising and decending, the rest of us hold IAS or Mach the old fashioned way. :confused:
Depending on the airplane most jets transition between IAS and Mach in the high 20,000 ft. area. It also changes from day to day depending on temperature.
In the G2 typically we climb at 300 Kts. IAS until .75M, decend at or slightly above cruise mach (.77-.81 typically) transitioning to 300-320 IAS depending on any ATC speed restrictions.
Why Mach instead of IAS? The everyday practical reason is that at higher altitudes an aircraft will overspeed the Mach limit (Mmo) before hitting the airspeed limit (Vmo).



Not from you. To automatically assume someone is giving an opinion when he is giving a fact from a source - a source that is already on the newsstands - is not customer service. It’s just another case of you being holier and smarter than everyone else.

Again, the guy asked a simple question. I gave a correct answer. Because YOU couldn’t find the answer correctly, you ASSumed I was wrong.

I found the answer in another reference - one from the FAA - in about .1 second. Why didn’t you find it if you said you searched for Mach number separation? It’s at FAA Order JS 7110.65S Chapter 8 Section 3.

Admit it, Allen. You just hate it when I’m right. For some reason that I cannot fathom, you have developed a real dislike for me. Perhaps, as a retired government employee, you are suppose to dislike a few people and I happen to be one of those. In any case, I don’t want an apology from you. In stead, I think you owe an apology to everyone in the forum for showing such hatred for anyone who disagrees with you.

Nope, no customer service there.