Interesting fact: The worlds currant fastest airliner.

As a (very rough) rule of thumb we fly relative to indicated airspeed in knots up to approx FL280 to FL300 and then Mach number above that. Consequently each jet airliner has 2 maximum speeds for their normal operation – Vmo (mx operating speed) and Mmo (max operating mach number). For the A340 Vmo is 330 kts and Mmo is M.86. Therefore up to FL 290’ish we would be limited to 330 kts but above that the controlling speed limit is M.86. The actual altitude that the changeover occurs at is the altitude where the temperature is such that M.86 equates to 330 kts.

One thing that needs to be borne in mind is that when the aircraft reaches high altitude the air molecules (which is what the instruments sense) become much more spread out (lower pressure) so instruments tend to massively underread. This is the difference between Indicated airspeed and True airspeed (any professionals please give me a little latitude in simplifying this explanation!). Consequently an indicated 330 kts at altitude might actually be a true airspeed of over 500 kts. This is how an aircraft can actually fly (‘true’ speed) much faster than Vmo (which is an ‘indicated’ speed).

As you can probably now see, it’s much easier to use Mach numer (easily simplified by pretending that the numbers are a percentage of the speed of sound) as a speed reference when in the higher parts of our atmosphere.

Steve, it’s not really possible to define a high altitude aircraft’s top speed in mph or ft/s as the speed is limited by its proximity to the speed of sound. This in turn is defined by the ambient temperature which is obviously variable. Consequently Mach Number becomes the only useful measure of speed for these aircraft types when up at altitude (where the top end speeds occur).

You learn something new everyday 🙂

Steve, it’s not really possible to define a high altitude aircraft’s top speed in mph or ft/s as the speed is limited by its proximity to the speed of sound. This in turn is defined by the ambient temperature which is obviously variable. Consequently Mach Number becomes the only useful measure of speed for these aircraft types when up at altitude (where the top end speeds occur).

The Convair 880 and 990 had a top speed of 600 mph or 880 ft/s and had a cruising speed of Mach 0.91

Thanks for the expalanation HJ. All my previous enquiries as to why this was had proven fruitless. On the occasions that I flew with some of the American guys it was apparant that they didn’t give a damn about the cost of fuel and charged around everywhere at close to M.84 in Europe and above it in the US. Very different from the fuel conservation techniques that are de riguer in Europe.

Yes that’s an interesting point. They do look like Mmo (maximum operating Mach number) rather than a normal cruising speed. The reason I think this is interesting is that this speed is often faster uf the aircraft is FAA registered than JAA registered. For example, JMC would put a couple of 757’s on the FAA register for winter operations in the US. At these times the Mmo went up from the JAA figure of M0.84 to M0.86

The reason the FAA speeds are faster is they way they calcuate Mmo for a given type.The FAA require a margin of 0.05M between Md (design limit dive speed) and Mc (design cruise speed). Whereas the CAA require the margin between Md and Mc be sufficient to cope with a head on gust of 45ft/sec at Vmo/Mmo which equates to 0.07M

In the case of the 757/767 Md is 0.91M so 0.91-0.05=0.86M under FAA rulings and 0.91-0.07=0.84M under CAA.

Shall I get my coat? Too warm for a coat. It’ll have to be an anorak.

i seem to remember a programme on National Geo about the 744…they got it up to the speed of sound in a shallow dive, the test pilot reported it handled rather well….apart from the winglets nearly being ripped off

Ren Frew, who’s Gary? :confused:

Not you obviously..? 😮

We used to cruise the 757 at a cost index that gave M.795. If you upped it to M.82 the fuel burn would go through the roof.
Our 744’s have always operated at a typical cruise of M.855 which I gather is the same as BA. My brother tells me that they typically cruise at M.85 in the 777. The typical cruise expected for the 380 is M.86 .
The VC10 and alot of the older low bypass jets did cruise at much higher speeds than modern stuff so in light of Concorde’s retirement I’d hazard that their statement could well be correct in this day and age.

Now if I had my way I’d fly as slow as I possibly can. Slow speed means longer flight times so as I am contracted to fly for a fixed number of hours a year the slower I go the fewer sectors I have to fly and the fewer times I have to go to work. QED 😎

Ren Frew, who’s Gary? :confused:

Dean Cross, sure the 744 isn’t a bit faster than that?
And I thought the 757 was about 0.82 or 0.83?

Is that one soooo dated or is it a figment of my imagination? :diablo:

Any more like that and the Mods will prune this thread.

I thought typical cruise for the 744 was more around .85 and the 777 .84? Of course depending on cost index, but .89 would be going like the clappers so to speak, not for normal on-time operation?

Is that one soooo dated or is it a figment of my imagination? :diablo:

Oh dear!

Paul

In normal cruise, the 744, 777 and TU154 are the fastest airliners. The A380 will be slightly slower than those 3 believe it or not.
744 = M0.89 or M0.90 typical cruise
773ER = M0.88
TU154 as 773ER
A380 = M0.87/8

Thanks for the info Gary…

He’s raisin an interesting point, IMHO. :diablo:

Is that one soooo dated or is it a figment of my imagination? :diablo:

In normal cruise, the 744, 777 and TU154 are the fastest airliners. The A380 will be slightly slower than those 3 believe it or not.
744 = M0.89 or M0.90 typical cruise
773ER = M0.88
TU154 as 773ER
A380 = M0.87/8

Are you aware that in the Military forum, the same subject is being discussed?

Remember Machmeters on subsonic aircraft are very inaccurate approaching Mach 1.

The BOAC VC10 alleged to have got to M0.96 Indicated, or whatever, in a dive was probably well short of that really.

Ah that fatefull day 2 years ago when i saw a vc-10 powering out of ncl !! awesome sight!!

Someone was paying attention to that Red Arrows programme last night 😉

I think Mach 0.886 is typical maximum cruising speed in the VC10, not necessarily the structural maximum speed. Those Boeing figures are Maximum Airframe Limit speeds I guess?

lol I saw that programme very good wish they did one about the Blue eagles 😀 What luke said is true 😉

JAmes