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Mach.84 At FL 280  
User currently offlineMr.BA From Singapore, joined Sep 2000, 3423 posts, RR: 22
Posted (12 years 1 month 1 week 3 days 11 hours ago) and read 4705 times:

Hi everyone,

Apologise for the simple question, I can't find answers to this from anywhere so i decided to ask here.

Can anyone enlighten me what's approximate the IAS at FL 280 cruise Mach.84/86?

Thanks!


Boeing747 万岁!
24 replies: All unread, jump to last
 
User currently offlineB747skipper From , joined Dec 1969, posts, RR:
Reply 1, posted (12 years 1 month 1 week 3 days 11 hours ago) and read 4679 times:

.84 at FL 280 is 332 KCAS
.86 at FL 280 is 343 KCAS
IAS about 2, or 3 knots more... depends on the airplane
(s) Skipper


User currently offlineMr.BA From Singapore, joined Sep 2000, 3423 posts, RR: 22
Reply 2, posted (12 years 1 month 1 week 3 days 10 hours ago) and read 4670 times:

Thanks Skipper, figures as I expected. Just wondering how Air France did Mach. 84 at FL 280 from Singapore to paris when it has a Vmo of 330.  Confused


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User currently offlineB747skipper From , joined Dec 1969, posts, RR:
Reply 3, posted (12 years 1 month 1 week 3 days 10 hours ago) and read 4667 times:

Which plane has a Vmo 330 with AF, a A-340 or a 747...?
Our Vmo - normal - is 383 KIAS - but
We also have a reduced Vmo, only when reserve tanks 2 and 3 are full...
But these are empty by the time we reach top of climb...
Then they can get faster...
Further - FL 280 is their initial FL, because heavy... therafter
They climb 310, then 350, then 390 at the end... as traffic/weight permits -
They probably told you .84... meaning that is what they will eventually do...
A 747-400 normally "files" for .85 as far as ATC is concerned.
What I personally like (200 series) is 85 initially, 84 for overwater crossing -
And 83, at the end, last couple of hours when light.
That is LRS "best" based on weight...
 Smile
(s) Skipper


User currently offlineTsentsan From Singapore, joined Jan 2002, 2016 posts, RR: 15
Reply 4, posted (12 years 1 month 1 week 3 days 8 hours ago) and read 4640 times:

I think that temperature plays a part in the Mach-to-IAS conversion.... so maybe its to do with temperature too?


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User currently offlineB747skipper From , joined Dec 1969, posts, RR:
Reply 5, posted (12 years 1 month 1 week 3 days 7 hours ago) and read 4637 times:

Dear Tsentsan -
xxx
I used my normal pilot slide rule - Jeppesen CR-3 - to convert Mach to CAS...
CAS calibrated airspeed has no temperature correction from Mach number...
Now, what you think about, is TAS true air speed - there, correction applies.
xxx
The numbers above, requested by Mr.BA would give -
about 504 Kts TAS for .84, and -
about 515 Kts for .86, that is assuming standard temperature -41 degrees
At these calibrated speeds, the temperature rise is about 35 degrees...
Any deviation in temperature from standard, each 1 degree change + or - will change the TAS by 1 kt approximately, warmer = faster, colder = slower.
These "old whiz-wheels" are still great, and mighty accurate...
In real life it is rare that the plane's TAS indicator shows more then 2 kts error...
This despite the fact I should get new prescription glasses...
xxx
(s) Skipper  Wink/being sarcastic


User currently offlineMr.BA From Singapore, joined Sep 2000, 3423 posts, RR: 22
Reply 6, posted (12 years 1 month 1 week 3 days 6 hours ago) and read 4618 times:

Hi Skipper,

Which B747 has a Vmo of 383? Does Vmo change according to weights?

It's neither an A340 nor a B747. It's a B777-200. Due to the large amount of traffic leaving Singapore around the same time for Europe on the same routes intially ATC always asks for cruising Mach number for a certain FL, which is FL 280 most of the time. Well KL doesn't want to give Singapore originated flights FL 310, they reserve it for MAS.

Every B747 responds with either Mach.85 or .86 which can be understandadble since it's Vmo is 365 but Air France responds with Mach.84, which exceeds 330 knots (anyway you won't want to go too cose to 330 either, you won't want to overspeed?). I think they have to maintain .84 intially in this case since their clearances are given and planes are separated judging from the speed given by pilots.

Thanks a lot!



Boeing747 万岁!
User currently offlineBroke From United States of America, joined Apr 2002, 1322 posts, RR: 3
Reply 7, posted (12 years 1 month 1 week 3 days 2 hours ago) and read 4550 times:

Mach number for a specific airspeed is depend on the ambient temperature. The higher the temperature, the lower the Mach number; the lower the temperature, the higher the Mach number. This is all for a constant airspeed.
Since compressability and shock wave formation are dependent on Mach number, many airspeed records were established at low altitudes and high ambient temperatures until supersonic airplanes were developed.


User currently offlineB747skipper From , joined Dec 1969, posts, RR:
Reply 8, posted (12 years 1 month 1 week 2 days 23 hours ago) and read 4518 times:

Dear Broke - correct on that - Thank you very much. The answer for Mach .84 and 86 was provided. IAS, CAS and TAS...
xxx
Dear Mr.BA - the 747 classic is VMO 383 KIAS and MMO .92 Mach...
VMO does not change with weight - correct - aerodynamic limit...
But one thing with 747 airplanes fitted with 9 tanks (with #2 and 3 reserves) these tanks are located on the outside portion of the wing span (outboard) and when full there are structural limits. So anytime these tanks contain any fuel - the VMO is reduced to 330 KIAS - and the "barber pole" VMO indicator is relocated on the airspeed is set at 330 by a knob at the bottom of the airspeed indicator...
These tanks are emptied first - during the climb, they get to be empty just about time the aircraft level off in cruise, then the VMO "knob" is switched back to normal - VMO 383...
 Smile/happy/getting dizzy
(s) Skipper


User currently offlineMr.BA From Singapore, joined Sep 2000, 3423 posts, RR: 22
Reply 9, posted (12 years 1 month 1 week 2 days 18 hours ago) and read 4466 times:

Thanks Broke and Skipper! I didn't know about the Vmo 383 thing, interesting, thank you!


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User currently offlineCx flyboy From Hong Kong, joined Dec 1999, 6642 posts, RR: 55
Reply 10, posted (12 years 1 month 1 week 2 days 17 hours ago) and read 4456 times:

Did you know that the designed VMO for the 777 is 360kts, but Boeing would have had to do additional flight tests to certify it at the higher speed. They weren't willing to do this, so Boeing said to the other airlines that it was going to be 330kts unless an airline would come out and pay for the additional flight tests. Nobody did, so the VMO is 330. If you overspeed a 777, the maintenance check for overspeeds up to 360kts is a simple walkaround! Interesting!

User currently offlineB747skipper From , joined Dec 1969, posts, RR:
Reply 11, posted (12 years 1 month 1 week 2 days 16 hours ago) and read 4435 times:

Dear Cx flyboy -
xxx
Mighty interesting fact... have to copy that one on my notepad thanks to you. Boeing is Boeing with their ways of doing things... I wish they had learned about the ways Douglas and Lockheed were doing as well... all they were happy to do is to drive competition out of business...
xxx
External inspection - I smile - i.e. overweight landing... then "sign off"... how do they know the main wing spar is not bent, or is not about to fail...
xxx
I admire Boeing... but up to a limit... Airbus does poopooh as well.
Cheers
(s) Skipper


User currently offlineMr.BA From Singapore, joined Sep 2000, 3423 posts, RR: 22
Reply 12, posted (12 years 1 month 1 week 2 days 15 hours ago) and read 4428 times:

Hi Cx_flyoby,

Since tests to certify a Vmo of 360 knots were not done, safely to say that B777 airplanes are only certified to 330? Anything above 330 is considered overspeeding? Didn't know that a simple walkaround would do! In that case, any explanations on how did AF managed to do .84 at FL 280 at all? I'm sure it will exceed 330.

Just thought of one more thing, (didn't want to tart another not so useful thread), regarding rudders, I had a chance to 'kick' the left rudder on the B747-400 once. After reading posts earlier discussing about the one engine landing, I come to know that when you release your foot from the rudder panel, the rudder will move back to the centre. But however when I elft the aircraft, the rudder was still deflected to the left. I kicked the left rudder and just released it, never made any efforts to centre it (it would centre anyway). Hydraulics was still turned on then. How did the rudder manage to stay at the 'left position'?

Thanks Capts

alvin



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User currently offlineTsentsan From Singapore, joined Jan 2002, 2016 posts, RR: 15
Reply 13, posted (12 years 1 month 1 week 2 days 13 hours ago) and read 4408 times:

Alvin->

Broke Said:
Mach number for a specific airspeed is depend on the ambient temperature. The higher the temperature, the lower the Mach number; the lower the temperature, the higher the Mach number. This is all for a constant airspeed.

Given the temperatures in our region are higher than the US/Europe, it is very possible that M.84 at FL280 would be bordering at the 315-330IAS level. Furthermore, M.84 could mean M.835 or M.833 and not necessarily always M.84 exactly.



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User currently offlineB747skipper From , joined Dec 1969, posts, RR:
Reply 14, posted (12 years 1 month 1 week 2 days 12 hours ago) and read 4405 times:

And Skipper says the contrary...
xxx
Cold temperature problem - (standard -10)
300 KIAS - FL 330 - OAT minus 61 - Mach number .845
Warm temperature problem - (standard + 10)
300 KIAS - FL 330 - OAT minus 41 - Mach number .845 still...
xxx
- I did not change IAS, but changed OAT 20 degrees... NO MACH CHANGE
xxx
However -
In the first case, TAS - would be 484 Kts -
Cold temperature - looks like LOWer speed...
In the second case, TAS - would be 502 Kts -
Warm temperature - looks like HIGHer speed...
Exactly the opposite of what you claim...
xxx
QED - Low temperature = low speed for same given Mach number
High temperature = high speed for same given Mach number
xxx
Tsentsan, please verify your statements before broadcasting them...
Or advise my airline, that the manager 747 training, should go to pilot school.
xxx
(s) Skipper  Wink/being sarcastic





User currently offlineB747skipper From , joined Dec 1969, posts, RR:
Reply 15, posted (12 years 1 month 1 week 2 days 12 hours ago) and read 4398 times:

P.S. Sorry
Forgot you, Broke -
Same applies to your theories...
(s) Skipper


User currently offlineEjazz From United Arab Emirates, joined May 2002, 725 posts, RR: 34
Reply 16, posted (12 years 1 month 1 week 2 days 11 hours ago) and read 4394 times:

Hi Mr BA,

Flights leaving Singapore for Europe always receive a NO PDC (no pre-departure clearance) Flight level of FL280. FL310 is not really reserved for MAS but for Malaysian Airspace. The NO PDC Flight levels were split amongst varies FIRs to allow originating flights from those FIRs to receive an instant clearance. Flight levels outside of those allocated to NO PDC can be requested, so if you are light enough going to Europe you could request FL350 since it is not allocated.

NO PDC was introduced to reduce the large delays on busy routes such as those to Europe because previously a clearance could take upto 1 and a half hours.

The B777 operating in ECON mode will normally fly at M0.84 at FL310 and above. At FL280 we'd be around M0.81-0.82 but when asked by ATC for our Mach number at FL280 most times I do find the F/O would respond with our normal cruise speed of M0.84 not realising the substantial difference. I suspect the Air France crew fell into the same trap.

Cheers



Etihad Girl, You're a great way to fly.
User currently offlineCx flyboy From Hong Kong, joined Dec 1999, 6642 posts, RR: 55
Reply 17, posted (12 years 1 month 1 week 2 days 10 hours ago) and read 4388 times:

Mr. BA,

You are correct, the 777 is only certified to 330kts, and this is where the "red army" overspeed tape starts on the PFD. However, the aircraft is DESIGNED (but not tested) to a VMO of 360kts, it just hasn't been flight tested to such that's all.


User currently offlineMr.BA From Singapore, joined Sep 2000, 3423 posts, RR: 22
Reply 18, posted (12 years 1 month 1 week 2 days 6 hours ago) and read 4359 times:

Thanks everyone again.

I guess it's right to say colder OAT results in lower GS. In winter a lot of flights seem to divert to alternates reason insufficient fuel with headwinds.

Thanks Ejazz for clearing up the atc clearance issue. If we are to compare under the same conditons, 2 B777s, one light at one heavy with the same cost index, would ECON CRZ be lower (say about .82 at FL 310) while the heavy one .84 since it's closer to Opt and max altitude?

Thanks Cx_flyboy!



Boeing747 万岁!
User currently offlineB747skipper From , joined Dec 1969, posts, RR:
Reply 19, posted (12 years 1 month 1 week 2 days 6 hours ago) and read 4352 times:

Dear Alvin -
Much of the problem comes from the different speed units -
TAS True Air Speed, IAS Indicated Air Speed, GS Ground Speed...
Many guys confuse one for the other...
I often see "correct statements" but "wrong unit" used in discussions here...
Shall we add EAS, CAS... I got a few more if needed...
xxx
And I learn, myself, a lot here, did not realise - that 330 VMO for the 777...
(s) Skipper


User currently offlineTsentsan From Singapore, joined Jan 2002, 2016 posts, RR: 15
Reply 20, posted (12 years 1 month 1 week 2 days 6 hours ago) and read 4351 times:

B747skipper n all,

Apologies for my incorrect statement. Big grin




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User currently offlineMr.BA From Singapore, joined Sep 2000, 3423 posts, RR: 22
Reply 21, posted (12 years 1 month 1 week 2 days 5 hours ago) and read 4344 times:

Talking about difference... I still have no idea why the difference here between the B772 and B773. Both aircraft have the same Vmo but different Mmo (.87 and .89 respectively). Well maybe like Cx_flyboy has said it's not tested further  Smile.

Well i also didn't know the A330 and A340 had such huge differences compared to the A340 too. (Vmo/Mmo).

A330: 360kias/M0.86
A340: 330kias/M0.86

By the way does anyone know if the Vmo of 383 or similar applies to the B744?

Dug up a bit of my memory... I think the A332 has a diving speed of 365 KIAS/M.0.93? Any known figures for the B747/777 or even the 767?

Cheers, fly safe!



Boeing747 万岁!
User currently offlineCPH-R From Denmark, joined May 2001, 6056 posts, RR: 3
Reply 22, posted (12 years 1 month 1 week 2 days 4 hours ago) and read 4319 times:

Mr. BA

Regarding the rudder question halfway up the thread, during a flight, wouldn't the wind speed be enough to push the rudder back to center? I'd imagine that while the aircraft is one ground, the wind isn't fast enough to do so  Smile


User currently offlineEjazz From United Arab Emirates, joined May 2002, 725 posts, RR: 34
Reply 23, posted (12 years 1 month 1 week 2 days 4 hours ago) and read 4330 times:

Mr BA

The FMS uses many parameters to determine the Mach Number but the wind would be the most important. The FMS has become so accurate that Mach Numbers determined and flown are now down to 100th of the local speed of sound.

I would think that a light and heavy B777 would have very similiar Mach Numbers, with the same wind, temperature and cost index. If there were a difference it would be in the region of M0.002-M0.005. On the other hand from my performance charts a B777 with a 150 kt headwind would be flying at around M0.845 and with a 100kt tailwind M0.834 so a much larger difference.

Cheers.




Etihad Girl, You're a great way to fly.
User currently offlineMr.BA From Singapore, joined Sep 2000, 3423 posts, RR: 22
Reply 24, posted (12 years 1 month 1 week 1 day 18 hours ago) and read 4255 times:

Thanks Ejazz. No wonder from pictures I see even in pretty strong headwinds the B777 doesn't have much changes in cruise mach, almost at .84 while the B747 at about .87?

CPH-R,

If the hydraulics was turned on, I think if you let go of the pedals it will return to the centre by itself. If the hydraulics were turned off, the wind can deflect the rudder to the left or right... the wind will then have control over it.



Boeing747 万岁!
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