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Author Topic: VMO on the speed tape - Part 2  (Read 793 times)

Simonijs

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VMO on the speed tape - Part 2
« on: Wed, 22 May 2019 14:03 »
Hi Hardy,

Now that we have established that MMO is indicated on the speed tape as a fixed CAS at one and the same Flight Level, independent of temperature changes, I wonder if you are able to calculate the corresponding TAS from this CAS, and hence the Mach-number.

From what I have seen (with two formulas now being in agreement) is, that the CAS number in PSX produces a Mach-number of ± 0,92. So: at FL380 in PSX, VMO is shown on the speed tape as 297 kts, the amber band starts at ± 290 kts. This corresponds to M0,92 and M0,9 respectively. I had the aircraft go to 290 kts yesterday, and it started to make noises as if things were about to fall off.

Below, I included two screenshots taken from the E6B (iPhone) app, confirming my Excel findings. This happens at all FL's (tested between FL300 and FL400 in 1000 ft steps).



Also on the Low Speed Side, the numbers seem to be too high: some 5-6 kts at the lower flight levels up to ± 20 kts at the higher flight levels. If you like, I can send you a table with all results between FL300 & FL400.

Regards,
Simon


Hardy Heinlin

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Re: VMO on the speed tape - Part 2
« Reply #1 on: Wed, 22 May 2019 17:35 »
Hi Simon,

I'm sorry, I don't understand your text. What parameter exactly do you think is wrong in PSX? Or is it just about the noise? These vibrations start at such high Mach numbers intentionally; the same happens in the big sims. It's similar to the low speed buffet vibration noise which starts before the stick shaker activates.


Cheers,

|-|ardy


I just did a PSX test re your 290 KCAS screenshot. My result:

290 current KCAS (PFD)
38000 ft current altitude (PFD)
-14°C current TAT (EICAS)
1013.25 hPa current QNH (PFD and weather model)
524 current KTAS (ND)
0.901 current Mach (PFD)
-50°C current SAT (EICAS)

Perfect agreement with your E6B output.
« Last edit: Wed, 22 May 2019 17:54 by Hardy Heinlin »

Simonijs

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Re: VMO on the speed tape - Part 2
« Reply #2 on: Thu, 23 May 2019 08:14 »
In the two screen shots that I posted earlier, 290 kts (at FL380) is where in PSX the amber band starts and where manoeuvre margin should still exist before actually getting to MMO. 297 kts (at FL380) in PSX is where the “barber pole” starts.

The bottom of the barber pole indicates maximum airspeed, so the point where - with flaps up - VMO/MMO is reached for the first time: i.e. 365 kts or M0,9 for the 747. In PSX and at FL380, however, the barber pole starts at a CAS value of 297 kts, resulting in a TAS that corresponds with M0,92. The PSX amber band CAS value of 290 kts results in a TAS that corresponds with M0,9 which is the value for MMO. Since I heard buffet noise at 290 kts, and since we now have both established that at 290 kts a TAS - consistent with M0,9 is reached - it looks like the aerodynamic behavior is modelled correctly, but the indications on the speed tape not.
I think, that the barber pole should start at 290 kts instead of at 297 kts. With the amber band extending down from 290 kts to ± 284 kts. At...: FL380 only. Other values of course for other Flight Levels, as shown below.

In the following PSX screen shot, at FL370 the barber pole sits at 304 kts; the amber band starts at ± 298 kts (SAT = -58 C).



At 304 kts CAS, the Mach-number equals M0,92; at 296,45 kts CAS the Mach-number would read exactly M0,9. So in this scenario: I think the barber pole should start here at ± 296 kts with the amber band extending down to ± 289 kts CAS.

This is the case at all flight levels between FL300 and FL400: the number that I read as VMO on the speed tape, produces a TAS that is equivalent to ± M0,92. At which point the aircraft in real life is well inside the high speed buffet region.

Regards,
Simon

PS - I just looked at a picture of the PFD in the FCOM; here, at FL310 the barber pole starts at ± 338 kts, the amber band extends down to 330 kts. In PSX, the barber pole at FL310 starts at 348 kts...

Hardy Heinlin

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Re: VMO on the speed tape - Part 2
« Reply #3 on: Thu, 23 May 2019 10:14 »
PS - I just looked at a picture of the PFD in the FCOM; here, at FL310 the barber pole starts at ± 338 kts, the amber band extends down to 330 kts. In PSX, the barber pole at FL310 starts at 348 kts...

I think you can't compare KIAS values when M0.92 is setting the barber pole. Can you see the Mach numbers and temperatures in your comparison?

In general, I don't see why you're making it so complicated with CAS/TAS/Mach calculations. I thought those fixed reference values from the books are understood now -- as discussed in the first thread (Part 1):

http://aerowinx.com/board/index.php?topic=5256.msg56579#msg56579

5th engine carriage mode:
330 kt or M0.85

Normal mode:
365 kt or M0.92

It's always this fixed KIAS or this fixed Mach of the pair -- whichever is lower on the tape.


Regards,

|-|ardy


Quote
365 kts or M0,9 for the 747

No, it's M0.92.

Simonijs

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Re: VMO on the speed tape - Part 2
« Reply #4 on: Thu, 23 May 2019 10:46 »
Quote
No, it's M0.92.

Ehhh...: this is what I see.



I was taking these speeds as a reference. And the note below it.

Regards,
Simon

Hardy Heinlin

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Re: VMO on the speed tape - Part 2
« Reply #5 on: Thu, 23 May 2019 10:49 »
See :-) Just one short sentence would be sufficient to point at the problem. Without any math :-)

Other manuals say 0.92.

Is this 0.90 a typo or a company option? And is it just on the paper or in the FMC as well?


|-|

Simonijs

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Re: VMO on the speed tape - Part 2
« Reply #6 on: Thu, 23 May 2019 11:16 »
That was taken from KLM's FCOM. This next picture is taken from the AOM, of course showing same numbers...



Will wing design for the 747 differ as an airline option?

Then one last question: with the amber band starting in PSX at 290 kts (FL380), should I be hearing buffet noise in straight and level flight (no maneuvering). My guess would be "No".

Cheers,
Simon


Hardy Heinlin

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Re: VMO on the speed tape - Part 2
« Reply #7 on: Thu, 23 May 2019 18:38 »
Since PSX version 10.0.6 the buffet noise starts with 0 dB at Mach 0.9, and this dB value smoothly gradually increases with the rising Mach. The KIAS value has no influence on this.

Quote
Will wing design for the 747 differ as an airline option?

No, but airline policies differ. Each authority has a different opinion, as we learn almost daily. -- Aside from that, on your picture the "design" refers to VA, not VMO. VMO is just a straight line set by the office, not by empirical physics.

One difference, however, lies on the hardware side indeed: The 747 freighter upper deck behaves differently to the non-freighter's upper deck in terms of Mach effects. That's why the freighter systems include a Mach trim function in addition to the usual speed trim function. I can't recall if there is a freighter specific VMO though.

By the way, the Mach number of the air flow above the upper deck is higher than the Mach indication from the ADC which refers to the aircraft speed as a whole. That's also the reason the 747 flight deck is so loud.


Cheers,

|-|ardy


P.S.: My questions in this discussion are:

(A) Which airlines use VMO M0.90 and which use M0.92?

(B) Do the systems (FMC, EIU etc.) always use the M0.92 value or can customers program this?

« Last edit: Thu, 23 May 2019 19:06 by Hardy Heinlin »

Simonijs

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Re: VMO on the speed tape - Part 2
« Reply #8 on: Thu, 23 May 2019 20:51 »
Well..., I just fell off my chair in surprise reading this document (FAA Type certificate data sheet for all 747 models - July 2008): http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/0/29f8f1f15b2b08b786257479004b50e1/$FILE/A20WE.pdf, with all eligible KLM serial numbers listed in it (747-406/406F). VMO/MMO: 365/0,92 indeed, and quite misleading to me with KLM documents giving other numbers...

So...: 0,92 it is !

Regards,
Simon (in need of some holidays now).






Hardy Heinlin

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Re: VMO on the speed tape - Part 2
« Reply #9 on: Thu, 23 May 2019 22:48 »
I guess KLM have decreased it to 0.90 in their internal policy, but their avionics still apply 0.92 probably.


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Hardy Heinlin

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Re: VMO on the speed tape - Part 2
« Reply #10 on: Sun, 14 Jul 2019 14:37 »
MMO 0.90 is now a selectable model option in PSX 10.82:

http://aerowinx.com/board/index.php?topic=4191.0


Regards,

|-|ardy

Simonijs

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Re: VMO on the speed tape - Part 2
« Reply #11 on: Tue, 16 Jul 2019 17:55 »
A very neat solution, Hardy. Thank you for this one!


Regards,
Simon

United744

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Re: VMO on the speed tape - Part 2
« Reply #12 on: Fri, 19 Jul 2019 01:57 »
M 0.92 is the 747-400 MMO.

M 0.90 MMO is for the 747-8.

If the MMO has been reduced for the -400 it is probably to match the -8 where there are mixed fleets.

Hardy Heinlin

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Re: VMO on the speed tape - Part 2
« Reply #13 on: Fri, 19 Jul 2019 07:02 »
Simon(ijs) did a lot of research on this subject recently across several airlines, and it ended up with a reply from Boeing. It has to do with the time the JAA started to exist and the time certain 744 variants started to exist. Until the date the JAA was founded, all existing 744 types got FAA certifications (0.92). Thereafter, non-US aircraft got JAA certifications (0.90) -- if I understand it correctly. Simon may jump in :-)


|-|ardy


However, what's confusing me now is that the JAA was founded in 1970 and disbanded in 2007:
https://en.wikipedia.org/wiki/Joint_Aviation_Authorities
« Last edit: Fri, 19 Jul 2019 07:12 by Hardy Heinlin »

skelsey

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Re: VMO on the speed tape - Part 2
« Reply #14 on: Fri, 19 Jul 2019 08:57 »
However, what's confusing me now is that the JAA was founded in 1970 and disbanded in 2007:
https://en.wikipedia.org/wiki/Joint_Aviation_Authorities

JAA was superceded by EASA in the early/mid 2000s, so the JARs became the EASA certification standards, JAR-OPS became EU-OPS etc after that point.

The/a big difference is that the JARs had no direct force in law (they were ratified by individual member states in to each member state's national law) whereas EASA regulations are written in to EU law and therefore legally binding on all member states.

Hardy Heinlin

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Re: VMO on the speed tape - Part 2
« Reply #15 on: Fri, 19 Jul 2019 10:54 »
As the JAA had existed since 1970, why did they apply the 0.90 MMO certificate only to later 744 aircraft long after 1988?

Maybe I've misunderstood that statement. Perhaps it didn't mean to say the JAA didn't exist but the JAA certificate (0.90) didn't exist. Perhaps the JAA changed their own limit from 0.92 to 0.90 a couple of decades ago.

Jeroen Hoppenbrouwers

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Re: VMO on the speed tape - Part 2
« Reply #16 on: Fri, 19 Jul 2019 11:59 »
Or, they simply did not want to mess with all the documentation you need to change if one simple number on the airworthiness certificate changes. It's usually much easier to apply new regulations forward-only.

Hoppie

Hardy Heinlin

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Re: VMO on the speed tape - Part 2
« Reply #17 on: Fri, 19 Jul 2019 12:10 »
Forward-only is it in any case.

I guess by the phrase "the JAA didn't exist" they just meant to say "the JAA certificate re 0.90 didn't exist".

It's like saying "the Hoppie didn't exist in 2008" instead of "Hoppie's PSX Router didn't exist in 2008".


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VolleyballCALVA

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Re: VMO on the speed tape - Part 2
« Reply #18 on: Sat, 20 Jul 2019 14:54 »
I think the VMO is still M 0.92
But within RVSM airspace the Maximum Speed is restricted to M 0.90
That's why it said M 0.90 as Maximum operating speed in FCOM ?

Hardy Heinlin

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Re: VMO on the speed tape - Part 2
« Reply #19 on: Sat, 20 Jul 2019 15:04 »
According to Boeing, the MMO is airline specific. That is, airline specific is just the red tape setting on the PFD. The EICAS overspeed message and siren is triggered at 0.92 in all models, even if the red tape starts at 0.90. This has been proven in Lufthansa sims.

MMO is an aerodynamic limit; if you exceed it, the aircraft may break apart.
An RVSM restriction is a strategic limit; if you exceed it, you may collide with another aircraft.


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