Your first thoughts ala the topic title might sound like, "dude, to many variables, let's not go there"
But I'll break it down a little.


A 727-200 has a MMO of mach 0.90.
A 777-200 has a MMO of mach 0.87.

But they are not allowed to fly that fast! Atleast not in normal flight operations as far as I'm aware.
Some things determine that. Is it actually structural? Is there a actual stress threshold that would cause potentially fatal stress on the body or aircraft part when flying up to MMO with full throttle!

Would that 'potentially' be fatal per flight or would that significantly increase maintenance? It would duh, hence the word significantly.

Or is the reason not structural or technological but just more economical. Lower throttle is less of a pay day.

How bad would it be if a 777-200 pilot goes full throttle at FL360 for i.e. half an hour. Would things fall of, as in 'guaranteed'
Or is everything likely to turn out A-ok and all that's needed is a immediate and extra thoroughly inspection?

I know many variables are involved. The simplest would be the durability of the airliner, even if little to no extra strain or damage would occur.
But obviously durability is also a variable in this.
Are there more variables? I like them, give them to me

If somebody can break down to me why one alluminium casket is a-ok at mach mach 0.84 and the other is okido at mach 0.82?
Because I would really like to know what determines those limits.
I can't imagine why a change in few knot extra speed is dangerous and why a few knot slower is not only safe but durable and acceptable.

Luckily there are knowledged people who can answer it for me.

There is the Vne speed (never exceed velocity), and it actually gets tested. The manufacturer has to prove that the aircraft remains fully controllable at Vne + 0.07 Mach.

The Cessna Citation X has a Vmmo of 0.935 Mach, and a Vne which could even be a bit higher. In flight testing, the Citation X had to break the sound barrier.

semiless wrote:

Is it actually structural?

Yes. Excessive aerodynamic forces will bend the wings and the fuselage. Also, the actuators which move the control surfaces must be able to overcome the aerodynamic forces that act on them. Imagine a simple Cessna 172, and having to use all of your muscular force to pull the yoke.

Vmmo and Vne are not related to maintenance or economical fuel use.


David

It is structural or aerodynamic - during flight testing the aircraft is tested up to Vd/Md (Design Dive Speed), which is above Vmo/Mmo. On the A320 Vd is 381kts and Md .86 (I think). These speeds are not relevant to the normal line pilot and in fact not published in the manuals (in the A320 they were some years ago, but only as background info not as need to know - they have since been removed). Above Vd there are no guarantees - the aircraft might have undesirable flight characteristics (tuck under, shock stall etc. - there are some nasty effects in the transonic regime that are not encountered in the certified flight envelope, but may be found outside of it), there may be flutter oder the structure may fail for other reasons. So the reason for the speed can be structural (stiffness of the structure, flutter protection - or just strength of materials) or aerodynamic (shock stall etc.). This video shows the flutter test of the A380 which is conducted at around Md - as you can see during the first attempt some part of the aircraft came off...so its pretty dangerous: https://www.youtube.com/watch?v=ImSuZjvkATw I somehow doubt however that you would be able to exceed Md (you can exceed Mmo, which happens from time to time if you are not careful in turbulent air, but I don't think you can exceed Md in these circumstances) in level flight at typical cruise levels - most aircraft don't have a lot of surplus thrust at cruise levels and its very slow to accelerate.

Thanks for the information. I didn't knew all of that testing was involved. I did know they tested aircraft for speeds, as seen in many windtunnel videos among other sources. But what I want to know is why limit a 737-200 for example to mach 0.82?
Is the 737-200 not structurally/aerodynamically sound to operate at speeds there above?
So let's assume we decide to operate all 737-200 up to mach 0.84 from now on? Meaning as in standard operating procedures.

Oh, and this is a hypothesis to validate my question. I know it is not allowed.

What would this cause in the short/long term?

flyingturtle wrote:

Vmmo and Vne are not related to maintenance or economical fuel use.

It does in the sense that higher speeds all the way up to i.e. Vne cause more stress and that aircraft would thus need more maintenance. That is what I was getting at. Sorry if it was unclear.
But I also wasn't necessarily talking about Vne. Vmo itself or there slightly above is not necessarily Vne, although often.
And I wonder how it would limit a aircraft if it were to only fly at Vmo or even near Vne throughout it's life. Instead of the often lower cruise speeds.

Would that lead to serious damage in xxx(x) flight hours?
With this thread I'm more inclined to know how it is bad rather then why. But any comment is appreciated obviously.

EDIT: @ FLOW: Thanks, nice video:)

In first place the margin between Mmo and Md is just this: a margin to avoid dangerous situations if Mmo is inadvertently exceeded (which happens a few times over the life of an airframe, due to inattention/turbulence/other factors) - if Vmo is exceeded the flight crew will (should) enter this into the technical logbook and the maintenance people will conduct an inspection based on their maintenance manuals (which will be depending on the aircraft type, there can also be different inspections depending on the severity of the overspeed - exceeding Mmo by 2 knots is a different thing than exceeding Mmo by 50kts in a high speed upset). I not in maintenance so I do not know which checks are done exactly. But I remember hearing that there was an issue with 737 classics (I fly Airbus, so this is just hearsay - 737 rated guys may be able to confirm) with the slats. Apparently the wing/the slats started to bend slightly during high speed cruise (i.e. higher than the usual cruise Mach number, but obviously still below Mmo) and would not fully return to the original shape before landing, leading to problems during slat extension. Cruise Speeds close to Mmo are fairly common, but this is depending on the airline. Most airlines request pilots to adhere to the cost index which usually result in a "normal" Mach number, but depending on how strict this is handled by the airline some pilots may decide to increase/decrease the cruise speed (increase to get home faster, decrease if their salary depends on the hours flown...). I know some pilots who fly the A320 at Mach .80 during most flights (unless there is turbulence) to save time (normal cruise speed, depending on the cost index is around .76 to .79, Mmo is .82). But it does not really make a lot of a difference - on a typical flight you will save around 3 to 5 minutes at most...its only Mach ,02 or ,03 more than the usual cruise Mach number...

I believe the 747-100 was flown to .99 mach in Vne/flutter testing. (and that's of course without the improved wing-body fairing of the -400)

semiless wrote:

Is the 737-200 not structurally/aerodynamically sound to operate at speeds there above?
So let's assume we decide to operate all 737-200 up to mach 0.84 from now on? Meaning as in standard operating procedures.

Vmo is a structural limit, while Mmo is an aerodynamic limit. Generally, when an aircraft gets to Mmo and above you start to experience things like Mach Tuck or other effects. While Vmo is a structural limit where there is a limiting design factor which will experience degradation should you operate beyond that speed for example wing flutter. The 747-200 with Reserve 2/3 tanks (IIRC) full had a different Vmo speed which was selectable on both the Capt and F/O airspeed indicators.

@ MMO. I lold. In the notification it said MMO replied to "how is MMO...." thread, XD.

Anyway, it means that aerodynamic effects don't really degrade an aircraft while at MMO. So it should be safe to fly up to that speed, or just above. Nobody is ever going above it since it is not allowed beyond test flights ofcourse.
I didn't know about mach tuck. I tried to delve into the term a bit but can't really find at what speeds mach tuck occurs per aircraft. That is probably only documented in aircraft manuals or in aircraft training. But it seems to happen far below the speed of sound and increase as a aircraft goes nearer to the sound barrier. It is potentially fatal so that alone would limit max operating speeds.
So there are indeed plenty of valid reasons for the max operating speeds pointed out in this thread.
Wing flutter destroyed a patch on the fuselage of a A380 as seen in the video above. I believe it was stressed to mach 0.95 so that is quite a increase in speed.

Did it experience mach tuck? I im sure it did, must have been good and brave pilots.

Mmo and Vmo are determined by three limits of design: the aircraft strength (maximum load), stiffness (maximum load without deformation), and flight control effectiveness.

The difference between maximum speed loads during flight testing and certificated speeds is based on two main considerations: flight control effectiveness/float/flutter and design loads, the latter known as the safety factor (expressed as a ratio) which is the margin that exists between design loads expected during normal operation (limit) and the ultimate load which is where structural failure is imminent. For commercial transport aircraft, the safety factor ratio is 1.5, whilst the dynamic load factor, based on a formula which at its core is simply lift divided by weight, is 2.5 and is expressed as a "G" force.

The load limits are calculated after flight testing based on the all up weight of the aircraft. Speed limitations alongside Vmo and Mmo, such as Vne (never exceed), Va (maneuvering), and Vb (turbulence penetration speed) are determined using a maneuvering envelope where the aircraft is always kept within the safety factor area of maximum load stress. Further considerations for Va include stall speed and full deflection of control surfaces, nominally the elevators and ailerons, while Vb considers a maximum vertical gust acceleration, and is nearly always below Va.

It is interesting to note that Mmo and Vmo are not interchangeable due to differences in air density according to pressure and temperature. Vmo can vary as much as 30 knots or more between MSL and cruising altitude and hazards such as bird strikes can affect the certificated number at lower altitudes.

Mach tuck speed minus 0.07M

semiless wrote:

@ MMO. I lold. In the notification it said MMO replied to "how is MMO...." thread, XD.

Anyway, it means that aerodynamic effects don't really degrade an aircraft while at MMO. So it should be safe to fly up to that speed, or just above. Nobody is ever going above it since it is not allowed beyond test flights ofcourse.
I didn't know about mach tuck. I tried to delve into the term a bit but can't really find at what speeds mach tuck occurs per aircraft. That is probably only documented in aircraft manuals or in aircraft training. But it seems to happen far below the speed of sound and increase as a aircraft goes nearer to the sound barrier. It is potentially fatal so that alone would limit max operating speeds.
So there are indeed plenty of valid reasons for the max operating speeds pointed out in this thread.
Wing flutter destroyed a patch on the fuselage of a A380 as seen in the video above. I believe it was stressed to mach 0.95 so that is quite a increase in speed.

Did it experience mach tuck? I im sure it did, must have been good and brave pilots.

Mach Tuck starts at the speed when part of the airflow is supersonic well below Mach 1. This speed is known as Mcrit and above it is the transonic speed range. Modern swept wing airliners cruise at transonic speeds, and are thus subjected to Mach Tuck almost every time they fly.

It is not dangerous for modern designs, but a routine part of the operating environment.

Many (most?) airliners have either a dedicated Mach Trim mechanism or this is incorporated in the FBW logic. Mach Tuck is one of the reasons why swept wing airliners use a trimmable horizontal stabiliser.

Fun fact: Mach Tuck was still not well understood when Chuck Yeager broke the sound barrier, and one of the discoveries of the X-1 program was the need for stabiliser trimming to combat "tuck under" in the transonic range. Without it, pitch authority is not enough to maintain control, which is why aircraft that unintentionally accelerated into the transonic range during WWII typically couldn't recover. A dive past safe speeds just got steeper because despite the best efforts of the pilots there was no elevator authority remaining to pull up.

Redbellyguppy wrote:

Mach tuck speed minus 0.07M

No.

FAR 25.335 b2

(2) The minimum speed margin must be enough to provide for atmospheric variations (such as horizontal gusts, and penetration of jet streams and cold fronts) and for instrument errors and airframe production variations. These factors may be considered on a probability basis. The margin at altitude where MC is limited by compressibility effects must not less than 0.07M unless a lower margin is determined using a rational analysis that includes the effects of any automatic systems. In any case, the margin may not be reduced to less than 0.05M.

Redbellyguppy wrote:

FAR 25.335 b2

(2) The minimum speed margin must be enough to provide for atmospheric variations (such as horizontal gusts, and penetration of jet streams and cold fronts) and for instrument errors and airframe production variations. These factors may be considered on a probability basis. The margin at altitude where MC is limited by compressibility effects must not less than 0.07M unless a lower margin is determined using a rational analysis that includes the effects of any automatic systems. In any case, the margin may not be reduced to less than 0.05M.

That talks about MC speed and the maneuvering envelope, not about Mach Tuck. As I read it, the regulation says that MC is MD minus 0.07M (or 0.05M in some cases) in case MD is limited by compressibility effects.

MC is the Mach number equivalent of VC in the maneuvering envelope diagram.

You've got it backwards. MC minus 0.07 becomes Mmo.

A practical example. Flying the D328 Jet we spent some time with the tech reps. Why is this jet so slow? It's mmo was .67. Because, the fixed stab tucks at .74 they told us. It could've gone real jet speeds had they put in a trimmable stab but they didn't.

It means that burning along at .81 in a NG just under barber pole I can encounter substantial mountain wave or severe turbulence and even though I'm well into upper barber pole I have 0.07 margin before I have to worry about anything other than a write up and an inspection.

Redbellyguppy wrote:

You've got it backwards. MC minus 0.07 becomes Mmo.

A practical example. Flying the D328 Jet we spent some time with the tech reps. Why is this jet so slow? It's mmo was .67. Because, the fixed stab tucks at .74 they told us. It could've gone real jet speeds had they put in a trimmable stab but they didn't.

I'm still disagreeing.

A 330, 350, 787, 777 or whatever flies above Mcrit. This leads to center of pressure movement backwards, which leads to Mach Tuck. Thus the onset of Mach Tuck is not limiting for aircraft designed for operation above Mcrit.

Why would airliners have Mach Trim mechanisms if they didn't experience Mach Tuck?

Redbellyguppy wrote:

It means that burning along at .81 in a NG just under barber pole I can encounter substantial mountain wave or severe turbulence and even though I'm well into upper barber pole I have 0.07 margin before I have to worry about anything other than a write up and an inspection.

Yes, but this doesn't mean that Mach Tuck is limiting MMO.

Sure you trim the stab to compensate for Mach tuck. But you can only trim it so far, at which point it continues to be a limiting factor, subject to safety margin.

Redbellyguppy wrote:

Sure you trim the stab to compensate for Mach tuck. But you can only trim it so far, at which point it continues to be a limiting factor, subject to safety margin.

Of course. But you write above that MMO was Mach Tuck speed minus 0.07M. This is not the case. The regulation states that MD-MC must be 0.07M. Neither of those speeds is necessarily Mcrit, which is the speed when Mach Tuck first manifests.

Starlionblue is correct. The 777/787/747 all fly above Mcrit.