FAA Speed Limit

[thinman]

I was flying the learjet again yesterday and was at an altitude of around 10,500 feet. Increased the speed to over 350 and the overspeed light and alarm came on. It went off after slowing below 350. I have read that there's no limit over 10,000 feet. Anyone else experienced this?

Bob

[JSkorna]

Hi,

It's not the FAA limit, but each plane does have a speed limit set. You discovered it for the Lear. All planes have a safe speed limit and just can't fly as fast as they want.

[opaplano]

Jim is quite right -

The speed at which the warning comes on is set in the aircraft.cfg file and can be edited if necessary.

Flying a particular aircraft above the set "speed limit" means you may be subjecting the aircraft to conditions it was not designed to handle.

If you find the Overspeed message intrusive, or wish to not have it display at all, check this out:

http://library.avsim.net/search.php?SearchTerm=ovsp_abb.zip&CatID=root&Go=Search

There are also instructions in the file which tells how to edit the aircraft.cfg file.

Hope this is helpful.

[thinman]

Thanks for the info, and about changing the limit. But I'm okay with the 350.
Another question. Would this structural speed safety limit increase at higher altitudes since the air is thinner?

Bob

[rickk]

Above FL180 (USA) you would change over to Mach Indicated speeds. You will find that the higher you get, the actual indicated speed in knots (KIAS) will decrease. Your speed over the ground, however, will be much greater.

[Flying Officer Jevans]

Another question. Would this structural speed safety limit increase at higher altitudes since the air is thinner?


No. It might seem like it if you look at the knots, but that's todo with knots not being a true measure of your speed.

The Vne - Never Exceed Speed - is about the G you are pulling on the plane when maneuvering. For instance, a Vulcan will happily fly extremely close to the speed of sound if you use full throttle. However, as soon as you start to turn, the wing might well start snapping.

[alastairmonk]

Hi Bob,

The true airspeed does vary from IAS as you gain height. Ignoring temperature and ambient pressure factors the following formula will give you a ballpark figure.

TAS = ((IAS x 0.02) x (Altitude/1000)) + IAS

Thus:

At 4,000ft 140kts IAS = ((140x0.02) x 4) + 140 = 151kts TAS
At 10,000ft 140kts IAS = 168kts TAS
At 25,000ft 140kts IAS = 210 kts TAS
At 45,000ft 140kts IAS = 266kts TAS

Similarly:

At 10,000ft 350kts IAS = 420kts TAS
At 45,000ft 350kts IAS = 665kts TAS !

Given that you're getting an "overspeed" light at around 420kts TAS, at cruise altitude (say 45,000ft) in order to keep below 420kts your IAS would have to be no more than about 220kts IAS and this would leave little room for a sudden tailwind that pushed your airspeed over the limit.

Alastair

[codebreaker]

I thought that aircraft fly and are damaged by IAS not TAS. Since IAS is a true measure of the dynamic forces on the fuselage and wings would that not be valid? I thought that was why extremely high flying but not very fast aircraft such as the U-2/TR-1 had such a narrow flight envelope at cruise altitude. At the altitudes the U-2 flies the engine cannot push it very fast indicated so even though the TAS is high enough to comfortably sustain flight, the max IAS the engine can propell the aircraft is only a little bit above stall. Hence the LOONNNGGG wings that bird has (had?). A sudden tail wind would be a pain at that altitude.

WARNING! War Story Alert!

While I was in the Air Force, I spent a lot of 1980 on temporary duty at Osan Air Base in South Korea. Officially there were no U2/TR1 aircraft stationed at Osan. That was why I never watched them take off at 0700 every morning (if they'd have been there you would've been able to set your watch by them) and of course I never watched them circling above Osan Base and Song Tan city to loose altitude. I couldn't have watched them take off or land since they were never there. I also never drove by the hanger they weren't kept in on the way to work every morning for over 3 months.

Anyhow, in the spring I did a small installation in the RAPCON at Osan (you old guys will know what a RAPCON used to be) and was baby sitting the new radios. The controller at the station I installed showed me where one of the pilots of the U2s that wasn't there forgot to turn off his transponder as he was climbing to altitude (or should I say as he WASN'T climbing to altitude since the U2 he was flying wasn't there - military double think can get confusing at times LOL). The transponder was squawking FL86. The controller quietly advised the pilot of the aircraft that he might want not to tell the world how high he was. The transponder signal then ceased. Of course, in reality that blip had never been there since there WERE NO U2s at Osan in 1980. FL86 and climbing. That's high, boys and girls. That's high.

[alastairmonk]

Hi Bill,

I think you'll find that the aircraft are damaged by excessive speed, however that is defined. As I'm sure you know, the reason that IAS decreases with altitude is because its measured by the flow of air into the pitot tube. As altitude increases the number of air molecules decreases due to the lower atmospheric pressure, thus the pitot tube / IAS unit measures a drop in speed even if in any objective terms the aircraft "speed" hasn't decreased.

Obviously as the air gets thinner air resistance also decreases so any specific quantity of motive force (jet thrust etc.) should propel the plane faster than when there is more resistance.

Equally, the motive force that keeps the plane in the air is the lift imparted by the differential pressure from the airflow over/under the wing. At high altitudes the quantity of air decreases so (as with the U2) at some elevated altitude the combined forces keeping the plane flying are only just enough and further climbing is impossible unless something changes (e.g. fuel is burnt and the plane becomes lighter - the principle behind "step climbing" in airliners.)

I'm not an expert but my understanding is that the performance envelope narrows at high altitude, so I can appreciate that at some point the Vne and stall speeds may be close.

As Jim has touched on, the published "maximum safe speed" is determined by a number of factors, including physical structural limits and a certain "speed buffer" that allows for unforseen factors such as sudden wind changes that can force the plane out of the safety envelope. In turbulent conditions it is prudent to slow down, so as to increase the margin between current, stable operation and "catastrophic failure".

Alastair

[Tim_A]

FL86 and climbing. That's high, boys and girls. That's high.

FL86? Even my Warrior can do that! Maybe you meant FL860 ;)
Nice story though. If you'd told it, which of course you didn't, because there were no....

[Captain Smithy]

Excellent explanation Alistair.

I'm not sure if FS simulates it but in real life there are a couple of other things that make the ASI read different to TAS in high-speed aircraft (i.e. jets).

One of the other things is compressability, which is something to do with the air being compressed at high speed as it enters the pitot tube, and it gives an erroneous reading. Compressability is insignificant below 300 Knots, so being a "spamcan" driver I have never experienced this and that's as far as my understanding goes. Perhaps one of our real-life jet drivers would care to elaborate/correct?

There are other factors that affect ASI readings... see the following Wikipedia articles for more info.

http://en.wikipedia.org/wiki/Indicated_airspeed

http://en.wikipedia.org/wiki/Calibrated_airspeed

http://en.wikipedia.org/wiki/Equivalent_airspeed

WARNING: Contains moderate content of mind-boggling maths and algebra!

Smithy

[lnuss]

You may also want to see this article (http://www.flightsim.com/fswiki/index.php/What_are_the_various_%22speeds%22_used_in_aviation%3F) in the Flightsim.com Wiki -- don't forget to click on the links to more info about various types of airspeed (yes, there are many types).

Compressibility is certainly a major factor in high speed flight, and this is especially true at higher altitudes where the speed of sound is slower than it typically is at sea level. In a nutshell, as you get closer to the speed of sound, the air has a tougher time getting out of the way, thus it gets more and more compressed as you get faster. At these speeds, air pressure can exert rather considerable force (even to causing an airframe to break up), as you get closer to the speed of sound (mach 1) even today's best non-supersonic jets can approach damaging pressures at low indicated airspeeds at high altitudes, thus each has not only a limiting indicated airspeed (most useful at lower altitudes), but a limiting mach number, as well (often in the range of .8 to .9 mach), beyond which airframe damage and/or loss of control can occur.

Because of the lower air density at high altitudes, a jet which has a Vne of, say 310 knots, can travel at very high true airspeeds (500-600 knots) when they are very high, but the mach limits then come into play, often before you've reached Vne on the airspeed indicator.

The "coffin corner" referred to above (very narrow band of available speeds) occurs with many jets as they approach their service ceilings which in slower aircraft is defined as the altitude at which they cannot climb better than 100 feet per minute (fpm), but in jets is often either being in or near to the "coffin corner" or being high enough that they have reached their maximum pressure differential between the cabin and outside, such that they can no longer maintain the required cabin pressure for people without oxygen masks. There are, of course, other factors that can limit an aircraft's service ceiling, which I won't go into here.

[tigisfat]

Aircraft and FAA speed limits are, as before mentioned, different. Allow me to elaborate on the two:

FAA speed limits;
250KIAS below 10,000MSL and 200KIAS when within 4NM of the primary class B airspace, also; 200KIAS when below class B airspace. These are the basics, they get way moer complicated. Please note that if there is a minimum safe flying speed or maneuvering speed in excess of any of these, you can fly at them.


Aircraft speed limits;
There are many different kinds, such as maximum mach numbers for overpowered aircraft such as private jets and some commercials, and there are also never exceed indicated airspeeds for most aircraft. There are many other speed limits, but an important one is the maneavering speed and/or the rough air penetrating speed.

[codebreaker]

FL86? Even my Warrior can do that! Maybe you meant FL860 ;)
Nice story though. If you'd told it, which of course you didn't, because there were no....

Yeah, I meant FL860. I just know that if the thing had actually been there it woulda been really high.

WARNING WAR STORY ALERT!

The most scared I have been in my life was while stationed in the NW Pacific. I had a job where I had to monitor all the secret messages coming through the comm center at Yokota AB, Japan and wake up my unit commander (a full bird Colonel who happened to be Deputy Commander for Communications for 5th Air Force and all U.S. Forces in Japan) if I saw anything I thought he ought to see. Since I worked 3rd shift (2300-0700), I was REAL careful about calling him seeing as how enlisted guys waking up full birds in the middle of the night can be bad for your carreer and quality of life and all. ;)

One night the North Koreans decided to get frisky and play a little game of chicken. They moved 5 divisions (in addition to the ones they already had) down close to the DMZ. Then they tried to shoot down the overflying SR-71s and U2s taking pictures of them. I figured this was something to wake the Colonel over and called him - at 0200!!! While he was coming in we got messages telling us to go to Defcon 2 from the normal 4 we stayed at. For the uninitiated there are only 2 levels of readiness between Defcon 2 and war - i.e. "stick your head 'tween your knees and kiss your a** goodbye".

When the Colonel got in I gave him the messages and continued the myriad of tasks needed to get the 1956th Comm Group up to Defcon 2. When I had a chance to catch my breath I turned to this senior officer, Deputy Commander for Communications for a Numbered Air Force and ALL U.S. forces in Japan and asked him "Sir, is the balloon gonna go up?"

He looked back at me and in all seriousness answered "Honestly, Billy, I don't know."

Now THAT scared the living FRACK outta me. If he didn't know I had no way to know.

BTW, in case you're wondering, the North Koreans blinked and stood down.

[tigisfat]

BTW, in case you're wondering, the North Koreans blinked and stood down.[/size]


I assumed that much; otherwise I would've missed out on knowing about a large chunk of history!!:D

[codebreaker]

I'm kinda glad they did, too. Those guys in the DPRK are nutzoid. My son got his FIRST Purple Heart while stationed in Korea in the Army. While he was walking guard duty on the DMZ some Dee-perk dips**t took a shot at him and didn't miss. Wasn't much of wound - the round was pretty well spent when it hit him - but since Korea is officially still a war zone (they have never signed enything more than a cease-fire) my son got a PH. He got his 2nd PH in Sandbox 1. He was rolling out accross the Kuwaiti desert in a semi and went over an anti-tank mine. The device detonated under the trailer wheels and flipped the whole rig over on the driver's side. The medics patched Ray up while they transferred the load onto another trailer, righted the tractor and connected the two. Then he got back in and drove on. He got his bronz star a couple of days later. But that is another story.