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Effect Of Altitude On V Speeds?  
User currently offlineKaitak From Ireland, joined Aug 1999, 12600 posts, RR: 34
Posted (11 years 8 months 1 week 5 days 12 hours ago) and read 8831 times:

I was watching a superb video produced by BL&P of Germany about Lufthansa's preparation for A340 flights into Quito. Superb, if you can get your hands on it.

Anyway, there was mention of the fact that the high altitude adds a few knots to the V speeds. That's not particularly surprising, given the lower air density, but I'm wondering if it might be possible to calculate the effect, or indeed the difference between speeds at say AMS (sea level) and JNB (c.5,500') for landing. Does anyone know what a typical Vref for a 747 landing at JNB (weight 250t) would be (leaving aside wind correction)?

22 replies: All unread, jump to last
 
User currently offlineIllini_152 From United States of America, joined Jan 2001, 1000 posts, RR: 2
Reply 1, posted (11 years 8 months 1 week 5 days 4 hours ago) and read 8621 times:

Hmmm.

As I understand it, granted, this is limited to small planes, indicated airspeed shouldn't change at all given changes in air denisty. While the air is thinner and the airplane has to go faster, the pitot-static system is also sensing that lower pressure air to the same degree.

Indicated airspeed won't change, but the TRUE airspeed for given indicated airspeed will be higher at higher density altitudes. If you want to calculate this differance, here's a formula I found in my files, don't know if it's correct or not, but I have reason to trust the source.

TAS = CAS/(sigma)^0.5

Sigma = (518.7/(OAT+459.7))*(1-6.875E-6*hp)^5.25635

where OAT is in F, hp is pressure altitude in feet.

Of course, pilots either use a chart, a flight computer, or have a built in air data computer to do this for them.

--
Mike



Happy contrails - I support B747Skipper and Jetguy
User currently offlineB747skipper From , joined Dec 1969, posts, RR:
Reply 2, posted (11 years 8 months 1 week 5 days 4 hours ago) and read 8621 times:

Vref speed (landing speed) for a given weight is the same if at sea level or high elevation, in all airplanes... There is no altitude consideration in the Vref tables - only a weight consideration...  Smile
(s) Skipper


User currently offlineB747skipper From , joined Dec 1969, posts, RR:
Reply 3, posted (11 years 8 months 1 week 5 days 3 hours ago) and read 8604 times:

Dear Kaitak -
Forgot to add, that at 286,000 kg (630,000 lbs) which is the maximum landing weight of a large number of 747s (200 or 300) the Vref speed (1.3 Vs) is 152 kts... that could be the Vref, at Amsterdam, sea level or... La Paz, Bolivia, at 13,700 feet above sea level...
xxx
(s) Skipper  Smile


User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 4, posted (11 years 8 months 1 week 5 days ago) and read 8598 times:

The required indicated airspeeds will remain basically the same with altitude. Basically is the key word here.  Smile

I'd have to hit the books to tell you exactly why though, and I can't promise you that I'll have time to do that anytime soon. One thing that'll change is the Reynolds number, and with it the lift/drag coefficient curves of the aircraft. Engine performance will be affected as well. But other than that, I'll hope for someone else to provide a more in-depth answer for now.

Cheers,
Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineJetguy From , joined Dec 1969, posts, RR:
Reply 5, posted (11 years 8 months 1 week 4 days 20 hours ago) and read 8580 times:

Let me throw a couple of other factors into the mix...

Assuming a given takeoff or landing flap setting, V1 will be determined by several different variables - aircraft weight, flap setting, field elevation and temperature (the old density altitude factors), runway slope, wind component, etc. The other V speeds are basically just a function of aircraft weight and flap setting.

While the "indicated" speeds remain constant, the "true" speeds will increase by roughly 2% per 1000 feet of density altitude. While it is true that Vref at Los Angeles and La Paz will be identical the true airspeed will be significantly higher (+/- 25%) at La Paz. So what? I'm not familiar with the various operational limits on the larger transport category aircraft, but on the stuff I fly there is a 182 knot true groundspeed limit on the tires. While it's not normally a consideration, it does become a factor at the higher altitude airports.

A lot of other variables are introduced into the equation as your true air and ground speeds are increased - for example, if you double your speed it takes 4 times the energy to stop. Braking distances are exponentially increased. The same concept applies to other things like takeoff profiles, etc.

Jetguy

[Edited 2003-04-23 13:23:43]

[Edited 2003-04-23 13:25:04]

User currently offlineTsentsan From Singapore, joined Jan 2002, 2016 posts, RR: 15
Reply 6, posted (11 years 8 months 1 week 4 days 19 hours ago) and read 8574 times:

Hi,

Would landing a 747 at say 152kts at Amsterdam or La Paz be very different since 152kts at sea level would be about 152kts ground speed, while 152kts at La Paz might be 200kts at sea level?

Thanks!
Tsentsan



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User currently offlineBoeing747_600 From United States of America, joined Oct 1999, 1295 posts, RR: 0
Reply 7, posted (11 years 8 months 2 days 16 hours ago) and read 8526 times:

Tsentstan,

The difference between airspeed and groundspeed depends on the prevailing wind, not altitude.

Remember that the B744 MLW approach speed of 152 kts is Indicated airspeed (KIAS), not True airspeed (KTAS). At AMS, KIAS and KTAS would be more or less the same, whereas at LPB, your KTAS would be quite a bit higher.

In either case, your groundspeed would depend on the prevailing surface wind and could be either higher or lower depending on the wind magnitude and direction.



User currently offlineMr.BA From Singapore, joined Sep 2000, 3423 posts, RR: 22
Reply 8, posted (11 years 8 months 2 days 16 hours ago) and read 8501 times:

In that case why is there groundspeed difference when you fly 330 knots at FL180 and at FL 270? For example you fly 250 knots at 5,000 feet and 250 at FL 170 for example in wind calm conditions I believe GS is different.


Boeing747 万岁!
User currently offlineBoeing747_600 From United States of America, joined Oct 1999, 1295 posts, RR: 0
Reply 9, posted (11 years 8 months 2 days 15 hours ago) and read 8492 times:

Mr. BA,

Are you sure that you're relating TAS to groundspeed and not IAS to groundspeed when you cite the speed numbers?


User currently offlineBoeing747_600 From United States of America, joined Oct 1999, 1295 posts, RR: 0
Reply 10, posted (11 years 8 months 2 days 15 hours ago) and read 8481 times:

A clarification is in order:

In a previous post when I wrote:

The difference between airspeed and groundspeed depends on the prevailing wind, not altitude.

I mean True airspeed.

If you notice a difference between GS and the same IAS at different altitudes but the same wind conditions, its because of the TAS variation with altitude at the same IAS.

Hope this helps!


User currently offlineRick767 From United Kingdom, joined Jan 2000, 2662 posts, RR: 51
Reply 11, posted (11 years 8 months 2 days 15 hours ago) and read 8497 times:

Like a wise old instructor once said to me "There is only one airspeed, and that is Indicated airspeed".

Everything else is just a derivative... we take-off and land at a given IAS, we fly around inside a flight envelope based on IAS. TAS/GS just gets us to our destination.

Vr speeds (IAS) are higher at high pressure altitude / high temperature airports. We have a chart with the relevant corrections.



I used to love the smell of Jet-A in the morning...
User currently offlineXFSUgimpLB41X From United States of America, joined Aug 2000, 4229 posts, RR: 37
Reply 12, posted (11 years 8 months 2 days 13 hours ago) and read 8473 times:

Rick makes a very pertinent point with that last comment.

A clarification.... in a zero wind situation, if you were to be flying final approach at 150 knots at sea level, and flying it at FL170, the groundspeed would be much higher at 170 because your TAS is higher. Groundspeed is TAS corrected for headwind/tailwind component.


A great acronym I found in one of my dad's Air Force training manuals: ICE-T.... indicated, calibrated, equivalent, true.....calibrated is indicated corrected for pitot static system error, equivalent is calibrated corrected for compressibilty at high altitudes/high airspeeds (only really becomes a factor above 200 knots indicated above FL200, true is equivalent corrected for non standard temp and altitude.




Chicks dig winglets.
User currently offlineRick767 From United Kingdom, joined Jan 2000, 2662 posts, RR: 51
Reply 13, posted (11 years 8 months 2 days 13 hours ago) and read 8434 times:

Example of Vr increase due to altitude / temperature:

Sea Level + 32oC New Vr = Vr + 1
5,000ft p.a. + 30oC New Vr = Vr + 3
7,500ft p.a. + 25oC New Vr = Vr + 5

Those are for a 757-200 with RB211-535E4 engines.



I used to love the smell of Jet-A in the morning...
User currently offlineBoeing747_600 From United States of America, joined Oct 1999, 1295 posts, RR: 0
Reply 14, posted (11 years 8 months 2 days 12 hours ago) and read 8468 times:

I'm a tad confused:

B747Skipper wrote:

Vref speed (landing speed) for a given weight is the same if at sea level or high elevation, in all airplanes... There is no altitude consideration in the Vref tables - only a weight consideration...

whereas Rick767 wrote:

Vr speeds (IAS) are higher at high pressure altitude / high temperature airports. We have a chart with the relevant corrections.

Assuming that Rick meant reference speed (and not Rotation Speed) when he wrote "Vr", only one of the above can be true, unless there are different operating procedures for the B767 and B747 that I'm not aware of.

Fom an analytical standpoint, here's how I see it:

For a given aircraft weight, maintaining the same approach profile at AMS and LPB with more or less the same thrust setting requires the lift and drag forces during approach to be more or less the same.

The lift and drag forces are functions of True Air-Speed (actually TAS-squared), not IAS.

Essentially

Lift Force L = 0.5 * CL * Density * TAS ^2 * Wing_Area

and

Drag Force D = 0.5 * CD * Density * TAS ^2 * Wing_Area

where CL and CD are the lift and drag coefficients (primarily functions of angle of attack and flap angle) CD is also affected by the wing aspect ratio (induced drag)

The density at LPB (13,700 MSL) is quite a bit lower than at AMS (~0 MSL)

To maintain more or less the same final approach profile at AMS and LPB, you would need a higher TAS at LPB than at AMS

Now, if B747Skipper is right, and you had a B744 on finals at LPB doing 152 KIAS, the true-airspeed would be around 190 KTAS (I'm assuming the 2% per 1000 ft density altitude rule of thumb) which is higher than the ~152 KTAS you would get at AMS.

The question then is whether this ~25% increase in TAS is regarded as sufficient to offset the decrease in density at LPB. It also depends on the degree of flaps used which affects CL and CD and as I alluded to earlier, on the thrust setting on finals.

I suspect that this whole discussion is largely academic, because I dont see a carrier planning operations with a B744 @ anything close to MLW to LPB anytime soon!  Smile

I'd love to get some feedback on my $0.02 worth.






User currently offlineRick767 From United Kingdom, joined Jan 2000, 2662 posts, RR: 51
Reply 15, posted (11 years 8 months 2 days 12 hours ago) and read 8447 times:

"Assuming that Rick meant reference speed (and not Rotation Speed) when he wrote "Vr", only one of the above can be true, unless there are different operating procedures for the B767 and B747 that I'm not aware of."

I was referring to Vr (Rotation Speed). Reference Speed (VRef) is not affected by changes in pressure altitude / temperature (in our operation at least). As has been explained, TAS will be higher at higher pressure altitudes and the GS is resultant of any wind effect on the TAS, in zero wind TAS=GS.

Hope this clarifies matters!



I used to love the smell of Jet-A in the morning...
User currently offlineBoeing747_600 From United States of America, joined Oct 1999, 1295 posts, RR: 0
Reply 16, posted (11 years 8 months 2 days 11 hours ago) and read 8438 times:

Rick767,

Thanks! It does clarify matters immensely and confirms my initial suspicion that the higher TAS you get for the same IAS at a higher altitude is more or less sufficient to compensate for the reduced density, thereby making the aerodynamic forces on the airframe more or less the same as those at a lower altitude.

I can also see how from an operational point of view, it is useful to have one single reference speed for all altitudes.

I really need to find a nice compact word for "more or less"! I've been abusing that phrase to death in the last couple of posts!

Can you comment on the flap settings at LPB and AMS for the same landing weight (B757 or B767)

B747Skipper, likewise for the 747, if the LPB scenario is even considered in any carrier's ops.


User currently offlineMr.BA From Singapore, joined Sep 2000, 3423 posts, RR: 22
Reply 17, posted (11 years 8 months 1 day 16 hours ago) and read 8398 times:

Thanks guys, something new learnt!


Boeing747 万岁!
User currently offlineB747skipper From , joined Dec 1969, posts, RR:
Reply 18, posted (11 years 7 months 4 weeks 11 hours ago) and read 8366 times:

V2 is one of these other speeds that is (generally for V2) only a function of weight... since V2 is equal to 1.2 times the stall speed in that configuration. The only variable would be a V2 which has been increased because of Vmca consideration (1.1 times Vmca is minimum V2)...
xxx
I realize that for a non-pilot, the concept of indicated airspeeds and true airspeeds are slightly difficult to grasp, however pilots "handle the plane" in reference to indicated airspeeds only... I once had a pitot tube damaged slightly by a bird, during takeoff, my indicated speed was showing less than the other pilot's airspeed indicator... so the other pilot continued the flight and performed the approach and landing...
xxx
Nowadays, with the equipment available on airplanes, I would, in case of a failed airspeed indicator on both sides, derive a true air speed, then a ground speed from the inertial navigation systems to perform an approach and landing if circumstances dictated... Besides, our manuals also give power settings to use, should the radome be missing, severely affecting airspeed indications...
xxx
Not the first time some of our friends here confuse Vr and Vref... I much prefer the designation "Vat" (velocity at threshold) which is sometimes used as an alternate name for Vref-landing speed... At PanAm, we used "Vth" (V "thresh") rather than Vref...
 Big grin
(s) Skipper


User currently offlineBoeing747_600 From United States of America, joined Oct 1999, 1295 posts, RR: 0
Reply 19, posted (11 years 7 months 3 weeks 6 days 18 hours ago) and read 8293 times:

Thanks B747skipper! If this thread ever evolves into a protracted discussion of Indicated altitude vs Pressure, Density and True altitudes, you have my sympathies in advance  Smile

The nice thing about IAS (confusing as it might seem at first) is that it simplifies operating procedure immensely. It would be a nightmare to have to look up the appropriate TAS for V1, VR, V2, Vref-landing speeds and so forth at each airport!

I wonder though, if True altitude will eventually replace Indicated Altitude if GPS systems become standard?! Any thoughts?!


User currently offlineJetguy From , joined Dec 1969, posts, RR:
Reply 20, posted (11 years 7 months 3 weeks 6 days 17 hours ago) and read 8293 times:

Dear Boeing 747-600...
Watch your mouth! Someone may be listening! We haven't even implimented RVSM here in the States and you're talking about using GPS based true altitudes?  Innocent

Personally, I don't see it happening for a long time, if ever. The present system is more than adequate. Operationally, what benefits would be gained?

Jetguy


User currently offlineBoeing747_600 From United States of America, joined Oct 1999, 1295 posts, RR: 0
Reply 21, posted (11 years 7 months 3 weeks 6 days 16 hours ago) and read 8298 times:

Jetguy,

This may be truly naive, but wouldnt a True-Altitude based RVSM ensure better accuracy in terms of actual vertical separation?

I guess the real question I'm asking is whether a 2000 ft Vertical separation between two aircraft at FL 260 and FL 280 also implies a 2000 ft separation in terms of True Altitude and if not, whether this difference itself varies with altitude.

Arent there altitude-dependent temperature effects that preculde a one-to-one correspondence between true and indicated altitudes?


User currently offlineJetguy From , joined Dec 1969, posts, RR:
Reply 22, posted (11 years 7 months 3 weeks 4 days 3 hours ago) and read 8394 times:

B747-600…
Sorry to take so long to get back to you, I’ve been out of town. You raise a good question, however you must understand that aircraft that are operated in the “Flight Levels” are actually being operated at a “pressure” altitude or level, not at an absolute altitude. Here in the United States, the flight crew changes their altimeters to 29.92” at 18,000’ msl and above regardless of the local altimeter settings. This altimeter setting results in an indicated altitude that only corresponds to the aircraft’s actual or true altitude if the temperature just so happened to be ISA. (Hey guys, I know that I’m really simplifying this, but this post is going to be long enough without going into a full-blown discussion of altimetry.) Operationally, this is really no big deal – the theory is that all aircraft, in a particular block of airspace will be subject to the same temperature and pressure fluctuations and thus maintain their relative vertical separation.

Altimeter accuracy becomes very important – in “pre-RVSM” days you were allowed about +/- 260’ altimeter error. This was more than adequate above FL290 where you had 2,000’ vertical separation. With the implementation of RVSM and 1,000’ separation above FL290, the previous standard was not acceptable. Think about it – a westbound aircraft at FL350 coming up on an eastbound flight at FL370. Assume the westbound flight had a maximum error of +260” and the eastbound aircraft had the opposite error, -260’. Their separation would still be 1,480’. Under RVSM, those tolerances would be unacceptable. The possible “worse case” separation would be only 480’, hence the need for tighter tolerances in RVSM airspace.

You VFR guys need to take note. Here’s a scenario for you. You’re VFR, flying eastbound on a victor airway at 5,500’ msl. You come upon a westbound IFR aircraft at 6,000’, coming down the same airway. With no altimeter error and assuming each pilot maintains his/her exact assigned altitude the aircraft will have 500’ vertical separation. Throw in a bit of altimeter error and sloppy flying and the margin of separation disappears rapidly. Holding your altitude +/- 200’ just doesn’t cut it from a safety point of view. You can see why, regardless of whether you’re flying IFR or VFR, “see and be seen” is still the primary method of aircraft separation and collision avoidance.

One more thing to consider. Back in the “good old days” (pre-GPS), navigation accuracy was simply good enough. Today, with everybody running around with GPS all that’s changed. VOR and VLF/Omega based navigation allowed for quite a bit of allowable error built into the system. Today, nearly everyone has at least a hand-held GPS. Knowing your position to within just a few meters is a great thing; but in these days of GPS and RVSM accuracy, it's getting to be pretty easy to be at EXACTLY the wrong place, at EXACTLY the wrong altitude, at EXACTLY the wrong time - if you get my drift. In fact, in non-radar international or foreign airspace, pilots will intentionally put a bit of offset into their FMS's to provide them with an extra bit of “insurance”.

Long story short, the current system works just fine without the added complexity that a GPS-based system would provide. Granted, there are some situations (extremely low temperatures and/or extremely high barometric pressures) where this would be an advantage when conducting instrument approaches. However, the old saying probably applies – “If it ain’t broke, don’t fix it.”

Jetguy


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