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XXXX10
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Vref At Higher Field Elevation

Thu Jul 20, 2006 6:23 am

Can anyone tell me if when landing at a high elevation pilots have to increase the vref speed

I understand that at a higher elevation the true airspeed and the ground speed will be higher for a given indicated air speed.

Would you use a lower flap setting for higher fields?- I presume that the stalling speed for any aircraft (at a given weight and flap setting) is higher as altitude increases

many thanks
 
DH106
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RE: Vref At Higher Field Elevation

Thu Jul 20, 2006 6:36 am

No - don't forget that Vref is in INDICATED airspeed. The true airspeed - and hence the ground speed of the aircraft - will rise at a higher field elevation because of the thinner air.
...I watched c-beams glitter in the dark by the Tanhauser Gate....
 
SlamClick
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RE: Vref At Higher Field Elevation

Thu Jul 20, 2006 6:54 am

Quoting DH106 (Reply 1):
Vref is in INDICATED airspeed

True.
And, keep in mind that excess speed over the fence is not your friend.
Happiness is not seeing another trite Ste. Maarten photo all week long.
 
saab2000
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RE: Vref At Higher Field Elevation

Thu Jul 20, 2006 7:02 am

Indicated speed will be the same (weight dependent of course) regardless of field elevation. But ground speed (because of the higher true airspeed) will be higher, significantly in the case of a place like Denver.
smrtrthnu
 
flightopsguy
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RE: Vref At Higher Field Elevation

Thu Jul 20, 2006 11:39 am

And landing performance (approach go-around with loss of one engine, and landing-go around with all engines) is controlled by reduced landing performance weight limits, although this limit is usually higher than structural landing, except at a "hot and high" airport with significant obstacles on a really hot day. Back when we dispatched 727-100's, and 732's with -7 engines it was a significant issue. Much less so these days.
A300-330 BAC111/146/J31/41 B99/1900 CV580 B707-777 DC8/9/10 L188/1011 FH227/28/100 SB340 DO228 EMB2/170 CR2-900 SH330-60
 
XXXX10
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RE: Vref At Higher Field Elevation

Fri Jul 21, 2006 2:43 am

Thanks for your answers,


Does this mean that the stalling speed for a particualr type for a given weight and config would be the same at regardless of the altitude?

[Edited 2006-07-20 19:44:15]
 
DH106
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RE: Vref At Higher Field Elevation

Fri Jul 21, 2006 2:54 am

Quoting XXXX10 (Reply 5):
Does this mean that the stalling speed for a particualr type for a given weight and config would be the same at regardless of the altitude?

You gotta be careful in what you're measuring here.
In INDICATED airspeed (how much 'blow' the aircraft feels) the stalling speeds are the same. However, the higher the altitude, the more difference between indicated and true airspeed because the air gets thinner - so to get the same 'blow' the aircraft has to move faster in a true sense.

In a nutshell - if you assume no wind, to a ground observer watching an aircraft the stalling speeds WILL increase because of the thinner air at a high airfield, but the airspeeds read from the pilot's ASI will be the same.
...I watched c-beams glitter in the dark by the Tanhauser Gate....
 
SlamClick
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RE: Vref At Higher Field Elevation

Fri Jul 21, 2006 3:32 am

Quoting Saab2000 (Reply 3):
Indicated speed will be the same (weight dependent of course) regardless of field elevation. But ground speed (because of the higher true airspeed)

If I could make one thing happen I could retire from this forum a happy man. I wish people would ONLY use the word 'groundspeed' when they are talking about NAVIGATION. It has nothing to do with this discussion. It has nothing to do with flight or performance. It is only a navigation figure.

The statement quoted is only conditionally correct, that condition being a specific wind factor must be stated. There is no such thing as ground speed in the absence of a wind factor. If there is no wind this must be stated.

What was true in that statement is that 'true' airspeed will be higher. That is the important part and that is the reason why margin above stall is still protected.
Happiness is not seeing another trite Ste. Maarten photo all week long.
 
DH106
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RE: Vref At Higher Field Elevation

Fri Jul 21, 2006 4:01 am

SlamClick - I know where you're coming from and agree from a purist viewpoint, but I think Saab2000 and myself are only using 'groundspeed' (with nil wind proviso in my case) because I think it would be easier for the forum poster to envisage the aircraft flying by at a certain speed from a ground position.
...I watched c-beams glitter in the dark by the Tanhauser Gate....
 
SlamClick
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RE: Vref At Higher Field Elevation

Fri Jul 21, 2006 4:22 am

Quoting DH106 (Reply 8):
I think it would be easier for the forum poster to envisage the aircraft flying by at a certain speed from a ground position.

Yes, and that is an important concept, the understanding that while the airpspeed indicator still shows the same 130 knots, say, it shows at sea level the plane IS actually moving faster.

But the importance to me is this: It is moving through the air faster because of the higher TAS. To leave it at "it is moving across the ground faster" leaves a dangerous loophole open in the mind of a less experienced pilot, that being well, if I have a tailwind I should then have greater stall protection. Absurd to you and me, of course, but it points up the importance of the mass of air flowing over the wing itself.

If I had a student sitting in front of me I could attempt to explain it a couple of different ways because we don't always 'get it' at the same time and in the same way. But I'm treating my post as a one-shot effort and so I get very picky about how I use a word.

I need to keep in mind that this forum is an information cafeteria. Some items look better than others but those that are actually bad quickly get redflagged by the straight-scoop police.

Slam
but not necessarily
Click
Happiness is not seeing another trite Ste. Maarten photo all week long.
 
saab2000
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RE: Vref At Higher Field Elevation

Fri Jul 21, 2006 6:03 am

SlamClick,

I guess I should have used "speed over the ground". I know exactly what you mean in terms of "groundspeed" being for navigational purposes. But there is a performance factor here too, though I have never heard it referred to in books.

The Vref speed for the CRJ at max landing weight is 141 knots. We add 5 knots to this speed and our approach speed is 146 knots. This is, of course, indicated speed. At PHL or DCA or ORF (the three domiciles of my company) the indicated and true are, for all practical purposes, the same as these airports are all at sea level.

But we used to fly to DEN all the time. That airport is at roughly 6000 feet. Using a rough rule of thumb, true airspeed and indicated airspeed will diverge at approximately 2 knots per thousand feet. Therefore, assuming no wind, my speed over the ground at 146 knots indicated, will be 158 knots. Right?

My issue here, and I know it is not the point of the thread really, is that there is therefore more kinetic energy to be stopped, assuming the same mass.

At least that is unless my understanding of something is wrong.

Anyway, the 2 knots per 1000 feet is clearly not right high and fast and this is where I need a lesson. Maybe you can help explain. I have a photo from my own plane last winter en route to ORD. The indicated speed is 300 knots. The TAS is 477 at Mach .82. We were at FL 320 and the OAT was -50C. The TAT was -20C. Assuming 2 knots per 1000 feet, the TAS at this IAS should be about 364 Knots. But it is 477.

I can only assume that I have been told wrong on the 2 knots per 1000 feet rule of thumb.

My basic aerodynamic training is failing me right now. What am I missing?
smrtrthnu
 
DH106
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RE: Vref At Higher Field Elevation

Fri Jul 21, 2006 6:07 am

Quoting Saab2000 (Reply 10):
Anyway, the 2 knots per 1000 feet is clearly not right high and fast and this is where I need a lesson. Maybe you can help explain. I have a photo from my own plane last winter en route to ORD. The indicated speed is 300 knots. The TAS is 477 at Mach .82. We were at FL 320 and the OAT was -50C. The TAT was -20C. Assuming 2 knots per 1000 feet, the TAS at this IAS should be about 364 Knots. But it is 477.

The reason is that neither lift increase with rising speed, nor pressure drop with altitude is a linear function. I suspect that 2 knots/1000' is a reasonable approximation for a fairly narrow band of altitude/airspeed ranges found with most airliners' approach speeds and at most airports altitudes, but not the extreme altitude/speed at cruise you mention.

[Edited 2006-07-20 23:10:04]

[Edited 2006-07-20 23:10:40]
...I watched c-beams glitter in the dark by the Tanhauser Gate....
 
SlamClick
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RE: Vref At Higher Field Elevation

Fri Jul 21, 2006 6:29 am

Quoting Saab2000 (Reply 10):
I can only assume that I have been told wrong on the 2 knots per 1000 feet rule of thumb.

I think the two knots per thousand rule of thumb has practical limits. It may be fine for extrapolating from sea level to a three thousand foot airport but I don't think I'd run it all the way to FL320 and expect much accuracy. Heck, if it was really that simple they wouldn't charge so much for an air data computer for your jet would they?

I'll take a look at your numbers when I have a moment but for now I'm betting on the airplane being right. Smile

Anyone with the good skinny care to jump in, feel free.
Happiness is not seeing another trite Ste. Maarten photo all week long.
 
XXXX10
Topic Author
Posts: 707
Joined: Mon Jan 03, 2000 7:10 am

RE: Vref At Higher Field Elevation

Fri Jul 21, 2006 6:39 am

I undertood that at higher levels the stalling speed and vref would be higher when expressed as TAS. I previously thought that the stalling speed would also be higher in IAS (therefore even higher in terms of TAS)

The reason for me thinking this is probably the assumption that the maximum altitude that an aircraft can nomally fly is the point when the mimimum speed (IAS) met the maximum speed (Also in IAS)
 
SlamClick
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RE: Vref At Higher Field Elevation

Fri Jul 21, 2006 6:50 am

Quoting XXXX10 (Reply 13):
I undertood that at higher levels the stalling speed and vref would be higher when expressed as TAS.

Well that started out okay but falls apart at the end. VREF is always Indicated airspeed. Many jetliners don't have a place to read True airspeed except as a gee-whiz number somewhere other than directly in front of the pilots. On the Boeing version FMS it is usually on the "Progress" page 2 or 3. On Airbii it is usually displayed, but again, only for info, not for use in flying the plane.

We fly with indicated or with mach. We flightplan (those of us who actually still know how) with True ± wind factor.
Happiness is not seeing another trite Ste. Maarten photo all week long.
 
bri2k1
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RE: Vref At Higher Field Elevation

Fri Jul 21, 2006 9:01 am

I guess somewhere in there, then, in the design of the aircraft, it must be calculated that groundspeed at touchdown (the only time it matters for the purpose of this discussion) will always be within safe limits if the correct IAS is flown, regardless of air density.

Thinking about it just a little more, the performance charts for my plane list different rollout distances for different density altitudes, always with the same IAS flown on final. So that does make sense. It's partly due to less effective aerodynamic braking, and partly due to the higher groundspeed that results from flying an approach at a high density altitude.
Position and hold
 
Mr.BA
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RE: Vref At Higher Field Elevation

Sun Jul 23, 2006 12:34 am

So in this case when deciding which autobrake setting to select, you consider groundspeed rather than indicated airspeed since for stopping distance it's reference to groundspeed?
Boeing747 万岁!
 
flymatt2bermud
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RE: Vref At Higher Field Elevation

Sun Jul 23, 2006 1:11 am

Quoting Mr.BA (Reply 16):
So in this case when deciding which autobrake setting to select, you consider groundspeed rather than indicated airspeed since for stopping distance it's reference to groundspeed?

I think you should consult your charts or the FMS data. We input winds into our FMS computer prior to landing which calculates a GS factor which affects the field length. You can use the charts too. Situational awareness should dictate how you determine which autobrake setting you prefer. In other words, consider all your options before you decide.
"When once you have tasted flight, you will forever walk the earth with your eyes turned skyward" Leonardo Da Vinci
 
bri2k1
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RE: Vref At Higher Field Elevation

Sun Jul 23, 2006 3:01 am

Quoting Mr.BA (Reply 16):
So in this case when deciding which autobrake setting to select, you consider groundspeed rather than indicated airspeed since for stopping distance it's reference to groundspeed?

I think you've completely missed the point.

In the absence of wind, at the same indicated airspeed, your true airspeed and hence groundspeed will be higher at a higher density altitude.

You still fly the plane at the same indicated airspeed.

Required braking performance depends on how much energy you need to dissipate in a given distance, and hence, period of time. On a longer runway with favorable winds, you can dissipate the momentum of the aircraft over a longer distance. If you want to make the first turnoff with a tailwind, well, dial in some more braking.

If you're landing in a snowstorm with gusty winds at DEN, you don't fly a lower IAS or ask for more braking.
Position and hold
 
SlamClick
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RE: Vref At Higher Field Elevation

Sun Jul 23, 2006 3:18 am

Quoting Bri2k1 (Reply 18):
Required braking performance depends on how much energy you need to dissipate in a given distance, and hence, period of time. On a longer runway with favorable winds, you can dissipate the momentum of the aircraft over a longer distance. If you want to make the first turnoff with a tailwind, well, dial in some more braking.

Quite right.

On old-fashioned 'web-of-death' type brake energy charts you could either enter with the airport elevation, solve for temperature then go on to approach speed IAS or you could simply enter the chart with landing TAS then go on to landed weight and solve the rest of the chart.

It should also be mentioned that autobrake yields a deceleration rate. So if you use reverse thrust, you will get less wheelbrake effort. Autobrake is not always used. Most of us were still permitted to use manual braking and modulate it as our judgement dictated.

However, same landing gross weight will give more kinetic energy at higher pressure altitudes because of higher TAS at the same IAS and that is really pretty much the bottom line.
Happiness is not seeing another trite Ste. Maarten photo all week long.
 
saab2000
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RE: Vref At Higher Field Elevation

Sun Jul 23, 2006 9:13 am

Quoting SlamClick (Reply 19):
However, same landing gross weight will give more kinetic energy at higher pressure altitudes because of higher TAS at the same IAS and that is really pretty much the bottom line.

So I WAS right!!! Ha!! I didn't word it as eloquently, but higher density altitude will give a higher true airspeed and therefore higher groundspeed for a given indicated speed, given no wind and no change of weight, over a lower density altitude.

We have a tire speed limitation and I have always argued that it is related to the max certified takeoff or landing altitude at the max certified temperature. Or better put, the zero flap landing at such altitude. The limitation is a groundspeed, not airspeed, limitation. However we have no groundspeed indicator that we read during a takeoff or landing.

I better quit now though.  Big grin
smrtrthnu
 
flymatt2bermud
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RE: Vref At Higher Field Elevation

Sun Jul 23, 2006 9:23 am

Quoting Saab2000 (Reply 20):
However we have no groundspeed indicator that we read during a takeoff or landing.

I know, this will get everyone worked up!! During your groundroll, weight on wheels, your IRS or GPS should give you a very close groundspeed reference. Not that anyone is going to be starring at it.
"When once you have tasted flight, you will forever walk the earth with your eyes turned skyward" Leonardo Da Vinci
 
saab2000
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RE: Vref At Higher Field Elevation

Sun Jul 23, 2006 11:08 am

Quoting FlyMatt2Bermud (Reply 21):
I know, this will get everyone worked up!! During your groundroll, weight on wheels, your IRS or GPS should give you a very close groundspeed reference. Not that anyone is going to be starring at it.

You are right. Nobody would look at this during a take-off roll. And then it goes back to SlamClick's point that groundspeed and the indication on the display, is for reference only and for navigation and nothing else.

All speeds we use for T/O or landing are indicated, obviously. I was only referring to the greater inertia of a a higher TAS are higher density, or pressure, altitude.
smrtrthnu
 
SlamClick
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RE: Vref At Higher Field Elevation

Sun Jul 23, 2006 11:19 pm

Quoting Saab200 (Reply 20):
We have a tire speed limitation

That has been in the back of my mind during this discussion. I flew the -10 series DC-9 for a few years. They have no leading edge devices and takeoffs are at flaps 10 or 20, the former down closer to sea level.

I made a lot of hot and high operations with this plane. We got our runway analysis pages from a small business and they were very good. The last digit of the allowable weight was the limiting factor, '2' for second segment for example. Only once was a takeoff limited by tire speed. VR for this particular roll was about 188 knots. (that nosewheel was singing when it hit the snubbers in the well)

The tires in question were rated at 225MPH. That of course is predicated on their being in contact with the pavement so it is GS. We are reading IAS. I've not done the conversion and the old Douglas Racer had no such fancy avionics as could tell me that number. Sea level standard day 225 MPH is 196 knots so allowing for altitude and temperature I'd bet we were very close to 225 MPH when we lifted off at the 188 KIAS.

In other words I've gone two hundred miles an hour steering with my feet!

I don't remember what V1 was for that takeoff but I assure you that if you could have been in the left seat looking out at the rapidly-approaching runway end when that call was made, you would have no doubt that the plane had more kinetic energy than at the same point on a sea level takeoff.

The chart says I could have stopped in the remaining runway! Would have been colorful.
Happiness is not seeing another trite Ste. Maarten photo all week long.

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