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nuNce

Flight planning and strong winds

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Hello,

lately I've been flying with a bigger involvement in the planning phase, especially in relation to winds during the cruise phase. Being aware of that I've started avoiding strong winds when obvious, but I still have some doubts in relation to choices regarding FL, step climbs and descent. I'd like to know your opinion on:

  1. When is it worth it to delay climbs due to strong winds? Are 20kts less headwind worth flying 4-6k feet below optimal?
  2. Once estabilished on cruise, is it worth trying to go to a lower FL to avoid unforcasted strong winds? How much headwind should you lose to make it up for say 2k ft of altitude lost?
  3. In general, is there any rule of thumb to follow when faced with strong winds? How do you evaluate alternate routes and different FL?

I understand this can be a really complex discussion, and that flight planning is surely a big part of corporate investment, but I'd like to hear your opinions regarding my question.

 

Thank you.


Mateusz Kapusta

 

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Hi Mateusz,

PFPX does wind optimization. For transatlantic flights, you should follow published NAT tracks. There are also PAC tracks for the Pacific Ocean.

Peter


(P3D v4, i7-3820CPU @ 3.6 GHz, GTX 1070, 16 GB ram)

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Worth it in terms of fuel burn (cost): or in terms of on-time performance?

A good flight planning tool will tell you.  Essentially fuel cost is the difference in time (ground speed) between the two flight levels.  With the simple example of no winds, TAS increases with increased altitude, and burn decreases with altitude. That is two factors of advantage.  That assuming that OAT or TAT differential is usually the same.  It is about -2°C per thousand feet, called the "normal lapse rate". 

The burn advantage (per hour) will be there,  The TAS advantage will be there.  So the issue is how much ground speed (time) is affected by the difference in wind factor at the two altitudes. The first step is to find, or develop, a burn profile at various altitudes.  I do that by attaining a normal cruise speed at a given altitude, then logging fuel burn for say 15 minutes. Multiple that X 4 for hourly burn, and put that and the associated TAS for that altitude into the profile. Then I change altitudes, take a few minutes to stabilize, then add that altitude to the profile.  I build the profile in 2,000 ft increments, then I enter the profile into an aircraft performance profile at Fltplan.com, which is where I do my flight planning.  With a properly built profile here is what can be done, Though this flight plan did not have much variance in winds.

(Ignore the drawn oval.  The image was originally for a different purpose)

 

12-10-17%20plan-L.jpg

Edited by fppilot

Frank Patton
MSI Z490 WiFi MOB;  i7 10700 3.8 Ghz CPU; Ripjaws 32 gb DDR4 3600; ASUS GTX 1070 TI Turbo 8GB; MasterCase H500M; Corsair H100i Pro cooler; Corsair RMX850X PSU; ASUS VG289 4K 27"; Former USAF meteorologist & ground weather school instructor; AOPA Member #07379126 
                        There is no limit to what a man can do or where he can go if he doesn’t mind who gets the credit! - Benjamin Jowett

 

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1 hour ago, fppilot said:

TAS increases with increased altitude

Up to a point.  Your maximum TAS is going to be at a level lower than your optimum flight level because, as you correctly point out, burn decreases with altitude but so does TAS as you create less thrust in the thinner air.  This discussion also needs to recognize different aircraft have very different profiles.  I always fly my B736 at FL400/410 but a fully loaded B744 may never rise above FL340/350 or lower.

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Dan Downs KCRP

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1 hour ago, downscc said:

our maximum TAS is going to be at a level lower than your optimum flight level because, as you correctly point out, burn decreases with altitude but so does TAS

Are you perhaps confusing Indicated Airspeed (IAS) with True Airspeed (TAS) ?  Here is a link to a quick and dirty calculator.  Try inputting values and then change only the altitude value and to a higher level and watch what happens to True Airspeed...(TAS)

Edited by fppilot

Frank Patton
MSI Z490 WiFi MOB;  i7 10700 3.8 Ghz CPU; Ripjaws 32 gb DDR4 3600; ASUS GTX 1070 TI Turbo 8GB; MasterCase H500M; Corsair H100i Pro cooler; Corsair RMX850X PSU; ASUS VG289 4K 27"; Former USAF meteorologist & ground weather school instructor; AOPA Member #07379126 
                        There is no limit to what a man can do or where he can go if he doesn’t mind who gets the credit! - Benjamin Jowett

 

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34 minutes ago, fppilot said:

Are you perhaps confusing Indicated Airspeed (IAS) with True Airspeed (TAS) ?

No, Dan is correct. 

You would be right IF you maintained constant IAS at all times. However, you don't: above about FL290 you maintain constant Mach, and Mmo becomes the limiting factor for airspeed.

Thus. your maximum IAS is likely to be significantly lower at, say, FL350 than it is at the crossover altitude.

Maximum TAS, therefore, is actually achieved around FL290 or thereabouts.


Simon Kelsey

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1 hour ago, skelsey said:

No, Dan is correct. 

You would be right IF you maintained constant IAS at all times. However, you don't: above about FL290 you maintain constant Mach, and Mmo becomes the limiting factor for airspeed.

Thus. your maximum IAS is likely to be significantly lower at, say, FL350 than it is at the crossover altitude.

Maximum TAS, therefore, is actually achieved around FL290 or thereabouts.

AH! So a GA pilot like me who never ventures above FL250 should stay out of conversations here in the PMDG Forum - LOL!


Frank Patton
MSI Z490 WiFi MOB;  i7 10700 3.8 Ghz CPU; Ripjaws 32 gb DDR4 3600; ASUS GTX 1070 TI Turbo 8GB; MasterCase H500M; Corsair H100i Pro cooler; Corsair RMX850X PSU; ASUS VG289 4K 27"; Former USAF meteorologist & ground weather school instructor; AOPA Member #07379126 
                        There is no limit to what a man can do or where he can go if he doesn’t mind who gets the credit! - Benjamin Jowett

 

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17 hours ago, fppilot said:

AH! So a GA pilot like me who never ventures above FL250 should stay out of conversations here in the PMDG Forum - LOL!

No no no, let me assure you as someone with nothing in their logbook but GA aircraft that actual piloting experience is very welcomed and it's okay if you get something technically wrong once in awhile.  I try not to be wrong more than once a day.  Former AF weather guy, cool; I was an AF Ground Communications-Electronics engineer and several of my assignment included the guys that worked on your equipment.  My last assignment in combat communications we also had tactical metro stations that we shipped along with control towers and TRACONs.  Ah, just though of a good story:  We were the first to get solid state anemometers at Scott AFB and boy they sure were nice until our first winter with them when the birds discovered a warm spot. I thought it was funny but it became a problem.

Edited by downscc

Dan Downs KCRP

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One thing that has not been mentioned in this thread is "ride."  Those folks who fly passengers will choose the best ride over the optimal FL. Passengers don't like and don't pay for bad rides and firm landings. Most freighter folks don't really  care so much about the ride, just the most optimal FL.  White caps on the coffee came with the territory. :smile: 

One of my flight planning mental checks was fuel required for the flight.  For example on the B707 I used 12.000 lbs an hour.  If the flight was going to be 4 hours, I knew that I would need at least 48,000 lbs of fuel.  

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I Earned My Spurs in Vietnam

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On 26 March 2018 at 6:04 PM, nuNce said:

Hello,

lately I've been flying with a bigger involvement in the planning phase, especially in relation to winds during the cruise phase. Being aware of that I've started avoiding strong winds when obvious, but I still have some doubts in relation to choices regarding FL, step climbs and descent. I'd like to know your opinion on:

  1. When is it worth it to delay climbs due to strong winds? Are 20kts less headwind worth flying 4-6k feet below optimal?
  2. Once estabilished on cruise, is it worth trying to go to a lower FL to avoid unforcasted strong winds? How much headwind should you lose to make it up for say 2k ft of altitude lost?
  3. In general, is there any rule of thumb to follow when faced with strong winds? How do you evaluate alternate routes and different FL?

I understand this can be a really complex discussion, and that flight planning is surely a big part of corporate investment, but I'd like to hear your opinions regarding my question.

 

Thank you.

Listen son...and listen carefully; the 737 NGX is like any other aircraft - it's going to be thrown around by the winds aloft - it's YOUR job to plan your flight and then fly your plan.

I use SimBrief - because it just works.

If you want to be "a child of the magenta line" then carry on as you are - no change required. Just tell the FMC what your Cost Index is.

If you want to get to grips with the most beautiful airframe that has ever been created within a consumer flight simulation environment then get out of your head and get on the ground with this beast.

Lesson 1) Learn to fly manually.

Lesson 2) Learn to fly manually.

Lesson 3) Get up there and feel the weather, winds aloft and figure out what the best altitude change is. Do your maths. Fly this thing like you're doing it for real.

Like this:

(Notice no greens on anything...Manual flight...winds doing what they will, stick rudder inputs and a feel for the bird)

I'm a drunken old has been pilot - but I know one thing - until you put your faith in the airframe - and really learn to fly it...you're just pretending; and there's no need to pretend with PMDG metal.

737_NGX_Manual.jpg

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On 3/26/2018 at 4:11 PM, downscc said:

Up to a point.  Your maximum TAS is going to be at a level lower than your optimum flight level because, as you correctly point out, burn decreases with altitude but so does TAS as you create less thrust in the thinner air.  This discussion also needs to recognize different aircraft have very different profiles.  I always fly my B736 at FL400/410 but a fully loaded B744 may never rise above FL340/350 or lower.

OK. Did some reading about this last evening and again this afternoon.  My statement and your reply centered on TAS.  The real point in time enroute and fuel however is ground speed.  I see that the calculation on the crossover point is influenced my the effect of temperature on the calculation of mach speed, which in turn affects the TAS calculation.  The lingering question is this. Up until the aircraft in use reaches it's maximum safe cruise speed, what is the effect on ground speed?  What I read seems to indicate that GS and TAS are the same, given no winds for the point of discussion (of course there are always winds at altitude).  But while TAS becomes limited and even decreases by the complex calculation of mach speed above the crossover point,  I saw nothing to indicate that the forward speed of the aircraft through space GS becomes limited or decreases other than the safe limitations of the aircraft.

So above the crossover point, which is not a constant by the way, TAS decreases, influenced by calculations.  But does GS decrease also? Because its calculation is only speed and distance.

Edited by fppilot

Frank Patton
MSI Z490 WiFi MOB;  i7 10700 3.8 Ghz CPU; Ripjaws 32 gb DDR4 3600; ASUS GTX 1070 TI Turbo 8GB; MasterCase H500M; Corsair H100i Pro cooler; Corsair RMX850X PSU; ASUS VG289 4K 27"; Former USAF meteorologist & ground weather school instructor; AOPA Member #07379126 
                        There is no limit to what a man can do or where he can go if he doesn’t mind who gets the credit! - Benjamin Jowett

 

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7 minutes ago, fppilot said:

But does GS decrease also? Because its calculation is only speed and distance.

The groundspeed will equal whatever the TAS is plus the wind component, tail winds good.... headwinds fact of life.

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Dan Downs KCRP

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29 minutes ago, fppilot said:

So above the crossover point, which is not a constant by the way, TAS decreases, influenced by calculations.  But does GS decrease also? Because its calculation is only speed and distance.

Well, this is in some respects the precise question being posed by the OP ;).

GS is simply TAS corrected for the effect of wind. Therefore if we assume still air, GS = TAS.

Of course, as you quite rightly point out the wind is likely to be different at different altitudes and therefore optimising your cruise becomes a question of what you want to achieve.

If you want to get to your destination in the shortest possible time and fuel usage is not a concern, then you are looking for the altitude which gives the best possible ground speed. Your best TAS will be achieved at lower levels, so the question becomes: is the tailwind component at the higher level greater than the reduction in TAS as a result of climbing? If so then it makes sense to climb: if not then it does not.

In practice of course in an airline operation one is generally optimising toward minimum fuel burn. Again, we are presented with a trade-off: all things being equal (ie still air) you will achieve a higher TAS and therefore get to your destination faster at lower levels. However, you will also burn more fuel per hour at these lower levels, so now the question is: am I saving sufficient time by flying at a lower level to offset the increased rate of fuel consumption? 

Mathematically, 1kt = 1 nautical mile per hour. So if you know your consumption rate at different levels and the GS you can achieve at each level, you simply divide the burn rate by the GS to give the fuel burned per nautical mile.

For instance, at FL350 the 747 burns 11,064 kg/hr at 501 KTAS

At FL280 you will burn 11,112kg/hr but achieve 511 KTAS.

11064/501=22.08 kg/NM.

11112/511=21.74 kg/NM

Therefore in theory it is more economical to fly at the lower altitude and take the higher TAS despite the higher rate of consumption. Of course, I have assumed still air here: however, as I say, the same sums work with GS instead of TAS if you know the wind component (get your whizz wheel out!).

Of course, aircraft weight will also affect the rate of burn and the achievable altitudes.

Looking in my 747 FCOM I note that Boeing provide a 'wind-altitude trade' table in the 'performance inflight' section which helps simplify things: I don't know if it is in the PMDG FCOM or, indeed, if one is provided for the 737.


Simon Kelsey

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30 minutes ago, skelsey said:

GS is simply TAS corrected for the effect of wind. Therefore if we assume still air, GS = TAS.

You are taking the complex formula-driven route and backing into GS by basing it on other calculated speeds that are dependent on additional influences . Leave TAS out of the question/answer

Ground Speed is a factor of time and distance.  Nothing else need enter the equation.  Not even winds.  Because in using strictly time and distance the effect of the winds are already considered.  Ground speed can be determined for a long distance runner.  There is no TAS in determining that. Only distance and time.

Edited by fppilot

Frank Patton
MSI Z490 WiFi MOB;  i7 10700 3.8 Ghz CPU; Ripjaws 32 gb DDR4 3600; ASUS GTX 1070 TI Turbo 8GB; MasterCase H500M; Corsair H100i Pro cooler; Corsair RMX850X PSU; ASUS VG289 4K 27"; Former USAF meteorologist & ground weather school instructor; AOPA Member #07379126 
                        There is no limit to what a man can do or where he can go if he doesn’t mind who gets the credit! - Benjamin Jowett

 

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9 minutes ago, fppilot said:

You are taking the complex formula-driven route and backing into GS by basing it on other calculated speeds that are dependent on additional influences . Leave TAS out of the question/answer

Ground Speed is a factor of time and distance.  Nothing else need enter the equation.  Not even winds.  Because in using strictly time and distance the effect of the winds are already considered.  Ground speed can be determined for a long distance runner.  There is no TAS in determining that. 

It is as simple as time-distance; however, one will not know the actual elapsed time until after the distance has been flown.  This is why flight planning uses TAS (kts or Mach) to determine time based on distance.  GS is the factor that can be ignored for planning purposes (it is in the time calculations and available but while flying we watch the computer update forecast arrival times and fuel remaining), because the wind predictions are the independent variable.  All aircraft have very predictable TAS based on weight, pressure altitude and SAT and this is where we start when planning for jets. In fact, the dispatchers have software that knows exactly each aircrafts performance based on past performance.  This and statistical analysis of a specific route allow dispatchers to use less reserves on routes flown repeatedly by the same aircraft.

Edited by downscc

Dan Downs KCRP

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