Jump to content


TigerTugsVRForumGen468Helicopter AcademyVOLO_VRGeneral468Frasca VRForum468
Photo
- - - - -

Pressure Altitude Clarifications


  • Please log in to reply
19 replies to this topic

#1 polywogkev

polywogkev

    Newbie

  • Members
  • Pip
  • 7 posts

Posted 27 November 2017 - 18:41

I had a question regarding pressure altitude and flight planning that I was hoping to get help with:

 

I understand how to calculate pressure altitude and that the standard pressure lapse rate is -1" per +1,000' altitude. On an R44's performance charts (OGE hover, for example), the left y-axis is pressure altitude. My question is, how would you calculate expected "pressure altitude" for an off-airport destination at a significantly higher altitude when there is no way to get a pressure reading from that location prior to arrival?

 

Here is a scenario:

We are departing an airport at 1,000' MSL with a pressure reading of 28.92" and therefore the pressure altitude is 2,000'. We are flying into the mountains and will need to do an OGE hover at 5,000' MSL (but relatively close distance to the airport). Would it be best practice to assume that spread of 1,000' between MSL and pressure altitude persists at the 5,000' location so for consulting performance charts we would look at 6,000' PA?

 

Thanks in advance.


  • TeoerryNug likes this

#2 Nearly Retired

Nearly Retired

    VR Veteran Poster

  • VR Member
  • PipPipPipPipPipPipPip
  • 551 posts
  • Gender:Male
  • Location:Pensacola, FL
  • Interests:Well, obviously flying and anything aviation...but motorcycling as well. And drinking (when I'm not riding of course!). Oh, and Facebook. Always on Facebook (Bob Barbanes).
  • Company working for:Golden Wings Aviation, Brewster, WA (part-time)

Posted 28 November 2017 - 10:28

Wait...what?

 

Here is a scenario:

We are departing an airport at 1,000' MSL with a pressure reading of 28.92" and therefore the pressure altitude is 2,000'...

 



#3 r22butters

r22butters

    VR Veteran Poster

  • VR Member
  • PipPipPipPipPipPipPip
  • 1,733 posts
  • Gender:Male
  • Location:Pluto,...at least until the next election :) ,...damn the wifi here is unbelievable!
  • Interests:Admiring the city lights and coastal sunsets from a thousand feet,...when I have the dough. :)

    ,...oh' yeah, and boobs!
    😕+🍟+🚁+🌃=☺
    .
  • Company working for:Just a happy casual renter! :)

Posted 28 November 2017 - 11:18

Get the temperature at your airport and subtract 2° per 1,000' up to 5,000'. Then go to the OGE chart, start at 5,000' move over to that predicted temperature, then go down and it will tell you how much you can weigh if you want to HOGE at 5,000'
  • TomPPL likes this
The only dream I have left is to live long enough to see the pilot shortage. Its been about fifteen years since they first told me it was coming, so,...

Aaaaaaaany day now! :D

#4 Hand_Grenade_Pilot

Hand_Grenade_Pilot

    VR Veteran Poster

  • VR Member
  • PipPipPipPipPipPipPip
  • 128 posts
  • Gender:Male
  • Location:USA
  • Company working for:GoM

Posted 28 November 2017 - 11:41

I had a question regarding pressure altitude and flight planning that I was hoping to get help with:
 
I understand how to calculate pressure altitude and that the standard pressure lapse rate is -1" per +1,000' altitude. On an R44's performance charts (OGE hover, for example), the left y-axis is pressure altitude. My question is, how would you calculate expected "pressure altitude" for an off-airport destination at a significantly higher altitude when there is no way to get a pressure reading from that location prior to arrival?
 
Here is a scenario:
We are departing an airport at 1,000' MSL with a pressure reading of 28.92" and therefore the pressure altitude is 2,000'. We are flying into the mountains and will need to do an OGE hover at 5,000' MSL (but relatively close distance to the airport). Would it be best practice to assume that spread of 1,000' between MSL and pressure altitude persists at the 5,000' location so for consulting performance charts we would look at 6,000' PA?
 
Thanks in advance.

Your best bet is to use the nearest station pressure, even if its not in the immediate area. Another option is to set the aircraft altimeter to standard pressure (29.92 hg) and note the difference between the reading and true elevation. Apply the difference to the true altitude that you are planning on hovering at.

For example:
Field elevation (departure point): 2,000MSL. Altimeter at 29.92 reads 2,400. There is a low pressure system over the region and you are experiencing a performance penalty of +400.

Your intended OGE hover point is 50 miles away and over terrain at 3,500. You plan to hover 500AGL. You will reference the y-axis of the performance chart at 4,400PA.

Fortunately, changes in atmospheric pressure due to high/low pressure systems are a relatively small part of aircraft performance. For example, lets look at the record low and high for New Orleans.

Record high: 30.83 hg (approx. -830)
Record low: 28.11 hg (approx. +1,717)

Those are extremes; for all practical purposes non-standard atmospheric pressure will not have a huge impact on performance.

Temperature is a much more important variable to keep track of as it will have a significant impact on your ability to hover OGE. Another very important consideration is wind/turbulence; while your calculations may look good on paper, turbulent air or wind coming from an unfavorable direction can prevent you from hovering OGE.

While OGE performance planning is useful for having a general understanding of whether youll be able to hover for the expected conditions, there are many variables that can ruin your plan. Always have a backup plan. For example, a staging area nearby where you can offload weight.

Also keep in mind that operating in a performance limited environment requires a lot of improvising and seat of the pants flying; the performance chart should never be relied on.
  • TomPPL, SBuzzkill and polywogkev like this
Aviation is a cruel mistress. When she's not taking your money, she's coming up with creative ways to kill you.

#5 Hand_Grenade_Pilot

Hand_Grenade_Pilot

    VR Veteran Poster

  • VR Member
  • PipPipPipPipPipPipPip
  • 128 posts
  • Gender:Male
  • Location:USA
  • Company working for:GoM

Posted 28 November 2017 - 11:52

Some other considerations for performance that you can’t necessarily calculate are humidity, condition of the engine air intake filter (wet vs dry, clean vs dirty), condition of the rotor blades (dirty vs. waxed), and whether or not the engine is producing optimal power. Can’t stress enough that the performance charts are nothing more than a basic guide, not to be taken for granted.
Aviation is a cruel mistress. When she's not taking your money, she's coming up with creative ways to kill you.

#6 polywogkev

polywogkev

    Newbie

  • Members
  • Pip
  • 7 posts

Posted 28 November 2017 - 17:01

Some other considerations for performance that you can’t necessarily calculate are humidity, condition of the engine air intake filter (wet vs dry, clean vs dirty), condition of the rotor blades (dirty vs. waxed), and whether or not the engine is producing optimal power. Can’t stress enough that the performance charts are nothing more than a basic guide, not to be taken for granted.

 

Thanks for both of the thoughtful responses - your point on pressure variance records (max / min) makes a ton of sense and I understand all the other variables. I would disagree with the other commenter on this thread about using the temperature lapse rate for the final density altitude calcs. I've found from personal experience that when mountain flying, albeit at higher altitude MSL, if your altitude AGL is still <1,000' then the temperature does not actually lapse at 2 degrees per 1,000' (better to be conservative).



#7 polywogkev

polywogkev

    Newbie

  • Members
  • Pip
  • 7 posts

Posted 28 November 2017 - 17:03

Wait...what?

 

 

 

I should have said "altimeter setting is 28.92" not "pressure reading". My bad.



#8 r22butters

r22butters

    VR Veteran Poster

  • VR Member
  • PipPipPipPipPipPipPip
  • 1,733 posts
  • Gender:Male
  • Location:Pluto,...at least until the next election :) ,...damn the wifi here is unbelievable!
  • Interests:Admiring the city lights and coastal sunsets from a thousand feet,...when I have the dough. :)

    ,...oh' yeah, and boobs!
    😕+🍟+🚁+🌃=☺
    .
  • Company working for:Just a happy casual renter! :)

Posted 28 November 2017 - 17:59

Thanks for both of the thoughtful responses - your point on pressure variance records (max / min) makes a ton of sense and I understand all the other variables. I would disagree with the other commenter on this thread about using the temperature lapse rate for the final density altitude calcs. I've found from personal experience that when mountain flying, albeit at higher altitude MSL, if your altitude AGL is still <1,000' then the temperature does not actually lapse at 2 degrees per 1,000' (better to be conservative).


Sorry, thought you were just going to HOGE 4,000' higher? As for AGL effecting the lapse rate, I have never heard of that?

,...but then again I don't fly in the mountains!
The only dream I have left is to live long enough to see the pilot shortage. Its been about fifteen years since they first told me it was coming, so,...

Aaaaaaaany day now! :D

#9 polywogkev

polywogkev

    Newbie

  • Members
  • Pip
  • 7 posts

Posted 28 November 2017 - 19:38

Sorry, thought you were just going to HOGE 4,000' higher? As for AGL effecting the lapse rate, I have never heard of that?

,...but then again I don't fly in the mountains!

 

I agree you're right on with the technical calculation. I've just noticed in So-Cal that the ground in the mountains seems to keep in the heat or something. Either way, I'd bias to being conservative (with a higher temperature estimate for planning purposes). Thanks again for all the insight guys.


  • TomPPL likes this

#10 SBuzzkill

SBuzzkill

    VR Veteran Poster

  • VR Member
  • PipPipPipPipPipPipPip
  • 1,763 posts
  • Gender:Male
  • Location:Northeast

Posted 06 December 2017 - 13:34

2 degrees per 1000 feet is the standard lapse rate and not really what you're going to see outside of text books.  It's a very basic model for temperature in the atmosphere and in reality there are many, many variables that will determine the lapse rate.  And even then you will probably only see it maintained for a few thousand feet before the lapse rate changes again.

 

In the mountains it's even more difficult to predict and can vary depending upon which valley you're in, shade vs sun, where the winds are, how steep the terrain is, whether the air is dry or moist, etc.

 

The best thing you can do is look at what the weather is being observed to do as close to your destination as possible and try to interpolate from there.  Leave yourself a good margin for error, have multiple "outs" if possible, and do your best to fly conservatively.  I have found that even in areas that I know well I am consistently surprised.


  • Wally, TomPPL and polywogkev like this

#11 polywogkev

polywogkev

    Newbie

  • Members
  • Pip
  • 7 posts

Posted 06 December 2017 - 13:45

2 degrees per 1000 feet is the standard lapse rate and not really what you're going to see outside of text books.  It's a very basic model for temperature in the atmosphere and in reality there are many, many variables that will determine the lapse rate.  And even then you will probably only see it maintained for a few thousand feet before the lapse rate changes again.

 

In the mountains it's even more difficult to predict and can vary depending upon which valley you're in, shade vs sun, where the winds are, how steep the terrain is, whether the air is dry or moist, etc.

 

The best thing you can do is look at what the weather is being observed to do as close to your destination as possible and try to interpolate from there.  Leave yourself a good margin for error, have multiple "outs" if possible, and do your best to fly conservatively.  I have found that even in areas that I know well I am consistently surprised.

 

Good stuff, thanks!



#12 WolftalonID

WolftalonID

    VR Veteran Poster

  • VR Member
  • PipPipPipPipPipPipPip
  • 551 posts
  • Gender:Male
  • Location:Bozeman, MT
  • Interests:Flying, helicopters, back country, archery elk hunting, wood working!
  • Company working for:Rocky Mountain Rotors - Mildly experienced stick wiggler/rookie expert

Posted 09 December 2017 - 10:56

Being a full time mountain pilot, the one thing I can tell you is we flight plan like crazy. Then we toss it out and go fly. In reality my flight planning is simple. I look to confirm wx wont sock me in. I look to see if we can get there based on fuel needs first, then on cargo weights. Next is winds. Winds at the airport mean nothing. I look at winds from 9000 and up. This is what will tell me somewhat what to expect in the high country. Wind is my absolute friend up there...to a point. Last I want to know my temps now, and expected high for the day. I may look at the altimeter as a factor but it has so little to do with my absolutes that its like a small high five with high pressure, and a ehhh we will watch power if its low pressure.

In flight we do a power check in hover prior to take off to see what a solid surface IGE is giving us for margin. If we are concerned that we could struggle, then a max performance takeoff will show you further limits on power checks.

Backing up to winds. An above post said winds will not let you hover well OGE due to turbulence? .....ohhh kay, well stay off the leeward side of terrain and it wont be an issue. Wind is our friend, like high water flow is to a kayaker. Mountain winds and high waters work exactly the same way too. Winds allow the aircraft to stay in ETL, reducing required power needed and at times I have hovered at 11k ft due to winds helping. This has been done in both R44s and 206b3s. When your hovering completely still at 14 MAP in an R44 at 11k feet to get good film shots of high altitude wildlife....you cannot tell me winds are making it harder. ;)

You need to realize there are ways to work winds in mountains that help you, and there are ways it will kill you. Stay on the windward sides. Avoid leeward sides. Winds compress against terrain like water does on rocks in a river. The windward side is smoother, provides tremendous lift, and reduces power when you stay tight to the mountain.(within 300-500 ft AGL) when you get above that mountain, or beyond the ridge on the leeward side, winds begin to roll, tumble, churn. Just like water does on the backside of boulders in a white water river.

If I see dead calm winds in flight planning...high altitude work must be reconsidered or extremely well planned to ensure safe margins up there. Winds 10-40 kts make life easy up there. Winds above 40 kts make it a bit harder to manage, and in some cases may inhibit safe operations all together. Usually though the later is a call decided by the pilot based on their personal skill limitations. Time will expand your capabilities but always back off if your unsure, because wind will kill you fast if your not doing things right in high country.

When you talk about this said mountain at 3500 feet? Your still in flat country son. ;) just ribbin ya.
Seriously we take off daily from 4500 feet at home base, and operate over 10k very regularly. If you want to come do some serious training, call me sometime, we offer mountain courses for those who need some experience.

Edit: I forgot to talk about the power checks. A great way to check available power margins is to get to the altitude your headed for, out in the open where you have outs, and pull into an OGE hover. See if it will hang there or if it drops or rotor droops right away. If the ship hangs there a bit or settles very slowly, staying in ETL on appraoch to your LZ using updraft winds will give you a very safe margin. If she falls immediately due to lack of power, then the chances of settling into the terrain as you loose ETL are a pretty solid reality. Its basic power check steps like this combined with good wind awareness and temps that make you into a safe pilot. Cold windy days are far better than calm hot ones.

Edited by WolftalonID, 09 December 2017 - 11:01.

  • Wally, TomPPL and polywogkev like this
Sometimes we think we know it all....only later to discover we only knew all we had learned. Never stop learning.

#13 TomPPL

TomPPL

    VR Veteran Poster

  • VR Member
  • PipPipPipPipPipPipPip
  • 244 posts
  • Gender:Male
  • Location:United Kingdom
  • Company working for:My Own

Posted 09 December 2017 - 11:34

That was an excellent explanation there WolftalonID, thanks for posting, it was a great read.

 

 

Being a full time mountain pilot, the one thing I can tell you is we flight plan like crazy. Then we toss it out and go fly. In reality my flight planning is simple. I look to confirm wx wont sock me in. I look to see if we can get there based on fuel needs first, then on cargo weights. Next is winds. Winds at the airport mean nothing. I look at winds from 9000 and up. ............


  • WolftalonID likes this

#14 Hand_Grenade_Pilot

Hand_Grenade_Pilot

    VR Veteran Poster

  • VR Member
  • PipPipPipPipPipPipPip
  • 128 posts
  • Gender:Male
  • Location:USA
  • Company working for:GoM

Posted 09 December 2017 - 12:18

Being a full time mountain pilot, the one thing I can tell you is we flight plan like crazy. Then we toss it out and go fly. In reality my flight planning is simple. I look to confirm wx wont sock me in. I look to see if we can get there based on fuel needs first, then on cargo weights. Next is winds. Winds at the airport mean nothing. I look at winds from 9000 and up. This is what will tell me somewhat what to expect in the high country. Wind is my absolute friend up there...to a point. Last I want to know my temps now, and expected high for the day. I may look at the altimeter as a factor but it has so little to do with my absolutes that its like a small high five with high pressure, and a ehhh we will watch power if its low pressure.
In flight we do a power check in hover prior to take off to see what a solid surface IGE is giving us for margin. If we are concerned that we could struggle, then a max performance takeoff will show you further limits on power checks.
Backing up to winds. An above post said winds will not let you hover well OGE due to turbulence? .....ohhh kay, well stay off the leeward side of terrain and it wont be an issue. Wind is our friend, like high water flow is to a kayaker. Mountain winds and high waters work exactly the same way too. Winds allow the aircraft to stay in ETL, reducing required power needed and at times I have hovered at 11k ft due to winds helping. This has been done in both R44s and 206b3s. When your hovering completely still at 14 MAP in an R44 at 11k feet to get good film shots of high altitude wildlife....you cannot tell me winds are making it harder. ;)
You need to realize there are ways to work winds in mountains that help you, and there are ways it will kill you. Stay on the windward sides. Avoid leeward sides. Winds compress against terrain like water does on rocks in a river. The windward side is smoother, provides tremendous lift, and reduces power when you stay tight to the mountain.(within 300-500 ft AGL) when you get above that mountain, or beyond the ridge on the leeward side, winds begin to roll, tumble, churn. Just like water does on the backside of boulders in a white water river.
If I see dead calm winds in flight planning...high altitude work must be reconsidered or extremely well planned to ensure safe margins up there. Winds 10-40 kts make life easy up there. Winds above 40 kts make it a bit harder to manage, and in some cases may inhibit safe operations all together. Usually though the later is a call decided by the pilot based on their personal skill limitations. Time will expand your capabilities but always back off if your unsure, because wind will kill you fast if your not doing things right in high country.
When you talk about this said mountain at 3500 feet? Your still in flat country son. ;) just ribbin ya.
Seriously we take off daily from 4500 feet at home base, and operate over 10k very regularly. If you want to come do some serious training, call me sometime, we offer mountain courses for those who need some experience.
Edit: I forgot to talk about the power checks. A great way to check available power margins is to get to the altitude your headed for, out in the open where you have outs, and pull into an OGE hover. See if it will hang there or if it drops or rotor droops right away. If the ship hangs there a bit or settles very slowly, staying in ETL on appraoch to your LZ using updraft winds will give you a very safe margin. If she falls immediately due to lack of power, then the chances of settling into the terrain as you loose ETL are a pretty solid reality. Its basic power check steps like this combined with good wind awareness and temps that make you into a safe pilot. Cold windy days are far better than calm hot ones.

I suggest you read what I said a little more carefully. I said winds and turbulence MAY prevent you from hovering OGE. Your LZ or hover point is not going to always be on the windward side of the slope. Nor will the airflow always be smooth on the windward side. Where you operate, 40kts winds may be an advantage to you. When I was doing utility work in Hawaii in R44s and 500s, 40kts of wind hitting vertical cliffs will kick the sh%t out of you.

3,500 was an arbitrary number. Replace it with 8,500 if youd prefer; the main point was to demonstrate how to calculate PA and that a high/low pressure system wont affect performance much.
  • TomPPL, SBuzzkill and WolftalonID like this
Aviation is a cruel mistress. When she's not taking your money, she's coming up with creative ways to kill you.

#15 Whistlerpilot

Whistlerpilot

    VR Veteran Poster

  • VR Member
  • PipPipPipPipPipPipPip
  • 281 posts
  • Gender:Male
  • Location:BC
  • Interests:Skiing climbing mountain biking kayaking flying and having a long term relationship!
  • Company working for:A 5 ship company in a small BC mountain town

Posted 11 December 2017 - 01:49

Great responses and an interesting topic. Ive been flying in Nepal this past year so getting to experience the highest landings possible. Because its subtropical conditions are not only high but hot. ISA plus 20 to 35 degrees C are common. I always set the altimeter to PA 1013 as soon as away from an airport with QNH. Then I use the Gyronomo IPad app which rocks. If Im going to Everest Base Camp for example I know the altitude and usually know or can infer the temperature. I can change the variables temp and pressure and work out what weight with fuel load to have IGE power. Wind is scouted during the recce and sometimes landings have to be postponed due to conditions on the day. So far my highest landing is 20500 actual altitude but Ive regularly landed up to 23000 DA. So performance planning and preparation is crucial. As is a careful mountain reccee and approach. I also carry an E6B whizz wheel for PA to DA conversions, indicated to true airspeed calculations (which are significant at these altitudes and temps), and fuel burn calculations. Honestly The Gyronomo app is a game changer. I can work out fuel burn at different altitudes with it and plan for the lightest amount of fuel possible for certain missions. 110 LPM at Camp One versus 190 LPM for a low bad WX flight back to Kathmandu. In my opinion all pilots should be using this readily available information, and all Helicopters should have a fuel flow meter. But you can still do all the calculations the old way. On another PA note, at the really high altitudes the pressure and temp do have an effect. So the H125 is flight limited to 23000PA which means I have been able to fly (not land) up to 23700 actual altitude and stay within limits.

Safe mountain approach is 200 feet above 2000 feet horizontal. Slow down to almost translation to get power set, and chug on in. With almost no collective changes approach angle can be adjusted with cyclic pressure on each edge of the ETL shudder. Forward pressure a little and speed increases and approach steepens as ETL lift happens. Back pressure a little and speed decreases and ETL effect lessens and approach descends, angle lessens. This gives the most stable approach with no surprise power grabs at the bottom. As WolftalnID pointed out a power check is a good idea and its all about the wind. Wind is your friend until its your enemy.

I was happy to learn that the Canadian Mountain Approach I had been taught works equally well high in the Himalaya. Last interesting note, at really high altitudes it takes way way longer to transition through ETL, sometimes several hundred meters. This gets your attention when departing at near power limits as the pucker factor lasts longer. I carry spare underwear and and sit on a special seat cushion. Joking... but not really...
  • TomPPL, SBuzzkill, Hand_Grenade_Pilot and 1 other like this

When life's path is steep keep your mind even.


#16 polywogkev

polywogkev

    Newbie

  • Members
  • Pip
  • 7 posts

Posted 11 December 2017 - 16:27

Edit: I forgot to talk about the power checks. A great way to check available power margins is to get to the altitude your headed for, out in the open where you have outs, and pull into an OGE hover. See if it will hang there or if it drops or rotor droops right away. If the ship hangs there a bit or settles very slowly, staying in ETL on appraoch to your LZ using updraft winds will give you a very safe margin. If she falls immediately due to lack of power, then the chances of settling into the terrain as you loose ETL are a pretty solid reality. Its basic power check steps like this combined with good wind awareness and temps that make you into a safe pilot. Cold windy days are far better than calm hot ones.

 

These are some solid insights - thank you for taking the time to share your stories. On the topic of power checks, my instructor once told me that we should OGE hover as a power check before a confined area approach and make sure there are at least 2" of additional power to pull (R44 Raven II) before making the decision to do it. I don't have any other context - does that feel about right, conservative, aggressive? Thanks!


  • TomPPL and WolftalonID like this

#17 SBuzzkill

SBuzzkill

    VR Veteran Poster

  • VR Member
  • PipPipPipPipPipPipPip
  • 1,763 posts
  • Gender:Male
  • Location:Northeast

Posted 11 December 2017 - 20:35

Just when I said this place was boring...

 

What does 2" translate to in terms of percentage of power?  For instance, is that 10% of your available power, meaning you're hovering with 90% of your power pulled?  Flying an OH-58, I'm personally not comfortable operating with limited maneuverability (such as in a confined area) if I cannot hover OGE at less than 90% torque (with 100% available).  This gives me good options based on power available.  

 

On the flip side, if I have a nice open LZ with a good surface I don't even need OGE power to feel comfortable.  In fact in Afghanistan many times I barely had IGE power but I had a nice solid surface to scrape and bump along during takeoffs and landings.


Edited by SBuzzkill, 11 December 2017 - 20:41.

  • TomPPL, WolftalonID and polywogkev like this

#18 polywogkev

polywogkev

    Newbie

  • Members
  • Pip
  • 7 posts

Posted 11 December 2017 - 21:08

Just when I said this place was boring...

 

What does 2" translate to in terms of percentage of power?  For instance, is that 10% of your available power, meaning you're hovering with 90% of your power pulled?  Flying an OH-58, I'm personally not comfortable operating with limited maneuverability (such as in a confined area) if I cannot hover OGE at less than 90% torque (with 100% available).  This gives me good options based on power available.  

 

On the flip side, if I have a nice open LZ with a good surface I don't even need OGE power to feel comfortable.  In fact in Afghanistan many times I barely had IGE power but I had a nice solid surface to scrape and bump along during takeoffs and landings.

 

I honestly don't know how to translate manifold pressure to % power. The limits change by PA and temperature (i.e., DA) and unclear to me whether 0" or some higher number would equate to 0%.

 

Either way, good reference points if I ever make it in a turbine!


  • TomPPL likes this

#19 Hand_Grenade_Pilot

Hand_Grenade_Pilot

    VR Veteran Poster

  • VR Member
  • PipPipPipPipPipPipPip
  • 128 posts
  • Gender:Male
  • Location:USA
  • Company working for:GoM

Posted 12 December 2017 - 08:40

I honestly don't know how to translate manifold pressure to % power. The limits change by PA and temperature (i.e., DA) and unclear to me whether 0" or some higher number would equate to 0%.
 
Either way, good reference points if I ever make it in a turbine!

Assuming there isnt a manifold pressure limit for the aircraft, your max MP available will be approximately 1 inch lower then ambient pressure, and then -1 for every 1,000 of altitude.

For example:
You depart from sea level, and station pressure is 30.00NG. You will be able to pull about 29 inches of MP. If you then climb to 5,000MSL, youll have about 24MP before reaching full throttle.

Having an extra 2 inches available is a good rule of thumb for departing an area that will allow you to obtain ETL; it will give you approximately 7-10% of your total power available to use for takeoff. A confined area requiring a towering takeoff (altitude over airspeed, staying below ETL) will require a larger power margin.

There are a few ways to do power checks; my preferred method is to load the disk well before reaching the point in your approach where you are committed (go-around may no longer be possible) and observe how much TQ, TOT or NP (or in your case MP) is available. Loading the disk is simply slowing below ETL (or as close to it as possible; if the wind is above 15kts you wont be able to get below ETL while flying into the wind) and applying enough collective to maintain a very low rate of descent (100/min or so). If you are able to maintain that with additional power available, you shouldnt have too much trouble departing the LZ (which is when you will need even more power). As an extra layer of security, once the disk is loaded you can do a go around and initiate a max performance climb; observe the angle you are climbing at and reference it against the angle needed to clear obstructions during takeoff from your intendended LZ (aka doing a dummy approach). If your climb angle seems shallow, you know you will have issues departing the LZ.

Edited by Hand_Grenade_Pilot, 12 December 2017 - 08:45.

  • TomPPL and polywogkev like this
Aviation is a cruel mistress. When she's not taking your money, she's coming up with creative ways to kill you.

#20 Whistlerpilot

Whistlerpilot

    VR Veteran Poster

  • VR Member
  • PipPipPipPipPipPipPip
  • 281 posts
  • Gender:Male
  • Location:BC
  • Interests:Skiing climbing mountain biking kayaking flying and having a long term relationship!
  • Company working for:A 5 ship company in a small BC mountain town

Posted 13 December 2017 - 08:03

Totally agree with your power check and dummy approach. This is what I was taught and still practice.

When life's path is steep keep your mind even.





1 user(s) are reading this topic

0 members, 1 guests, 0 anonymous users



ColoradoHeliOpsGeneral200PrecisionVRForumGeneral200NFCVRGeneral200Able_VRGeneral200MaunaLoaSoftwareVRGeneral200HeliHelmetsGenesys VRForum200_GeneralSpectrum_VRGeneral468DevoreGeneral200