Temperature and pressure height calculations.

[Sapphire]

Looks like it's R rated as well-been disabled for me. Utube has lots of choices on this subject

 

 

[flying dog]

No, I had trouble watching it. Just click on the "Watch on youtube" option when the "denied" thingy pops up.

 

 

[motzartmerv]

Watching it again, it seems there was one very particular attitude where some altitude was being gained, but the poor guy kept trying to sneak the nose up just that little bit which pit the performance back in the red. Mind you I doubt much more height could have been gained regardless. Perhaps just a bit more time to find a clearer crash site.

 

It's hard to say for sure what went on without being there, but you have to give the guy a wrap for not stalling or attempting a turn.

 

DA calcs are not as close to the wire in aus and in our form of flying, but still very important particularly for those operating in the warmer climates. Temp has the biggest effect on DA and a calc that is not included is humidity. Humidity decreases density even further so a hot humid day with a low qnh, really have a long hard think about what your doing.

 

Cheers

 

 

[djpacro]

Heres a video that clearly demonstrates what can go wrong when these sort of calc's are not done, or done incorrectly...

Please explain.So, if he had calculated density altitude correctly, so what?

 

 

[motzartmerv]

Ummm... Surprised that you are asking that question dj. I dunno, maybe lightened the load, made more trips, not taken off?? That is of course if that's what went wrong.It could have been an engine issue, or an overloading problem. Who knows. The comments from the passengers refer to an 'air pocket' which sounds like lay terms for a downdraft perhaps. Regardless it never looked like flying did it.

 

 

[djpacro]

But how does knowing density altitude itself prompt or enable such decisions - especially in this case as that is the one held up as an example?

 

 

[motzartmerv]

Well, from memory, the first box in takeoff chart is the DA box. If the chart has you maxed out then the takeoff weight would be severly reduced would it not. Also perhaps in extreme cases the service ceiling could be exceeded at very low altitude (agl). There a number of documented cases where this has been blamed as the cause.

 

Actually one not so long ago at Mittagong, which is only 2000ft ish, a worrior couldn't get out of the ground effect, and crashed on takeoff on a high temp low pressure day. Loaded with people and fuel.

 

Am I on the right track? Not exActly sure what your looking for.

 

 

[djpacro]

Well, from memory, the first box in takeoff chart is the DA box. .... Not exActly sure what your looking for.

Motz, I only see charts like that in CASA tests these days.Seems to be plenty of criticism of that pilot in the video but this thread is specifically only about density altitude calculations so I am looking for what people believe that particular pilot (as an example now, but it can be a broad question) should have done after doing the density altitude calculations.

 

Who has take-off charts, for airplanes or aeroplanes that they fly, with a DA box? Not me.

 

Who has take-off distance information which would be applicable to that situation? Not me.

 

 

[motzartmerv]

Yes ok. Well without all the facts it's hard to use the example in any mathematical way.

 

We have takeoff data charts for the jabs. Regardless some consideration should be given. While you may not nee to do any calcs in your Pitts, a bush strip at high altitude (presumedly) full of pax and gear, I'm no expert, but I'm sure it would need to be addressed. I'm still unsure as to what it is your trying to say. DA is not something that should be addressed? Discussed? Or is there some secret you have to share?

 

 

[turboplanner]

Seems to be plenty of criticism of that pilot in the video but this thread is specifically only about density altitude calculations

Well it actually says "Temperature and pressure height calculations'

 

Since we are working in equations, the P&O calculations which start with calculating pressure height, density height and temperature variation from ISA allow us to calculate:

 

• Take Off distance required if the take off weight is known
   
   
  
• Maximum all up weight if the airfield data is known
   
   
  
• Take off speed
   
   
  
• Landing speed
   
   
  
• Maximum landing weight taking into account fuel burn (Some aircraft have a lower landing weight than take off weight)
   
   
  

 

 

Without going into a Thesis, this is one of my calculations:

 

Data1

 

Airfield Elevation = 1500', QNG = 1009, OAT = 17 deg C

 

Calculation

 

ISA QNH - 1013

 

QNH - 1009

 

Variation 4 MB x 30' = 120'

 

Add to elevation as pressure is lower than ISA

 

1500

 

120

 

1620' = pressure height

 

Data2

 

Pressure Height = 1620'

 

Calculation

 

ISA for this altitude = 15 degrees - 3 = 12 degrees

 

OAT is 17 "

 

Variation is 5

 

x 120'

 

600'

 

Add to Pr Ht since temp is hotter, air thinner

 

1620'

 

600'

 

1720' density height

 

Then refer to aircraft performance data, or further equations for the selected information.

 

In the case of a Cherokee its just quickly ruling four lines on a chart with a set square.

 

I'm assuming the clever answer in the case of the Stinson was that the manufacturer hadn't provided charts so better they crash.

 

So let's play the game, and record our aircraft's performance, after calculating density altitude as we fly. One way to get an accurate take off point at your local strip is to throw an egg over the side, another is record threshold altitude.

 

There will be occasions where you fly to a friends location which is either much higher or much hotter or both and you may also have refuelled just a short distance back. Cross country flying is like that - you get surprises you shouldn't have got.

 

The relevance to a flight similar to the Stinson flight is this:

 

With five or six minutes of calculations, taking into account the altitude and temperature, which can be hot in those regions, the pilot could have assessed that he would have to drop passengers, fuel or both to get a normal roll and climb.

 

There's no need to risk your life on a try out.

 

 

[djpacro]

I agree it is important - having operated quite a bit from strips at density altitudes around 8000 ft (just a guess because I have never calculated density altitude since doing my pilot theory exam).

 

We have discussed how to calculate density altitude now I suggest we discuss whether that calculation needs to be done and if so, what to do with that information.

 

Just had a look at a Jabiru manual - seems to have some good data there but no need to calculate density altitude.

 

Many aircraft don't have such comprehensive information. We can use this chart to get the effect of altitude and temperature but again no need to calculate density altitude or even use that phrase.

 

 

[facthunter]

By using the expression density altitude it contains a concept of what the plane will fly like. We all know what a plane goes like when there is a highpressure around and it is a cold morning at sea level. It is not hard to notice how poorly most planes climb when at height on a hot day. Once you get used to applying temp and height to a standard atmosphere or just set your subscale to 1013 and do the temp calc adjustment you can get an idea if you can expect problems and should take precautions by going into it further. At Hotham with a Mooney I didn't trust the figures and determined a reject point for a normal lift-off to happen. Since the engine also has much lower power available all other factors like slope and grass, soft earth etc become more of a problem too. I suspect most who get into difficulties have never done much thinking about it. Nev

 

 

[Sapphire]

The person/business that designed your small plane does not seem to be required to supply complete performance data. That could be hard to supply because people put in different engines, esp if home built. Make up your own figures for your a/c. Measure landing, t/o distance, rates of climb at different temps and weights-if you like to fly by the numbers.

 

 

[turboplanner]

Always remember Snoozers are Losers.

 

It's not the person who trains and operates at 6000', or around Maree; it's the person who flies in with a fully loaded aircraft.

 

Checking the runway required for that aircraft when the temperature is around 43 degrees will tell you whether you can go. I can't see the resistance to a couple of small calcs and four ruled lines.