QNH-The silent threat

[Louis Cole]

Friday morning.Perfect flying conditions by the looks of it. Haven't flown since Secunda trip 3 weeks ago. Preflight done, CEX starts first time, flightsuit on and just the last minute call to Lanseria ATC and I'm on my way to Aero 57.Guess what...QNH 1012.I briefly consider taking on my 180m rwy. with powerlines at the end of 05 and a slight tailwind and with my one remaining braincell humming Pink Floyd's "Is there anybody out there ?"I suddenly remember the day ZS-WLA and I parted company under similar conditions.She went to the scrapheap and I went to ICU. Nice to have Flo as a soundingboard when you consider putting on your bulletproof vest and tempting fate.QNH not to be underestimated.Well both CEX and I are still intact and tomorrow is another day.

[Big-D]

Good call

1012 seriously low.

I know the feeling of trees approaching and not yet airborne, not nice

As you say, tommorow is another flying day

D

[nicow]

Good call

1012 seriously low.

I know the feeling of trees approaching and not yet airborne, not nice

As you say, tommorow is another flying day

D

Wise call
The QNH at Potties yesterday at 17h00 was 1011 and it was 32 deg

[Big-D]

A nice Ipad app to have is "Aeroweather" - Shows aviation weather of numerous airports, even now FALA is showing a QNH of 1013....

D

[Tribal Croc]

The QNH at Potties yesterday at 17h00 was 1011 and it was 32 deg

Eina That would have given you a DA of 6359ft

Good call to abort

[Loco]

A nice Ipad app to have is "Aeroweather" - Shows aviation weather of numerous airports, even now FALA is showing a QNH of 1013....

D

Thanks very cool app!!!

[HansH]

So how does QNH affect take off distance?
The standard sea level QNH is 1013.2 so why is 1012 seriously low?
Your silent threat is temperature. Altitude and temperature combined give you the
density altitude and if the temp is high then the density altitude is a lot
higher than your stated airfield altitude.
Some Examples: Assuming your field is at 5,000 ft.
At an OAT of 0 deg your density alt is about 4,200 ft. Climb like an angel.
At an OAT of 10 deg your density alt is about 5,800 ft
At an OAT of 20 deg your density alt is about 6,900 ft
At an OAT of 30 deg your density alt is about 7,800 ft. Argue with the trees
and possibly join the angels.
Temp is what you need to worry about not QNH.
Might be worth your while to invest in an E6-B flight computer and use it to work out the
density altitude for every take off you do.

[John.com]

So how does QNH affect take off distance?
The standard sea level QNH is 1013.2 so why is 1012 seriously low?
Your silent threat is temperature. Altitude and temperature combined give you the
density altitude and if the temp is high then the density altitude is a lot
higher than your stated airfield altitude.
Some Examples: Assuming your field is at 5,000 ft.
At an OAT of 0 deg your density alt is about 4,200 ft. Climb like an angel.
At an OAT of 10 deg your density alt is about 5,800 ft
At an OAT of 20 deg your density alt is about 6,900 ft
At an OAT of 30 deg your density alt is about 7,800 ft. Argue with the trees
and possibly join the angels.
Temp is what you need to worry about not QNH.
Might be worth your while to invest in an E6-B flight computer and use it to work out the
density altitude for every take off you do.

I took off from Aeroden this morning at 5:20am with QNH = 1007mb (the lowest EVER experienced) and took off in exactly the same take off distance as normal!

The QNH of 1010mb shown below was at 03:51 and was falling!

21C at 03:51!!

Aeroden Airfield Weather Station:
19 Oct 03:51
Wind:S T178 0-4kph,0-2kts
Temp:21C
Hum:33%
QNH:1010mb FAL
Dewpoint:5C
CU Height:6952ft
D.Alt:5073ft (FYI: Aeroden is at 4000ft)
Sunrise-05:31
Sunset-18:17

Safe Skies!

John.com

[Bundy]

Hi Louis,

Wow, I thought Rny 09/27 at Microland was short.... haha, nothing compared to yours. I'll have a good look at it when I come visit.

Although all three factors (OAT, QNH and Relative Humidity) will affect the DA when they change, Temperature seems to have the biggest effect as HansH says.
Prob in the formula... I looked it up on Wikipedia and well....let's just say that's way to complex for my brain to understand Here it is for the fundi's. http://en.wikipedia.org/wiki/Density_al ... alculation

I have used this website in the past: (DA for dummies )

http://www.pilotfriend.com/calcs/calcul ... ensity.htm

Try putting in the current weather and your altitude and calc the DA.
Now drop the QNH by 10 milibars....not much difference.
Now add 5 degrees to temp....and see how much more changes in temp affect the DA.

EDIT: I just though I'd add.... Louis on a runway like yours with obstacles etc etc, the DA is critical. Good on you for erring on the side of caution.

[Big-D]

Very interesting conversation and I certainly have learned something, thanks for everyones input

[Louis Cole]

I guess in theory the temperature is the determining factor wrt to the QNH and the temperature was on the high side when I had the accident some years ago but hell, those powerlines at the end of a shortish rwy.make a powerful argument against flying when the QNH is low especially after an accident.There was a time in my bulletproof days when ignorance was still bliss,when I would have been up like a shot or maybe not, but nowadays I tend to err on the side of caution.I just don't bend so good anymore and CEX really looks so much better than my last glimps of WLA.

[Tumbleweed]

So if you were spoilt with a 1km tar runway without obstacles you could eventually reach take off speed, but just as you settle in, that low qnh brings such unstable turbulance that you're going to wish for the same long strip when you land.

[Bundy]

So if you were spoilt with a 1km tar runway without obstacles you could eventually reach take off speed, but just as you settle in, that low qnh brings such unstable turbulance that you're going to wish for the same long strip when you land.

I love the way that TW can change the whole dynamic of a conversation in one sentence. What he says is very true. It's def not just about the DA and takeoff performance. a massive drop in pressure will only affect the DA slightly....but think of the general conditions accompanying a major low.....usually not so nice

Reminds me of that saying:

"Dont put your aircraft in a position that your mind did not get to 5 min earlier"

Great point TW.

[Louis Cole]

I couldn't agree more with Bundy and TW.Taking off is optional but landing is compulsory.Big D alluded to the dreaded scenario where the trees are approaching faster than your runway is running out of meters.That "Oh F#*#*k why didn't I...." feeling is slightly worse than the eerie silence of an engine out with little or no place to land.... but enough of the doom and gloom.Speaking of which I'm looking forward to Bundy's end of time flight on 21December.