Aviation meteorology..........?

Question

What is the importance of the right QNH pressure for an aircraft to take-off or to touch down? I believe the standard measurement range is +/- 1hPa.....

Answer 1

Altimeters calculate your altitude by measuring the air pressure (lower pressure = higher altitude). For it to function correctly, you need to set the local sea level pressure - QNH.

If you don't, it will indicate a wrong altitude! For example, if QNH is 1030, and you set 1013 instead, it will have an error of 450 ft, and that is quite dangerous, especially if you do a non-precision instrument approach in IMC.

Answer 2

at my base/ 1200ft AMSL/ the difference in setting of 1 hPa means difference of about 50 ft readout -rough estimate!!. should the base be higher, so would be the readout difference. this value is not the critical one for both takeoff and landing, because no landing system used today requires the QNH setting for its operations. the "best" systems used today use combination of ILS beam and radioaltimeter to estimate proper flaring of the plane during the landing.

systems using the readout of the altimeter terminate the instrument approach procedure well above the value of 50 ft or so, typically the NDB-DME /NDB-GPS combinations terminate in the height of 300 ft above the terrain or HIGHER. so you should see the surface and the base from that height and u do not need the altimeter readout as soon as you manage to aquire the visual contact.

enroute separations of the flight levels is 500ft, so this value is providing you a protection from mistaken altimeter setup, as well. Though things may go wrong and the setting mistake might be bigger than just 1 hPa.

Answer 3

Setting qnh in the kolsman window calibrates the altimeter to read the field elevation.

Answer 4

It's not important for takeoff or touchdown.

On takeoff you are simply looking for a positive rate of climb, you know where the ground is so the altimeter can read anything as long as it is going up.

On landing you know where the ground is (right below you coming up slowly if it is all good) because you can see it.  If you are doing a blind landing you are monitoring the radar altimeter, which cannot be adjusted (other than the DH bug).

You need to set the altimeter to QNH so your altitudes for climb, approach, and low altitude cruise will not conflict with other aircraft in the area (ie, your altimeter says 5000, the other plane's says 4000, but you are at the same altitude because you have different altimeter settings).  It also ensures obstacle clearance for IFR and simplifies flight planning for VFR.

Above 18,000ft, all altimeters are set to 29.92 since the planes would have to set their altimeters continuously if they used the ground readings and planes might conflict with each other if they hadn't set their altimeters quickly enough.  Also there are no obstacles at these heights so a standard pressure makes things simpler.