{"id":14,"date":"2011-07-31T16:50:29","date_gmt":"2011-07-31T15:50:29","guid":{"rendered":"http:\/\/www.fortyfivehours.co.uk\/?p=14"},"modified":"2011-12-10T11:04:30","modified_gmt":"2011-12-10T10:04:30","slug":"barometric-pressure-qhn-qfe","status":"publish","type":"post","link":"http:\/\/www.fortyfivehours.co.uk\/?p=14","title":{"rendered":"Barometric Pressure: QNH \/ QFE"},"content":{"rendered":"

\"Cessna<\/a>Altimeters give a measurement with respect to air pressure, they are therefore essentially a sensitive barometer.\u00a0 It’s the age old way of measuring an aircraft’s altitude and as such the equipment is arguably more robust than ‘modern’ advances such as GPS. \u00a0 Additionally not all light aircraft have GPS fitted, so it’s important to know how it works and what it is actually telling you (not necessarily your altitude!).<\/p>\n

Altimeters can only tell you:\u00a0 Vertical distance above the datum selected<\/span>.<\/p>\n

So if you leave the altimeter alone for long enough and just keep flying at a ‘constant’ indicated altitude, as pressure changes – so will your real world \/ true altitude.\u00a0 This is because if the pressure goes down and you keep flying a ‘constant’ altitude on the altimeter, you’ll actually be descending.\u00a0\u00a0 Thus the saying “High to Low, down you go….”. <\/em>The reverse is equally true, giving the saying “Low to High, up you fly.”<\/em><\/p>\n

It’s what can cause pilots crash into hill sides.<\/p>\n

To avoid this problem, the altimeter needs to be regularly calibrated against a known pressure (datum).\u00a0\u00a0 There are three types available:<\/p>\n