A330 Air France GIG - CDG AF447 31ST MAY 2009

[David Morton]

Mike, I always tried not to add speed for the kids but I was invariably guilty of carrying an extra 5 tons . Why? Because I could. On a ten hour jag it only left me with about 3 tons which in the latter stages only gave me about 20 mins extra in the cruise or 45 mins holding instead of 30, but it worked in my favour more times than not. The fleet office gave up trying in the end, but they wanted everyone to go around on minumum (Sword) fuel. Sword was our flight planning system at the time and those who lived by the Sword usually diverted by the Sword as well. Average cost of a div, passengers sorted out and repositioning with another crew on a 747 with landing and handling fees? About £125,000. Cost of 5 tons AVTUR ? then about $1200. QED

 

[Sid]

Is GPS capable of adding a layer of safety to the pitot static method of measuring airspeed ? As a back up mode to drive the indicator in an emergency or as a 'reality check' on the primary system ?

 

[David Morton]

Absoultely not. Great for Navigating and reduced seperation on the Atlantic but as a back up for pressure instruments unfortunately no.

Perhaps I should try and explain. The GPS systems are fitted and I suppose an easy example is 20kt wind on the nose while sitting at the end of the runway completely stationary. The Pitot will register 20 knots but the aeroplne is going nwhere.
Now, think of up at FL330 (33,000FT ISH) with a tailwind of say 100 kts.
The IAS (IndicateD Air Speed)would be about 275 knots and the TAS (True Air Speed ) about 550knots (ish). (because of a whole range of things - temperature, compressability, the altitude [the air is much thinner)etc etc. The tailwind will push this up to 650knots. This is what the GPS will register - a speed basically across the ground but it's so far removed from anything the pitot and pitot static system is measuring. Hence the GPS is a navigational tool only.