Enough to stop the exhaust bubbling the paintwork around the flightdeck and being ingested into the engines! Longer if you are going into a headwind behind the other jet. You can see the effect on the TAT probe readings which can go up alarmingly. Longer for a big widebody, and enough to stop the jet in front buffeting you. Short enough to allow the airport to have more than about 2 jets taxiing at the same time! There is no set figure.
In Australia a common criteria is the requirement in CAO 20.9, section 5.1.4; that is 46 m behind a jet at breakaway thrust, and 30m behind a jet at ground idle thrust.
Well blow me down, there is a figure! I have been using common sense for 37 years. It tends to get a little bit windy to get out and measure 30 or 46 metres, not that I am any good at judging distances like that anyway, especially from a Boeing flightdeck (of all models!). The darn exhaust blows the tape measure to hell. Guess that's why the hair is thinning!
Taxiing once in FRA some years ago in LVP conditions, I discovered a good reason that rearward facing white position lights should remain fixed to the tail and not be put on the wingtips, as is now the fashion. The smell of badly burned parrafin wafted into the cockpit and caused me to slow down. On looking UP, I saw the APU exhaust of a B747 about 3m in front of me! He has stopped, presumably to check his position, and not told the Ground freq. I would suggest that that was too close. Some choice exchanges followed between myself and Grnd on the issue of monitoring the Ground radar. They sent a tug to move him to prevent blast damage to my little 737.
EXACTLY the same thing happened to me! I was saved by FO who simply slammed on the brakes. My initial response was "WTF are you doing"" followed immediately by "Oh my God" when I saw an Airbus tail cone barely a metre in front of me!
Needless to say I bought the beers that night. Wing-tip "tail" lights are fine, but in my humble opinion, they should reinstate the proper TAIL light.
Just continuing the thread drift prompted by rubik
Quote:
Some choice exchanges followed between myself and Grnd on the issue of monitoring the Ground radar.
Idle curiosity really, but what makes you expect ATC to monitor the surface movement radar - and what do you think ATC's responsibilities are if they do?
The smell of badly burned parrafin wafted into the cockpit and caused me to slow down. On looking UP, I saw the APU exhaust of a B747 about 3m in front of me! He has stopped, presumably to check his position, and not told the Ground freq. I would suggest that that was too close.
See and avoid still works, on the ground, too.
Perhaps the problem is in the "looking up." One ought to be heads up all the time, especially if one is in a position where one needs to "slow down."
I've never seen a standard or requirement setting an in-trail distance during taxi.
