Modesto2 From United States of America, joined Jul 2000, 2634 posts, RR: 6 Posted (11 years 5 months 1 week 5 days 6 hours ago) and read 899 times:
I was listening to my scanner last night at LAX. An American Eagle Saab 340 departed 24L and tower cleared it for take-off and added "offset approved." What does this phrase mean? If it makes any difference, the aircraft departed behind a JAL 747. Thank you.
MightyFalcon From Oman, joined Jun 2001, 384 posts, RR: 10 Reply 1, posted (11 years 5 months 1 week 5 days 6 hours ago) and read 853 times:
The offset procedure is used both for approach and area air traffic control, although for different reasons.
In the case you've witnessed, approving the Saab to fly on an offset track (from the original one flown by the JAL 747) is just a mean of keeping him out of the turbulence of the preceding heavy 747. His departure track will be parallel to the jumbo's by x Nm. The distance from the original track can be either specified by the controler or proposed by the pilot and then authorised or not by the controller.
En-route, the offset is mainly used when you have 2 flights, following the same airway for some time and too close to each other to provide them with the same cruising level. Later on, their routes are going to split so, not to penalise the traffic at the lowest level, we "push" one of them on an offset track (5,8 or 10 Nm to the original one) in order to get them both the same level.
Let's say the original route is: xxxxx-yyyyy-zzzzz. The traffic on the offset track will then fly on a stricly parrallel route 5Nm (for instance) off xxxxx-yyyyy-zzzzz.
Modesto2 From United States of America, joined Jul 2000, 2634 posts, RR: 6 Reply 2, posted (11 years 5 months 1 week 5 days 5 hours ago) and read 830 times:
MightlyFalcon, you have confirmed my suspicions. In this particular case, would the Saab execute the take-off with the offset? Or would the pilots make a slight turn AFTER takeoff? Thanks for the insight.
MightyFalcon From Oman, joined Jun 2001, 384 posts, RR: 10 Reply 3, posted (11 years 5 months 1 week 5 days 5 hours ago) and read 825 times:
I wouldn't like to give you wrong figures about the approach offset procedure but I can tell you how this is performed en-route: Either the ATC gives a heading to split or he let the pilot do the standard procedure which is a 45 degree turn to join the offset track.
In the case you described, the Saab turned L/R short after take-off. One thing is sure, the Saab used the same runway than the 747, of course.
Jetguy From , joined Dec 1969, posts, RR: Reply 4, posted (11 years 5 months 1 week 4 days 23 hours ago) and read 809 times:
Most, if not all, FMS's have offset capability built into their programs. The pilot simply selects the NM offset (left or right) and plugs it into the box. It's a capability that is becoming more and more important - particually in non-radar international or foreign airspace. Pilots will intentionally put a bit of offset into their FMS's. In these days of GPS and RVSM accuracy it's getting to be pretty easy to be at EXACTLY the wrong place, at EXACTLY the wrong altitude, at Exactly the wrong time - if you get my drift.
However, Modesto, I doubt if it were anything as formal as this the other night. It's normal to offset slightly upwind (less than a mile) when following any aircraft where wake turbulence would be an issue. The pilot simply asks to offset slightly and ATC normally will be accomodating.
MightyFalcon From Oman, joined Jun 2001, 384 posts, RR: 10 Reply 7, posted (11 years 5 months 1 week 7 hours ago) and read 689 times:
An offset and a deviation are two different things, used for different purposes.
Once again, an offset is just a track strictly parallel to the original one and very unusually used for weather deviation. Furthermore, we're not using km as a distance unit in aeronautics but Nm (nautical miles).
Deviations are used for weather avoidance, of course but mostly to provide enough separation between crossing traffic at the same level or climbing/descending through the level of another one. Deviations are expressed in degrees.