Standard Visual Circuits
Hi, I wonder if you experts can help me. I’m an ex-military pilot. I have no GA experience, but recently I went to my local flying club, as I wanted to get checked out on a light aircraft. I have flown with two flying instructors. They have left me totally confused about a couple of things; amongst them the ‘standard’ visual circuit. Each seemed to have a different idea about how it should be flown.
In an attempt to clarify things, I’ve been looking up various references, on and off-line, to see how to fly a ‘standard’ powered visual circuit. The first thing that I have discovered is that there does not seem to be any such thing; something which seems to be borne out in the real world! The references are all delightfully vague and contradictory. But further than that, I can find none that actually ‘work’. By work, I mean that the described geometry actually works out.
Let’s take RD Campbell, author of the AOPA Flying Instructor’s Manual, which I understand is often considered an authority.
Campbell suggests climbing at Vx to a minimum of 500 ft, a climbing turn onto the crosswind leg and then levelling at circuit height (800 or 1000 ft). The turn downwind is made level using 30ºAoB ‘when at a suitable distance out from the runway’. He gives no clues what he considers the downwind spacing should be.
He suggests the turn onto base should similarly be made level at 30ºAoB ‘at a suitable distance from the touchdown area’. Elsewhere he mentions that the ideal point to turn is ‘when the intended touch-down point lies at an angle of approximately 45º behind the wing’. Presumably this is in still air conditions.
He then says that ‘the approach will be prepared at some stage along this [the base] leg’. So, at an unspecified point after rolling out on base leg, one should ‘select required power, select flap and adjust the attitude [for the descent].
There is no indication precisely where the finals turn should commence, other than the height should be 500-600 ft. He has already stated that all turns (other than the climbing turn onto crosswind) should be flown at 30ºAoB. After rolling out on finals the approach is flown straight in.
Now, this is my problem with this description. A normal approach is flown at 3º. This means approximately 320 ft per NM travelled. Let’s use a typical light aircraft base leg speed of 75 KT. The rate of descent required to achieve 3º is approx 400 fpm. At 30ºAoB the aircraft has a turn radius of 1860 ft, circumference of 1.92 NM and will take 23 sec to complete a 90º turn, in which time it will have lost about 150 ft. If it started at 500 ft, it will then roll out at 350 ft which equates to 0.875 NM. If the turn were started at 600 ft, the roll-out would be at 450 ft/1.125 NM. So far, so good (ther than the fact that we now have 2 'standard' circuits).
Between commencing descent and start of turn the aircraft must lose 400 or 500 ft, taking it 1 or 1.25 NM. The turn onto base leg and the finals turn will take the aircraft a further 3720 ft or 0.6 NM, plus the unspecified distance travelled after rolling out on base leg to commencing descent. Let’s say that is brief: in 10 seconds the aircraft would travel 0.2 NM. Add that all together and we get a downwind spacing of 1.8 to 2.05 NM. Does that sound right?
Well how about this. Let’s plot out the start of base leg turn point(s). If X is the distance along the extended centreline and Y is the distance in the direction of the downwind leg, then we know that Y is 1.8 or 2.05 NM. As the 2 turns cancel each other out in direction X, then the start of the base leg turn is the same distance from the touchdown point in direction X: 0.875 or 1.125NM. So the angle from the touchdown point to the start of base turn is either 23º or 32º aft of the wing. But wait a minute, didn’t Mr Campbell tell us that it was 45º?
I’ve tried other references. Some day that the downwind spacing should be 0.5 to 0.75 NM (woah, that’s some difference from what Campbell’s works out at). Many agree on 45º being the cue to start the base turn. Some say do a descending base turn. Most agree on about 500 ft being the turn onto finals height. Several suggest rate one turns (specifically argued against by Campbell). However, none are specific; they are all vague about the detail, and the reason is that none of them actually work, when one plots out where the aircraft would actually travel if their descriptions were followed.
Then another can of worms is opened when it comes to how to space from other traffic. I’ve read extend upwind (most would agree with that one), extend crosswind (so presumably have varying downwind spacing and delay descent on base leg by the same amount) and extend downwind (a total no-no for others).
The fact is that (assuming a 1000 ft circuit height) one must travel 3.14 NM from descent to touchdown (2.5 NM for a 800 ft circuit) unless doing a steeper approach. The final approach will account for 300-450 ft of descent, the 2 x 90º turns another 300-400 ft, so the base leg descent must be between 150 and 400 ft, which equates to 0.5 to 1 NM. Added to the turn radius x 2 (0.6NM at 75 KT and 30ºAoB, more at higher speeds/rate one) and you can see that the downwind spacing is a minimum of 1.225 NM (achieved by doing a descending turn with 30ºAoB and commencing the finals turn at 600 ft). Incidentally, this pattern is the closest to 45º from the touchdown point; but it is still a little less (42.5º). Most are considerably more. The only way to achieve closer spacing is a steeper descent or to extend downwind.
So, after all that waffle, my questions: what do you regard as a ‘standard circuit’? What do you use to judge one? What do you teach, and what are your sources?
And what has the esteemed Mr Campbell been doing all these years: dive bombing the runway or flying bomber circuits?
I have to say I’m totally bemused by this approach to aviation. Both in the military and the airline I now work for, standard procedures have been, well, standard. No grey, just black and white! That’s the point; isn’t it?