View Full Version : AoB in Base Turn

4th Jun 2003, 14:19
I am just a simple line pilot on the 737.

Does any body have a formula or rule of thumb to figure out the Angle of Bank required in a base turn in order to roll out pretty close to established?

If it has already been covered here I apologise. Could someone post a link? (I couldn't find exactly what I was after in a search)


4th Jun 2003, 17:23
If you will pardon an old fella here, the formula is AIRMANSHIP and EXPERIENCE.

The variables are infinite, distance of parallel downwind, head or tail wind on base, airspeed and weight to name a few.

You can't do a base turn on numbers alone you have to look out on a visual circuit. Most SOPs state 25 deg normal/30 max, so start off hard and ease out to final, monitoring the sink and C/L closure rate. Remember that the sink can sneek up on you in the turn.
Radar vectors being a different kettle of fish as the vector is usually 30/40 angle off and to an ILS or radial.
Most flight directors predicate at approx 25 deg bank achieved at 3 per sec average in and out of the turn in still air, which we never have.
Flight directors can be used in a visual circuit but have to be adjusted to what you are doing rather than you following what they tell you to do, which can be "fun" if you dont change your usual mindset.
I came from a jet operation in Aust where 95% of our approaches were visual 3 legs of a circuit. I now do 99% vectors to final and the rare visual ones are sometimes not pretty to watch, it's all experience and recency of the operation.

PM me if you wish.


7th Jun 2003, 21:50
In a base turn; suggest you keep your head outta the cockpit and don't inside, except for a quick glance at the fast/slow indicator ;)

John Farley
8th Jun 2003, 06:27
When you find out let me know what angle I have to turn the steering wheel to on my car to park it at the kerb.

8th Jun 2003, 18:42
There is an interesting thread by Capt Stephen Hawkins on Wannabees/Pro Training UK......

8th Jun 2003, 19:48
Effect of Bank on Radius of turn
Radius of turn is inversely proportional to the tangent of the angle of bank.

r is inversely proportional to Tan(b)
As the Tan of b approaches zero (wings level) tan(b) approaches zero and the radius becomes infinite. Thus, the equation correctly shows that there is no turn with the wings level.

As the Tan of b approaches 90, tan(b) approaches infinity and the radius of turn approaches zero. Thus, the equation indicates that a turn of zero radius can be made at 90 degrees of bank.

Previously we developed the equation LF= 1/cos(b). This equation indicates that the LF becomes infinite at 90 degrees of bank.

Thus, these two equations indicate that it is not possible to make a turn at or near 90 degrees of bank because the forces involved become infinite. Most of us have seen aircraft flying at 90 degrees of bank (and not making infinitely tight turns.) This is possible only by slipping, which was assumed not to be the case when we began this section (examine the first diagram and consider how it would change if the aircraft was slipping.)

Now what was the question again?


9th Jun 2003, 07:38
... ah, Gimpgimp, the real problem lies in having a sufficiently robust tape measure attached to your wingtip to determine just how far you are out from the centreline on downwind ... so that you can relate bank to radius of turn to the actual base turn diameter you might be trying to achieve ...

.. could it be said that we might be .... "going around in circles" ?

Like JF, I too would like to find out how to reduce my tyre wear and tear during parking operations ....

9th Jun 2003, 10:57
It appears I may have been misunderstood.

I was referring to Base Turn as the turn from outbound to inbound leg in a reversal procedure (ie Non Precision Approach).

However, after a little bit of asking around it appears that what I am after is probably too academic.

Stay safe all.

9th Jun 2003, 11:40
The procedures are designed assuming a 25 deg angle of bank OR 3deg/sec rate of turn, whichever requires the LESSER angle of bank.

9th Jun 2003, 15:56
Why didn't you say so first time around ..... now the problem becomes relatively simple.

(a) figure the included angle between the outbound and inbound legs

(b) figure the distance out from the station at which you commence the turn

(c) it is a simple matter to figure the diameter of the circle which is tangential to the legs at that distance

(d) knowing the aircraft speed, it is a simple matter to figure the bank to achieve the diameter.

If you really want this stuff (it is of use in link trainer work especially) do send me an email and I will send you the relevant formulae and a graph or two to make it easier.

In practice, wind complicates things sufficiently to make an easier procedure -

(a) figure the bank as above and, if less than 25-30 degrees, use your preference of those bank angles

(b) turn to the mid-heading position and note the relative bearing

(c) use (b) to figure a lead bearing to commence the turn to final

(d) soft shoe shuffle the turn onto final but it will be pretty close.

If the calculated bank is greater than 30 degrees (not often, but sometimes the designers goof off and this happens) consider extending the outbound leg within maximum speed parameters to increase the distance and diameter .... and reduce the bank to achieve the diameter ... maybe even reduce speed by configuring a bit earlier than standard for your type before or during the turn to reduce the radius for a given bank ...

9th Jun 2003, 16:21
Well - a liitle bit of v squared over g tan theta sometimes helps....

Gulf War 1, flying at strange speeds and needing to know at what range to turn our tanker to roll-out in front of the receiver formation, tap tap on the calculator and it worked just fine.

I also decided in one bored moment to work out the ideal angle of bank to fly the outbound-to-inbound turn on our local NDB approach in still air. It works out at 12 deg. Since doing that, I found NDBs an absolute doddle as using 15 deg and allowing for the reduced rate of turn in the descent from base to final, even with significant cross-track wind you're normally pretty well in the cup and only minor corrections are needed to tow the tail of the needle to the right value! Mind you, having the FMS inbound course and track error displayed on the HSI as a back-up helped - particularly with a Y-code GPS and LINS feeding the FMS!!

12th Jun 2003, 05:03
I`m sure there is a formula to calculate the required angle of bank to achieve a rate one turn according to your airspeed which is probably what you`re after, however, I`ve forgotten it!so my advise is keep reading this thread until one of those clever chaps gives you the answer you`re after or just use 25 degrees like everyone else.

12th Jun 2003, 06:11
Standard rate of turn rule of thumb:

Bank angle= TAS/10 + 7 (in knots), +5 (m.p.h.)


12th Jun 2003, 18:37
Maybe I should have dug the file out last post ....

R = KTAS/(62.8 Rate)

R= KTAS squared/(68637 tan(bank))

and, for the base turn geometry

R = distance out tan(included angle/2)

Notes ...

(a) R in nm, turn rate in deg/sec

(b) the numbers will vary very slightly depending on the actual values you insert for the constants but not to any great extent

(c) you can play with the equations to get whatever result you need and, in turn, develop any rules of thumb which might give you some sensory pleasure .....

(d) the derivations are very straightforward if you are really into playing with sketches and elementary mechanics/trigonometry ..... I prepared a set of graphs of all this sort of stuff for a link trainer instructor colleague many years ago and I can email that to anyone who likes pretty pictures .... not a great deal of use, I guess, but some people like that sort of thing ...

(e) while I have tried to avoid any ... if I have made any typos, please do tell me and I will correct the post ...

13th Jun 2003, 21:40
If the calculated bank is greater than 30 degrees (not often, but sometimes the designers goof off and this happens)
Shame on ya for even suggesting such a thing J_T! We'd never make such a stuff-up! :p

Depending on the alignment of the approach - and whether or not you need to circle at the end of it - it's conceivable that you could have either a headwind or tailwind while heading outbound toward the reversal. Most times it'll be pretty easy to work out in advance, so you know that a substantially opposite effect will apply at the end of the reversal.

Now, so long as you make a reasonable allowance for head or tailwind prior to the reversal, you'll be around about the right distance away from the navaid. The distance IS the key to the radius of turn - so long as you're somewhere near that distance, a 25-degree bank turn will work out pretty nicely.

I configure the B200 for 170 knots until established inbound from the base turn. In practice, I'm actually starting to reduce thrust (and therefore speed) during the reversal. But I hold 25 degrees throughout the turn. Mostly, I'm able to roll off a bit of bank from about half way thru the turn to make a nice intercept of the inbound track.

But, basically, if you want to fly faster than that (and provided that the approach allows it, of course) a 25 degree bank turn will work out 9 times out of 10. The thing you've got going for yourself here is that the turn is protected for an omnidirectional wind of a fairly high speed. Now, remember what I said about how the wind effect is most likely to differ between the outbound and inbound leg? You're hardly likely to have a complete head or tailwind right thru the turn.

The long and the short of it is, therefore, adjust outbound timing for a head or tailwind, then initiate a 25 degree bank turn. I'm confident that you'll find it works out right... 9 times out of 10.

13th Jun 2003, 22:28
... oops ... sorry about that, mate ... forgot that you were following the thread ... the tight one which comes to mind was one which my link trainer mate used to throw at his victims .. can't recall now just which letdown, but somewhere in SE Oz ....