Pivotal Altitude

J

JordanD

Here so I don’t get fined
I'm a little confused on the pivotal altitude during eights on pylons. The Airplane Flying Handbook says to calculate it using your true airspeed squared divided by 15 (for mph, which is what the airspeed indicator is in). Almost all of the other training material and CFIs have taught me to use your groundspeed (in knots) squared divided by 11.3. Which is correct? I've been using the groundspeed method and it comes out so so, part of the problem is the pylons are a little close, but it always feels like I'm almost diving to catch the pylon after the first turn.
 
Pivotal altitude is always determined by ground speed, which is why it changes throughout your turn.
Whether you do it in mph or knots is up to your instrumentation of course.

Of course, unless you've got a GPS with GS reading out and are on an extended stabilized entry you're pretty much guessing anyway...
 
But the FAA Airplane Flying Handbook explanation is definitely unclear; it sounds like the author of this doesn't even know the difference between ground speed and true airspeed:

"An explanation of the pivotal altitude is also essential.There is a specific altitude at which, when the airplane turns at a given groundspeed, a projection of the sighting reference line to the selected point on the ground will appear to pivot on that point. Since different airplanes fly at different airspeeds, the groundspeed will be different. Therefore, each airplane will have its own pivotal altitude. [Figure 6-12] The pivotal altitude does not vary with the angle of bank being used unless the bank is steep enough to affect the groundspeed. A rule of thumb for estimating pivotal altitude in calm wind is to square the true airspeed and divide by 15 for miles per hour (m.p.h.) or 11.3 for knots."

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I would advise: 1) calculate the pivotal altitude before you fly--it's purpose is to give you an entry altitude for the maneuver; 2) demonstrate your aircraft control and refined commercial flying skills by flying a coordinated, constant-bank turn while maintaining situational awareness (including stall awareness, etc.) and keeping your sight line on the pylon during the turn.

-A.S>
 
based on groundspeed, formula is a kinda-sorta-guesstimate to get you started based off airspeed.
 
I guess I missed the phrase true airspeed "in calm wind". They could have just written groundspeed and save some words. Good ol FAA publications.
 
Here's a good rule of thumb. In your typical 150/172 with a TAS of 100-110, your calm wind pivotal altitude will be 900-1000 AGL. Just take it down there and you will eventually "find" your pivotal altitude by starting through the maneuver. No need to make it harder for yourself.
 
A150K said:
TAS of 100-110, your calm wind pivotal altitude will be 900-1000 AGL. No need to make it harder for yourself.
Click to expand...

100 * 100 / 11.3 = 885

110 * 110 / 11.3 = 1071

I cannot fathom this being "hard" on a person. IMO use some simple math. Your 900 was close, your 1,000 was near 100 feet off. Pick a speed and do a downwind and upwind calculation for the forecast wind.

I.E. Let's pick 100 knots and say forecast wind is ~20 knots. Direction doesn't matter as, at some point, you will be flying directly into that wind and you'll start on downwind. So we calculate for 80 (upwind) and 120 (downwind):

120 * 120 / 11.3 = 1274
80 * 80 / 11.3 = 566

Altitude for the day will start at ~1300 and shouldn't go lower than 550. Is this really making it hard on yourself? Bonus!, now you've measurable standards to compare how you, or your student, is doing. Other than just guessing.
 
Remember, having instant access to GS is relatively new.
Many trainers still have no GPS.
Could you imagine students and instructors wasting time to calculate their GS just to get a rough estimate for "8s-on-pylons"?

If you see an undesirable trend in training, sacrifice one "8-on" doing the other extreme (i.e. do the calculation and purposely come in low). "Bracket your target", sorta speak.
 
I simply printed a small excel sheet with about 5 rows and 2 columns, with 80 - 120 knots in 10 knot increments in the first column and the respective pivotal altitude in the second column...taped to a knee board. The altitude formula is just to get you in the approximate range for the maneuver and you'll quickly adjust as needed visually... hence the purpose of the maneuver.
 
Douglas said:
Remember, having instant access to GS is relatively new.
Many trainers still have no GPS.
Could you imagine students and instructors wasting time to calculate their GS just to get a rough estimate for "8s-on-pylons"?

.
Click to expand...

Or you could calculate it ahead of time... during the pre-flight briefing, for instance

"100x100/11.3=885, so lets start out at 900 agl and see how much we need to climb or descend throughout the maneuver"
 
Mike H said:
Or you could calculate it ahead of time... during the pre-flight briefing, for instance

"100x100/11.3=885, so lets start out at 900 agl and see how much we need to climb or descend throughout the maneuver"
Click to expand...

That is the point.
 
From The Airplane Flying Handbook

EIGHTS-ON-PYLONS (PYLON EIGHTS)

The pylon eight is the most advanced and most difficult of the low altitude flight training maneuvers. Because of the various techniques involved, the pylon eight is unsurpassed for teaching, developing, and testing subconscious control of the airplane.

As the pylon eight is essentially an advanced maneuver in which the pilot’s attention is directed at maintaining a pivotal position on a selected pylon, with a minimum of attention within the cockpit, it should not be introduced until the instructor is assured that the student has a complete grasp of the fundamentals. Thus, the prerequisites are the ability to make a coordinated turn without gain or loss of altitude, excellent feel of the airplane, stall recognition, relaxation with low altitude maneuvering, and an absence of the error of over concentration.

Like eights around pylons, this training maneuver also involves flying the airplane in circular paths, alternately left and right, in the form of a figure 8 around two selected points or pylons on the ground. Unlike eights around pylons, however, no attempt is made to maintain a uniform distance from the pylon. In eightson- pylons, the distance from the pylons varies if there is any wind. Instead, the airplane is flown at such a precise altitude and airspeed that a line parallel to the airplane’s lateral axis, and extending from the pilot’s eye, appears to pivot on each of the pylons. [Figure 6-10] Also, unlike eights around pylons, in the performance of eights-on-pylons the degree of bank increases as the distance from the pylon decreases.

The altitude that is appropriate for the airplane being flown is called the pivotal altitude and is governed by the groundspeed. While not truly a ground track maneuver as were the preceding maneuvers, the objective is similar—to develop the ability to maneuver the airplane accurately while dividing one’s attention between the flightpath and the selected points on the ground.

In explaining the performance of eights-on-pylons, the term “wingtip” is frequently considered as being synonymous with the proper reference line, or pivot point on the airplane. This interpretation is not always correct. High-wing, low-wing, sweptwing, and tapered wing airplanes, as well as those with tandem or side-by-side seating, will all present different angles from the pilot’s eye to the wingtip. Therefore, in the correct performance of eights-on-pylons, as in other maneuvers requiring a lateral reference, the pilot should use a sighting reference line that, from eye level, parallels the lateral axis of the airplane.

The sighting point or line, while not necessarily on the wingtip itself, may be positioned in relation to the wingtip (ahead, behind, above, or below), but even then it will differ for each pilot, and from each seat in the airplane. This is especially true in tandem (fore and aft) seat airplanes. In side-by-side type airplanes, there will be very little variation in the sighting lines for different persons if those persons are seated so that the eyes of each are at approximately the same level.

An explanation of the pivotal altitude is also essential. There is a specific altitude at which, when the airplane turns at a given groundspeed, a projection of the sighting reference line to the selected point on the ground will appear to pivot on that point. Since different airplanes fly at different airspeeds, the groundspeed will be different. Therefore, each airplane will have its own pivotal altitude. [Figure 6-12] The pivotal altitude does not vary with the angle of bank being used unless the bank is steep enough to affect the groundspeed. A rule of thumb for estimating pivotal altitude in calm wind is to square the true airspeed and divide by 15 for miles per hour (m.p.h.) or 11.3 for knots.

Distance from the pylon affects the angle of bank. At any altitude above that pivotal altitude, the projected reference line will appear to move rearward in a circular path in relation to the pylon. Conversely, when the airplane is below the pivotal altitude, the projected reference line will appear to move forward in a circular path.

To demonstrate this, the airplane is flown at normal cruising speed, and at an altitude estimated to be below the proper pivotal altitude, and then placed in a medium-banked turn. It will be seen that the projected reference line of sight appears to move forward along the ground (pylon moves back) as the airplane turns.

A climb is then made to an altitude well above the pivotal altitude, and when the airplane is again at normal cruising speed, it is placed in a medium-banked turn. At this higher altitude, the projected reference line of sight now appears to move backward across the ground (pylon moves forward) in a direction opposite that of flight.

After the high altitude extreme has been demonstrated, the power is reduced, and a descent at cruising speed begun in a continuing medium bank around the pylon. The apparent backward travel of the projected reference line with respect to the pylon will slow down as altitude is lost, stop for an instant, then start to reverse itself, and would move forward if the descent were allowed to continue below the pivotal altitude.

The altitude at which the line of sight apparently ceased to move across the ground was the pivotal altitude. If the airplane descended below the pivotal altitude, power should be added to maintain airspeed while altitude is regained to the point at which the projected reference line moves neither backward nor forward but actually pivots on the pylon. In this way the pilot can determine the pivotal altitude of the airplane.

The pivotal altitude is critical and will change with variations in groundspeed. Since the headings throughout the turns continually vary from directly downwind to directly upwind, the groundspeed will constantly change. This will result in the proper pivotal altitude varying slightly throughout the eight. Therefore, adjustment is made for this by climbing or descending, as necessary, to hold the reference line or point on the pylons. This change in altitude will be dependent on how much the wind affects the groundspeed.
Click to expand...
 
I do eights on pylons in the metro all the time, just saying, your career hinges on you correctly understanding and performing this maneuver. To answer your second question, yes I've been drinking.
 
JordanD said:
I'm a little confused on the pivotal altitude during eights on pylons. The Airplane Flying Handbook says to calculate it using your true airspeed squared divided by 15 (for mph, which is what the airspeed indicator is in). Almost all of the other training material and CFIs have taught me to use your groundspeed (in knots) squared divided by 11.3. Which is correct? I've been using the groundspeed method and it comes out so so, part of the problem is the pylons are a little close, but it always feels like I'm almost diving to catch the pylon after the first turn.
Click to expand...

Know how to teach it, technically, but for the most part, isn't it around 800-1000 AGL?

I hated "lazy" eights and saw no purpose, but it's what I had to do to please the FAA so...
 
rframe said:
I simply printed a small excel sheet with about 5 rows and 2 columns, with 80 - 120 knots in 10 knot increments in the first column and the respective pivotal altitude in the second column...taped to a knee board. The altitude formula is just to get you in the approximate range for the maneuver and you'll quickly adjust as needed visually... hence the purpose of the maneuver.
Click to expand...

That's what I do. The maneuvers are a joke. The only one that makes any sense, IMO, is a chandelle.
 
Houston said:
Why does that one make sense to you?
Click to expand...
I heard supposedly it's a maneuver to simulate getting yourself out of a box canyon. But if you're the kind of person flying into those in an Arrow I'm not sure you need a Commercial certificate in the first place.
 
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