Pressurization can be one of the most frustrating things to figure out. Do I set it at my destination field altitude? Do I set my cruise altitude? Do I leave it at my departing field elevation if I’m going to return without another stop?
Let’s look at setting the pressurization controller and then explore running a test on your pressurization system to make sure it is working the way it is supposed to.
Setting your pressurization controller – takeoff
When going through your run-up prior to departure, one of the checks the pilot’s operating handbook, or POH, calls for is a pressurization check. Here is the procedure it lays out:
Pressurization……………………………CHECK AND SET
Bleed Air Valves…………………………OPEN
Pressurization Controller…………….SET
Adjust cabin altitude knob to indicate 1,000 feet below field pressure altitude.
Set rate knob to 12-o’clock position.
Cabin Pressure Switch………………..HOLD IN TEST POSITION
Cabin Altimeter and VSI………………CHECK FOR DESCENT INDICATION
Cabin Pressure Switch………………..RELEASE TO PRESS POSITION
Pressurization Controller…………….SET
The inner scale (ACFT ALT) should indicate planned cruise altitude plus 1,000 feet. This setting must not result in an outer scale (CABIN ALT) indication of less than 500 feet above departure field pressure altitude.
Once we have reached our cruise altitude, say FL250, we would have set 26,000 feet on our pressurization controller, resulting in a cabin altitude of a little over 6,000 feet at max differential (6.5 PSI) in a B200. With either flow pack operating, max differential should be able to be achieved; together they should easily be able to hold max differential.
Setting your pressurization controller – landing
The POH calls for setting the pressurization controller to 500 feet above the landing field pressure altitude. That works if the pressure is standard (29.92 inches Hg). What should we do if the field atmospheric pressure is not standard? We need to adjust our controller setting to account for non-standard pressure. Thankfully, we have been given a chart that does just that!
As you can see in Figure 1, going through all those lines on the chart while flying can be challenging.
We can also do the math in our head to get the correction to add or subtract from our destination field elevation. Remember the old private pilot rule: 1 inch of mercury equals 1,000 feet. We can also say: 0.1 inches Hg = 100 feet on a non-standard atmospheric pressure day. If the current local altimeter setting is 30.56, it is .64 higher than the standard 29.92, resulting in 640 feet lower than standard. We round it to the nearest 100 feet.
Due to the controller’s inaccuracy, we always start by adding 500 feet to the field elevation then correct for non-standard atmosphere.
Figure 2 shows a chart with the same information in an easier-to-read format.
Troubleshooting
Now that we have the normal setting of our pressurization for takeoff and landing covered, what happens if our pressurization system is not working the way we expect it to?
Is it a flow pack problem? Is it a leak rate problem? Is it a combination of both? How do we determine where to start looking?
What follows is a procedure you can use to check the health of your flow packs and the leak rate of your King Air.
1. Establish level cruise flight at maximum pressure differential (∆P), using normal cruise power setting, between 15,000 and 18,000 feet. (To do this, set the controller’s cabin altitude for a sea level or lower and climb until the cabin begins to climb also.)
2.Record:
Aircraft pressure altitude: __ feet
Indicated cabin altitude: __ feet
Indicated differential pressure (∆P): __ psid
Indicated cabin rate-of-climb: __ fpm (should be 0)
Engine speed: __ / __ % (L/R)
Conduct steps 3 through 6 for the left side only, recording the results in the appropriate spaces. For now, leave the results concerning the right side blank.
3.Watch the cabin rate-of-climb indicator as you turn the left bleed air switch to the “Envir Off” (center) position. The indicator should rapidly rise to a maximum peak, then descend.
4.Record:
Cabin’s peak rate-of-climb: ______ / ______ fpm
(left off) (right off)
Rapidly move the left power lever to idle while watching the cabin rate-of-climb indicator. If the flow pack is properly shut off, there should be no change. Return to normal cruise power. Circle the appropriate answer on one line below:
Does the left flow pack indicate that it is shut off completely? YES/NO
Does the right flow pack indicate that it is shut off completely? YES/NO
5.When the cabin stops climbing or descending, with the rate-of-climb indication stabilized at its original reading:
Record:
Indicated ∆P (left pack off): __ psid
Indicated ∆P (right pack off): __ psid
(Should be maximum, same as before.)
6.Turn the bleed air switch back on. Wait until there is an indication that the flow pack has reopened successfully (such as a momentary cabin descent surge, louder airflow noise, rise in ITT or reduction in torque) and until all parameters return to their initial values. Sometimes this takes 10 minutes or more, and occasionally a flow pack won’t reopen at all during this flight. In that case, terminate the check until another flight can be made.
7.Repeat steps 3 through 6 for the right side, recording the values in the places you left blank before.
8.With both bleed air switches back on and all parameters at their original values, watch the cabin rate-of-climb indicator as you move both bleed air switches to the center simultaneously. (DO NOT go to the bottom position, you will lose your door seal.) It should rapidly rise to a maximum, then show a slow, continual reduction. (With decreasing ∆P, the air doesn’t leak out as fast, so the cabin doesn’t climb as fast.)
Record:
Cabin’s peak rate-of-climb: __ fpm
This is your airplane’s leak rate. It is excessive if it exceeds 2,500 to 3,000 feet per minute. However, it is not uncommon to find leak rates well above 5,000 fpm. This is not necessarily dangerous, but it does imply that were an engine to fail or a bleed switch be turned off in flight, the airplane would not be able to maintain proper pressurization. Also, with a high leak rate one can expect to experience more pressurization irregularities than are typical (e.g., a cabin climb when power is even slightly reduced during descents).
9.Optionally, you may wish to keep the bleed air switches off until the cabin climbs high enough to trigger the “Alt Warn” annunciator, to verify that it is functioning properly. It should illuminate at 12,500 feet ± 500 feet.
To prevent the passenger oxygen masks from dropping if the cabin accidentally goes above 12,000 feet, pull the Oxygen Control circuit breaker, under Environmental on the right CB panel.
10.Turn the bleed air switches back on; the test is complete.
This test will tell you if one of your flow packs is weak or not working and if your plane has an excessive leak rate. Another thing you can do is have a pilot with good hearing (they exist, right?) get into the cabin of your plane while at max differential and listen near the emergency exit and the door for leaks. This information can greatly help your maintenance shop diagnose and repair your pressurization system.
With a pressurization system in good working order those painful ears should be a thing of the past!
your flow packs is weak or not working and if your plane has an excessive leak rate. Another thing you can do is have a pilot with good hearing (they exist, right?) get into the cabin of your plane while at max differential and listen near the emergency exit and the door for leaks. This information can greatly help your maintenance shop diagnose and repair your pressurization system.
With a pressurization system in good working order those painful ears should be a thing of the past!
Zach Cleaver, a Gold Seal flight instructor since 2009, started teaching in King Airs in 2010. He has worked for King Air Academy in Phoenix, Arizona, since 2013 and flies all models of King Airs.