Safety Publications/Articles


The mystery of the stopped climb

Sometimes, none of the clues found in an instructional mystery add up. Such was the case of The Mystery of the Stopped Climb, which involved one very puzzled pilot and a set of circumstances that--on the surface, at least--just didn't make sense.

Jim had purchased a pressurized six-place twin, and he had employed me to provide his transition training. Jim was a skilled pilot, and the transition was easy.

The mystery started a few months later when Jim called me, and in a highly agitated voice announced, "It just stopped climbing!" With words tumbling out rapid-fire, he explained that shortly after takeoff on a flight from his home airport, his airplane just quit climbing. He was alone in the airplane. Climb rate should not have been a problem.

The elevation of his home airport is about 800 feet. At the time of his takeoff, the ceiling was about 1,000 feet, and the temperature was one degree Celsius. As he entered the clouds the airplane stopped climbing. He had the manifold pressure set at 35 inches and the rpm at 2,400, and was maintaining about 130 knots with a 5- to 10-degree pitch-up on the attitude indicator. He had turned on the anti-ice equipment and had verified it was working before takeoff. There was no ice accumulation.

Just after he entered the clouds, he told me, his altimeter showed he was level at 2,500 feet. He checked with ATC, and they verified that his Mode C readout showed him level at that altitude. Landing gear and flaps were up. He used the attitude indicator to level off, and maintained a pitch-neutral attitude with a cruise power setting. He tried switching to the alternate static source; the needles twitched, he reported, but the altimeter still showed that the airplane had quit climbing. The airspeed indicator continued to work normally.

ATC vectored him back for the ILS approach. He noticed that he intercepted the glideslope about 12 miles from the airport instead of the normal five miles. When he broke out of the clouds the altimeter started unwinding, showing the airplane descending normally. On the ground the altimeter displayed the correct field elevation.

Pitch plus power equals performance. With that power setting, pitch attitude, weight, and density altitude, the airplane had to be climbing. The question is, why did the altimeter show the airplane had stopped climbing? Jim had his mechanic check several things, including the static system plumbing. "Oops," said the mechanic. "I disconnected the altimeter from the static line when I did some work behind the panel last week. Looks like I forgot to retighten the nut." (The mechanic also seems to have missed the regulation that says anytime the static system is opened the system must be tested again.)

The loose nut on the altimeter static source allowed the static system to sense cabin atmospheric pressure, not outside atmospheric pressure. Since the cabin pressurization controller is normally set to kick in at field elevation plus 500 to 1,000 feet, the cabin started to pressurize at about 1,000 feet agl, or just about the same height as the bottom of the overcast.

The overcast did not cause the "stopped climbing" problem, despite what it looked like from Jim's seat. The problem with the altimeter was caused by the pressurization point selected on the cabin altitude controller, which just happened to be the same altitude as the bottom of the overcast. Jim had set the cruise altitude on the cabin altitude controller to maintain a cabin altitude of 2,500 feet, and--thanks to the loosened static line nut--the altimeter was reading the altitude of the pressurized cabin.

The reason Approach Control saw the same "stopped climbing" condition was that the aircraft's altitude encoder was connected to the same (disconnected) static system, and so referenced the same cabin pressure altitude.

In an unpressurized airplane, the alternate static source is inside the cabin. That placement wouldn't work in a pressurized airplane because the alternate static source would be sensing cabin altitude--the alternate static source must be outside the pressurized cabin. The reason nothing changed when Jim switched to the alternate static source was that the leak in the static plumbing was inside the cabin. The altimeter continued to show cabin altitude regardless of which static source he used.

Jim probably was at an altitude much higher that he thought he was. First, he maintained climb power and airspeed for several minutes. His airplane would normally climb at 1,000 feet per minute. That would put him at around 4,000 feet. When Jim leveled off he flew the attitude indicator for pitch neutral. That meant his altitude did not vary much. Only about 10 minutes had passed until he was established on the localizer. The fact that he intercepted the glide slope at twice the normal distance from the airport and was on the glide slope for a considerable period of time indicate he was considerably above the usual 2,700-foot intercept altitude.

The altimeter began working as he came out of the overcast, which was just about at the altitude set in the cabin altitude controller. Coming out of the overcast was entirely coincidental to the revival of the altimeter function.

A pilot of any airplane, pressurized or not, may be faced with a situation where the altitude of the airplane is in question. In this case, Jim's airplane was certified with a single altimeter installed in the pilot's panel, and that altimeter is electric. There was no back-up altimeter. In the event of failure of that altimeter, the pilot has no barometric altitude information at all.

One option is to use GPS. Most all GPS units, IFR certified or not, have an altitude readout on at least one page. Another possibility would be to check the altitude readout on the transponder or TCAD system, which shows the encoder reading. In Jim's case that wouldn't have helped, but it could in other cases. For aircraft that have a radar altimeter, the reading on that equipment would be a possibility if the airplane is within 2,500 feet of the ground.

Pressurized airplanes have another alternative. If altitude permits, the cabin can be depressurized. It is a simple procedure but is rarely practiced outside of an FAR Part 135 operation. If Jim had depressurized the cabin the static system would be sensing normal atmospheric pressure and his altimeter would have started working correctly. Depressurizing the airplane would also allow the pilot to use the little altimeter that shows cabin altitude as a reference for the altitude of the airplane. It is only accurate within a few hundred feet, but it does provide a meaningful reference.

Good systems knowledge is essential, since checklists cannot be compiled to cover everything. It is just not possible to cover all possible failure modes of all systems. For example, 21 pilots of pressurized airplanes were given details about this flight, and not one reported ever being given a discussion about a static system problem where the alternate static system was not the answer. The knowledge of the system should lead to the fix.

Instructors should be teaching pilots ways to stay alive when things go wrong. A complete knowledge of an aircraft's systems is invaluable when the chips are down, and creativity is required.

David Dewhirst is president of Sabris Corporation, a firm engaging in aircraft sales, rental, instruction, and consulting in the business of aviation. He has 7,000 hours, including 5,000 as a flight instructor. He lives in Wichita, Kansas.

By David Dewhirst

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