On Friday, October 21, an instructor and a student were on a long cross country in N65842. The student pilot was receiving VFR flight following from ATC into Savannah. While waiting for ATC to instruct him to begin his descent, he missed his top of descent checkpoint and as a result, was very high on his approach to the airport from a cruising altitude of 7,500 feet. In an attempt to salvage the approach, the student initiated a descent at idle power. He turned the carburetor heat on, per the POH’s recommendation, but remained at idle in the descent for several minutes. The approach and landing were normal, but after he exited the runway and was preparing to complete his after landing checklist, the engine quit. After several attempts he was able to restart the engine and taxi to the FBO, but was understandably concerned.
When the instructor called me to discuss what happened, I asked him how much fuel was onboard, were the fuel caps secure, and was the primer in and locked. Inadvertent fuel starvation was my first thought because historically speaking, this is one of the top reasons why aircraft engines quit spontaneously. They took off with full tanks, but could there have been a leak in flight? After determining they had plenty of fuel, I told them to go get some lunch and relax and call me when they returned to the airport in an hour. After topping the fuel tanks, a thorough preflight and a successful engine start and runup, the instructor and student flew the plane back to Craig without further incident. A postflight inspection by our mechanic revealed nothing abnormal.
What could have caused the engine to quit after landing? The most likely culprit is carburetor ice. The images below, from the FAA Pilot’s Handbook of Aeronautical Knowledge, illustrate the basic function of an aircraft float-type carburetor and how ice can form inside the carburetor or on the throttle valve.
After the instructor and his student had returned to Craig, I asked the instructor what the outside air temperature was at 7,500 feet, and he said it was about 40 degrees, with surface temperatures that morning ranging between 60-70 degrees. That is within the range for carburetor ice. I also asked him if the mixture was leaned for cruise while in the descent at idle with the carburetor heat on, and he said it was leaned until the student performed the before landing checklist after descending through 3,000 feet, when the mixture was returned to the full rich position.
I spent the weekend researching other possible causes, including carburetor flooding (or “loading”) at idle due to the design of the idle circuit of the carburetor. (Here is a link to a very interesting video on aircraft carburetors, presented by A&P mechanic and author Mike Busch: https://www.youtube.com/watch?v=zxmG_QEO7_s; the relevant discussion begins at approximately the 16 minute mark.) After hours of scouring the internet, watching YouTube videos and poring through the FAA pilot training handbooks, all signs still pointed to carburetor ice. Still, I noted some serious deficiencies in the relevant guidance provided to pilots in the FAA handbooks.
The FAA Airplane Flying Handbook, in Chapter 10, describes the process for executing a steep spiral maneuver, which is required for commercial pilots, not private pilots. Because this maneuver involves a rapid descent with the throttle at idle, the handbook cautions: “Operating the engine at idle speed for any prolonged period during the glide may result in excessive engine cooling, spark plug fouling, or carburetor ice. To assist in avoiding these issues, the throttle should be periodically advanced and sustained for a few seconds.” Chapter 9 states: “During all simulated emergency landings, keep the engine warm and cleared.” But it doesn’t say what “cleared” means, or how or why a pilot should do this.
The FAA Pilot’s Handbook of Aeronautical Knowledge doesn’t explain what “clearing” means at all, though it does offer the following advice regarding managing the fuel mixture during a descent: “During a descent from high altitude, the fuel-air mixture must be enriched, or it may become too lean. An overly lean mixture causes detonation, which may result in rough engine operation, overheating, and/or a loss of power. The best way to maintain the proper fuel-air mixture is to monitor the engine temperature and enrich the mixture as needed. Proper mixture control and better fuel economy for fuel-injected engines can be achieved by using an exhaust gas temperature (EGT) gauge. Since the process of adjusting the mixture can vary from one aircraft to another, it is important to refer to the airplane flight manual (AFM) or the POH to determine the specific procedures for a given aircraft.”
The Cessna 172 POH says virtually nothing about making descents at idle power, other than a recommendation to use carburetor heat to prevent carburetor icing, but gives no reference to any possibility of flooding the carburetor.
Search the NTSB aviation database for “carburetor ice” and you’ll get hundreds of reports of engine failures in various phases of flight. This one seems particularly relevant:
The pilot reported using carburetor heat during the descent; however, the pilot did not periodically apply engine power (clear the engine) during the descent. According to FAA Advisory Circular 20-113, Pilot Precautions and Procedures to be Taken in Preventing Aircraft Reciprocating Engine Induction System and Fuel System Icing Problems, “Heat should be applied for a short time to warm the induction system before beginning a prolonged descent with the engine throttled and left on during the descent. Power lever advancement should be performed periodically during descent to assure that power recovery can be achieved.
It’s worth noting that this AC, written in 1981, is not referenced anywhere in the current edition of the Airplane Flying Handbook, the Pilot’s Handbook of Aeronautical Knowledge, or the FAA Private Pilot Airman Certification Standards. How, then, is a pilot today expected to know that this 40-year-old document exists, let alone be familiar with its contents?
Another interesting element of our instructor and student’s experience has nothing to do with carburetor ice, but rather with the way VFR pilots utilize ATC services on a cross country flight. The FAA Aeronautical Information Manual (AIM), Chapter 4, Air Traffic Control, describes Radar Traffic Information Service, otherwise known as VFR flight following, in section 4-1-15.
This service allows air traffic controllers to provide traffic advisories to pilots on a workload permitting basis, and controllers have the discretion to discontinue the service at any time. It’s also important to remember that receiving VFR traffic advisories from ATC does not constitute an air traffic clearance, though it is possible for a pilot to request and receive an ATC clearance through certain airspace (such as Class Bravo) while receiving VFR flight following. Pilots must maintain VFR and are free to change altitude at their discretion, airspace and weather permitting, though the AIM states that pilots should inform the controller when changing VFR cruising altitude. In the case of our student pilot, he could have avoided the prolonged idle descent by simply initiating a descent at cruise power further from the destination, and advising ATC of the altitude change.
I think we all learned a lot from this experience. Thank you to the instructor and the student for sharing.