Writing about hypoxia in the December issue of Aviation Safety had me reminiscing about my days doling out high-altitude endorsements during a Part 135 training program. The Pilatus PC-12 was a great transition aircraft and boy, was it capable of rapid descents; flown close to the limit, we would regularly see 6-7000 fpm down and some impressive deck angles. It was great fun in the aircraft, but dealing with ATC was always a challenge. We had to file IFR to get into the flight levels, only to attempt to depart those flight levels as rapidly as possible.
Emergency descents are a useful maneuver even outside the flight levels. Smoke/fire/fumes or a medical emergency can justify an emergency descent and proper execution in time-sensitive situations can be lifesaving. Emergency descents are not something convenient to practice and in my experience are rarely brought up during flight reviews.
Execution By The Book
Often the trigger for an emergency descent may require action before deviating from the aircraft’s flight path. In a pressurized aircraft with a cabin pressure issue, the first step will generally be a memory item calling for the crew to don oxygen masks. In a smoke/fire/fume situation, a mask or other smoke removal technique may be prudent. If the smoke is so bad that the cockpit visibility is poor, pointing the aircraft straight at the ground with minimal ability to perceive the instruments would be far from ideal.
Once ready to initiate the descent, it’s imperative that the POH/AFM guidance for emergency descents be adhered to. I am guessing that most of us have a basic idea of how to conduct the maneuver, but if it is not a memory item in your plane, it’s worth a quick review. Some are short, simple and to the point. For example, the Cirrus SR22 is three items: power lever to idle; mixture, as required; airspeed at VNE.
Conversely, the emergency descent checklist for the Pilatus PC-12 consists of 10 steps, along with many sub-steps that we mere mortals would have trouble committing to memory on top of all the other stuff bouncing around our heads. Some aircraft, like the SR22, call for an emergency descent to be conducted clean. This simplifies the maneuver, because in smooth air the only speed that needs considering is VNE. Flaps, gear or other drag devices can increase the descent rate and allow the maneuver to be flown at lower airspeeds, which can reduce the overall stress of the airframe. If configuration is required, it will require slowing down to below the VFE or VLE before pointing the nose down. Some aircraft may descend right below flaps or gear speed, others may be able to speed right up to VNE once the configuration was set. For example, in the PC-12, you would slow to 180, put the gear down and once the gear was down and locked, you could accelerate to VMO, the aircraft’s maximum speed.
Beyond The Book
Another consideration is bank angle. Here is the Airplane Flying Handbook’s (FAA-H-8083-3B) opinion: “When initiating the descent, a bank of approximately 30 to 45° should be established to maintain positive load factors (G forces) on the airplane.” I checked six different AFMs I had handy to see if any call for banking the aircraft: none provided guidance one way or the other.
My semi-educated guess is that the lack of guidance from the manufacturers plus the guidance in the AFM means dealer’s choice: whatever action is required to meet the extent of the emergency. There are pros and cons to each.
Banking the aircraft will reduce the vertical component of lift and increase overall descent rate. If the emergency occurs over an airport or otherwise ideal landing spot, it helps to not drift away, especially during an engine fire where power may not be available and descent planning is critical. If the issue is related to cabin pressure, the emergency is substantially mitigated once the aircraft is under 10,000 feet msl and in breathable air. Most high-altitude aircraft are equipped with oxygen masks and can descend fast enough to reach 10,000 while keeping the wings level. Avoiding the bank can prevent disorientation, which is especially concerning if you are potentially hypoxic. Keeping the wings level also can help eliminate any tendency for the airplane to enter a “graveyard” spiral.
Care must be taken when leveling the aircraft after an emergency descent. The biggest threat would be terrain concerns, especially in mountainous areas in conditions of reduced visibility. Having a general idea of the minimum safe altitudes along a planned flight route can quell any uncertainty, and ATC should be able to assist as well. In this situation, cabin pressure issues are also more manageable, as leveling off at any altitude with breathable air is an acceptable outcome so there is no need to rush the level-off or extend the descent as long as possible.
If the aircraft is on fire, and the emergency descent was no help fighting the fire, the goal should be to thread the needle. Level off too early, and all the work done to get down rapidly is negated and there is a chance that an approach will be flown too high, resulting in an overshoot. Level off too late, and there is a risk of not making the runway at all, or accelerated stall/structural damage.
In a perfect world, the emergency descent requiring an immediate landing will roll out around pattern altitude (+/-300 feet) in a tight downwind, close enough that an engine-out approach is possible. Easier said than done, right?
Common Errors
As we can all imagine, emergency descents can often rapidly devolve into a goat rope. My list of common errors is non-exhaustive but contains most of the ones I would see conducting this maneuver.
The first was poor or even no communication during the descent. I know what you are thinking: it’s aviate, navigate and then communicate, right? The pilot probably had their hands full aviating and navigating and never even made it to the communication step! I do appreciate this struggle, but few maneuvers are more unexpected to ATC and fellow pilots than suddenly pointing the nose of the aircraft at the ground as rapidly as structural integrity will allow. This process is further muddled by the challenges of communicating with oxygen masks on, if applicable. At some point during the maneuver, whether it is while configuring or during the descent, try to squawk 7700 and alert ATC or other pilots in the vicinity. The last thing anyone needs in this situation is a near-miss in a busy traffic pattern that could have been avoided.
Usually, pilots are great at following the checklist and properly configuring the aircraft for the emergency descent. Two errors can occur at this point. The first is related to airspeed. I would say most common and least concerning is the pilot wanting a buffer. I would frequently have to verbalize, “Push the nose down…little more…little more!” to get the plane within even 20 knots of Vmo/Vne. Depending on the situation, a little buffer may be completely acceptable. Either way, when practicing the maneuver, there is a benefit to pushing the envelope and seeing exactly what the aircraft and pilot are capable of. It was not atypical to see a cowboy push the nose right down, requiring intervention to prevent the aircraft from exceeding structural speed limits. Goldilocks-ing these two scenarios leads to a well-flown maneuver within standards.
The second common error that I would see during the actual descent was getting tunnel vision, often flying a great maneuver but completely disregarding the direction of the aircraft. I liked to simulate a fire or smoke above or around a very suitable landing point. We have already discussed whether banking is appropriate, and the upshot was that it depends on the situation. What benefits nobody is executing a fabulous emergency descent while barreling away from a perfectly good landing spot.
Look Out Below
While emergency descents are rare, we all know about the “prior preparedness” alliteration. Academically, reviewing any POH/AFM guidance that exists supplemented by information in the Airplane Flying Handbook is a great starting point. Practicing in the aircraft is even better.
My recommendation would be to not try this with any first-time or nervous flyers, but reaching your destination airport at cruise altitude and simulating an emergency descent would be phenomenal practice. Just take care to coordinate this with tower or on CTAF, because I can attest to the fact that, when flown properly, this is a far cry from a normal pattern entry.
There is nothing better than being in an airplane, improving airmanship and having a blast doing it.
Maximum Range Vs. Descent Rate
This article is primarily concerned with a maximum rate emergency descent, but both the descent and the range left over afterward are important and can often become intertwined. For example, consider flying a turboprop at FL280 and experiencing an engine failure in cruise. Often there is enough oxygen for about 10 minutes for all occupants, which is sufficient to descend to 10,000 feet msl at or greater than 1800 fpm. But what if you are operating over water and a rapid descent will take the aircraft outside the meticulously planned glide distance to land?
This was a perpetual concern operating turboprops in the Caribbean, especially on longer flights. Emergencies are often dynamic and often choices need to be made between outcomes nobody would consider ideal. The initial emergency descent may have helped fight the fire or allow the cabin to fill with breathable air, and the best use of the excess energy earned on the way down may be best suited to an optimal outcome by transitioning to a low drag, best-glide configuration once level.
