Lighten the load of an aircraft

All aircraft have a maximum gross weight for safe operations. Weights are checked before the airplane takes off, and the pilots occupy themselves with all that is involved in flying the airplane. Sometimes something goes wrong in flight, and the plane is may be too heavy.

Allow me to tell a couple of stories to illustrate.

I have been reading about WWII bombers including the B-17 and the B-24 for some future articles for this blog. I have read some accounts of a heavily damaged bomber making her way back to base. Perhaps an engine was out and the performance of the bomber was suffering. Once they back in friendly territory and no longer at risk for being shot down in enemy territory, the pilot had to find a way to lighten the aircraft to have the best odds of returning to base. So, he would order the crew to dump equipment, armament, etc. overboard. The now lighter aircraft may be able to make it back to base. Or at least land or ditch in friendly territory.

Several months ago a commercial airliner got in trouble shortly after take-off. If memory serves, they had a hard time retracting their landing gear. They had to return to the airport, but they were too heavy to land. Procedures called for the pilots to jettison some fuel until a safe landing weight was reached. They circled at a safe altitude for something like 20 to 30 minutes. The fuel evaporates in the air, well before it reaches the ground. After the safe landing weight is reached, the pilot returns to the airport for an emergency landing. Emergency vehicles are on alert. The landing gear holds and the pilot brings the plane down safely. No injuries and fatalities. The passengers deplane and catch a later plane to their destination.

I believe that I was about three or four months into writing about aerial wildland firefighting when I first learned that all (or most) tankers and helo pilots have the capability to jettison a full load of retardant when an emergency arises. Such emergency situations where the tanker pilot will jettison a load of retardant include an engine failure, and getting caught in bad winds. Of course there are important considerations here before jettisoning the retardant because the weight of falling retardant can kill people on the ground so a tanker pilot will not make an emergency drop if there are folk on the ground in the path of the emergency drop.

I have some numbers for you to demonstrate the weight of the retardant. That will come in my next post. If the weather and the power grid allow, I hope to be making this post on Wednesday.

Engine failures with happy endings

You will recall from my first post in the density altitude series that low air density -- which equals high density altitude -- affects engine performance. Engine failure in any plane can lead to disaster. In ideal conditions, engine failure in a multi-engine plane does not have to end in a crash. But in a tanker carrying a load of retardant, engine failure often means dumping retardant in order to avoid a crash. Most air bases used by tankers have a designated area where retardant can be safely dumped without harm to people, property, or the environment. But in an emergency situation, the pilot is not always able to reach the designated dump area.

Air density can affect how much time the pilot has to dump the retardant in the event of engine failure. My friend G (air tanker pilot) has this story:

I lost an engine in a P-2 on take-off out of Missoula on a fairly cool day, about 78 degrees.  It was almost a non-event.  I feathered the bad engine and continued to climb out to a spot where I could jettison the load without endangering anybody on the ground.  On a hot day, it would have been an entirely different story.

Engine failures can require quick action and a measure of luck irregardless of air density. Here are some more stories from G:

1. I saw a DC-4DC-6 lose an engine right at rotation, about six inches above the runway. The DC-4 DC-6 is not overpowered and that guy had to pitch the load in a heartbeat.  His retardant tank couldn't have been more than three inches off the runway and he just painted the Kalispel, MT airport runway all the way down.  That shut down the airport for two hours, but the crew lived to tell about it.  That crew was Canadian and I made sure he got the safety award that month for averting an airplane crash with quick thinking.

2. I lost an engine at Chico, CA in a P-2 right when I got the nose wheel off the ground and I knew we couldn't fly out of there.  I had to decide and act in about a second and a half, so I shut it down and did a panic stop, using every inch of available runway and a little more.  But, we were alive.

3. I lost an engine in the PB4Y2 out of Jeffco, CO and I had to find a spot to pitch the load pretty quickly.  Fortunately, there was a big field behind some houses right off the end of the runway and it went there.

Density altitude and helicopters (part 3 of 3)

Density altitude affects all aircraft equally, the higher the density altitude, the lower the performance of fixed and rotor-winged aircraft. In helicopters a higher density altitude will, among other things, translate to

(1) a lower hovering ceiling at a given gross weight, and

(2) the higher the density altitude the more power is needed for a vertical take-off and under certain weight and density altitude conditions, there may not be enough power for a vertical take-off. if that is the case, the helo pilot may need to do what is sometimes known as a running take-off.

For more information on the affects of density altitude on helicopters see this article

Density altitude defined, effect on airtanker performance (part 2 of 3)

Low density air means high density altitude. An air tanker pilot friend, G, explains:

High density altitude is called that because the low density air found in hot air mimics the air found at HIGH altitudes. At HIGH altitudes and HIGH temperatures, there is not only less air for an engine to "breathe", but there is less air for the wings to ride on as well.

High density altitude is also known as performance altitude. High density altitude caused by high temperatures and high humidity can occur at lower elevations but is more of a problem at airports at higher elevations such as those in California and the southwest. If you are based at an airport at 3,000 feet above sea level and the air temperature is hot (e.g. 95 degrees) your performance altitude is going to be higher. The higher the density altitude the longer runway you will need to take off at a given gross weight. This can be computed and G explains how this works:

The entire exercise of computing density altitude is used to predict aircraft performance on hot days or in high places or both. All the airplane manufactures publish charts that help you determine how much runway you'll need at a particular gross weight and a particular altitude.  You add up the weights, figure the density altitude, (which tells you the altitude your airplane will act like its flying at), go to the performance charts and you find that at 8800 ft and full gross weight, your airplane needs, say, 6500 feet of runway to get airborne.  If the airport you are parked at only has 4000 feet of runway, you're hosed.  You either get rid of some weight, or wait until evening when the air cools.

What this means for air tanker pilots is that tankers working wildfires, e.g. in California or the southwest, may be forced to carry lower loads of retardant in order to compensate for higher density altitudes.

In my final article on density altitude, posting this Sunday, I will write a short piece on how density altitude affects helicopters.