Diversions Part 1.

This is a two parter (or maybe three) on diversions. Enjoy! Like the book, for simplicity, pilots are male.

When pilots talk about diversions, chances are they're not talking about a fondness for Harlequin romances, basement model train sets or making homemade freezer jam. A diversion to a pilot most often means the unplanned arrival of his airplane at a place other than what it said in the "destination" box on the flight plan before departure.  At its simplest it goes like this; airplane flies from A to destination B where weather is forecast to be within landing limits at ETA. Airplane arrives and finds it unable to land at B because of sudden crappier than forecast weather. Airplane diverts to airport C and lands.

It all sounds simple but diversions can be forced upon crews for a variety of reasons and can be quite stressful for a variety of reasons. These reasons usually occur near the end of the flight when fuel becomes a more pressing issue, especially if the diversion airport's (known as "alternates"} weather has deteriorated as well. Many "to divert or not to divert" decisions are not cut and dried. The Captain must weigh many factors in the decision with overall safety of pax and crew the number one priority. There is pressure on these decisions, even though they remain unspoken. Massive extra  costs to the company. Inconvenienced, delayed and upset passengers. Crew Duty Day limitations grounding the crew and precluding a quick fill up and return to destination requiring an overnight stay. "Get-Home-Itus", the insidious mind set to "land where it says on the ticket!" found in some tired and cranky Captain. Get-Home-Itus has been responsible for much mayhem, incidents and accidents since flying began. NONE of the above should alter the decision to carry out what is the safest course of action. But I'll bet all who have been there will honestly admit that such things cross their  minds as they strive to arrive at their decisions.  Some random factors;
What is the weather here? Is it improving or not? How much extra fuel do I have? How many airplanes are stacked up ahead of me?  Our duty time? The Flight Attendants' duty time? How much fuel again? What is the landing rate? What are the runway conditions? How much fuel do I have? How is the weather at my alternate? Is it holding? Worsening? How much fuel do I have? How long do I hold for an attempt to land? Fuel?
But eventually, the decision is taken out of the Captain's hands. Unless he is wheels down and cleared to land (or VERY close to it) he MUST divert to alternate with his flight planned fuel to alternate plus an approach plus his minimum in tanks (plus any extra for the wife and kids as we say) as the Captain so decides. To hang on further in a destination holding pattern to  below this level with your fingers crossed is asking for trouble.    
Or, a reason to divert can come as a total surprise (airport suddenly closes....later on that one) and suddenly, the Captain and crew, rather than thinking about that pint at their local layover pub have a whole lot of new things to do and think about and not a lot of time in which to do it. 

What helps with the completion of a successful diversion is that the airports are picked very deliberately and filed as such on the flight plan before departure. They are not pulled out of the Captain's ear at the last minute or decided upon by flinging a dart at the local map. And during flight, it is the joint duty between flight crew and their "Star Fleet Command" flight dispatcher to monitor the weather at these chosen alternates in case they fall below limits. They do on occasion and I have had to change official alternate airports several times in flight during my career. It's no big deal but the vast majority of commercial aviation flights MUST "carry" a legal alternate airport. And for many years now, given the programmable magic flight management computers, air crews will often program their planned flight routing to their alternates from their destinations even before taking off. It's for "just in case" reasons, good airmanship and serves as a great time and work saver when things get hectic as you find yourself off to some place you hadn't planned on.
As mentioned, alternates are carefully chosen. They must be "legal" alternates which is simply pilot speak short form for an alternate airport that meets all requirements both on the ground and weather wise. On a long trip, an alternate may be selected even though the weather at take off time at the alternate may disqualify it. But if the weather is forecast to improve enough for it to qualify at the estimated time of arrival, it becomes a legal alternate again. A legal alternate must be able to accommodate your aircraft type with appropriate approach aids and lighting, (CAT III ILS is very nice!) sufficient runway lengths, ramp weight limits, fuel, water and lav service, catering, passenger handling, customs (sometimes) as well as the weather. Even something as simple as lack of a tow bar or a mule incapable of pushing back your big airplane will disqualify an airfield from the list unless a dire emergency warrants landing there. Filing a cross border alternate and actually having to use it can be a customs problem but probably less so these days with passports being the air travel ID document of choice world wide. So in short, you can't file Barney's Sunny Acre Air Patch, Petting Zoo and Flying School as your alternate from Heathrow to Toronto Pearson if you're flying a 747...even if the weather's fine at Barney's and he's a great guy. The reasons are obvious. 

Speaking of legalities, the regulations about minimum fuel on board (enroute, approach, missed approach, fuel to alternate and reserve fuel) are strictly spelled out for all commercial aviation. And money is a big factor in these planning exercises. Fuel equals weight and the old saying is that it takes fuel to carry fuel.  The heavier the aircraft, the more fuel it will burn to carry that weight to destination. That is simply physics. And if the alternate is 400 miles away rather than 30, then you must carry the fuel required to reach it plus the specified reserve PLUS the extra fuel required to carry that extra weight for the entire flight duration!   We're talking literally tons of fuel here.

A ridiculous example would be to fill a 747 to the brim to fly from Toronto to Montreal. Makes no sense whatsoever. The fuel used to carry that extra weight would far exceed the normal burn for such a flight....costing the airline money...and not to mention that it would have to hold and burn more fuel or dump enough of it to reduce down to the big ship's max landing weight. Many modern jets do not have fuel dumping capabilities anymore. I know the B-747-400 did not. The additional plumbing required for fuel dumping is heavy and the extra weight takes more fuel and more fuel costs....well...you get the picture.  

We did this once when I was a second officer on a B-747-200 at AC. An engine ran rough on climb out of Toronto on a trip overseas. The Captain decided to return to Toronto which was the best decision. It was close by, weather was fine, maintenance available, it was probably "home" to the majority of passengers on board if the flight had to be cancelled,  there was a place to dump fuel and lastly, one does not venture across oceans if one of your engines is not playing nice. 
We set up in a race track shaped holding pattern over the middle of lake Ontario at around 10,000 feet and I started the fuel dump procedure. I flicked the switch and fuel blasted out each wing tip like two gigantic pressure washers. I forget the rate but it was impressive. It was also not the environmental disaster you are envisioning. The spray is so fine, and we being 2 miles up, the fuel was all but completely evaporated long before it reached the water.  But there would've been a certain lingering odor...which is why we did it over the lake!

Until Part II

Over 'n Out