By Andy Cooke
Where am I? How fast am I going? How high am I?
All questions that you really want to be on top of at all times, especially if visibility is not good. A mild haze can bring it down quite startlingly - if you're in what looks to be an upturned hazy-milky bowl with the edges only a mile or two away, you might well not be able to see any useful landmarks.
So, you'll be relying on your instruments and your knowledge of the wind, and factors that can affect air density (such as temperature and humidity).
All of which can be unreliable, of course.
Your airship captain will be looking at a compass, an air-speed indicator (ASI) and an altimeter at the minimum. They will probably also have a vertical speed indicator (VSI) as well and, if they want to be able to fly in instruments alone, an attitude indicator (artificial horizon). In the modern era, probably GPS and radio navigation equipment as well. I'll cover just the basic three here, as your airship captain will almost certainly have these three at least, whatever era or timeline you are in.
Compass - shows your heading with respect to magnetic north. Magnetic heading must be adjusted to give the true heading (usually by 0 to 2 degrees or so). The aircraft itself will create a distortion in the magnetic field and give a further deviation; the captain should know what it is for his own aircraft (measured on the ground) and this should be between zero and four degrees or so. To add to the frivolity of flight, magnetic compasses have their foibles, and an inexperienced captain will be interested to sometimes see the compass heading changing just by the aircraft accelerating or decelerating. Furthermore, when turning to chase a new heading, the compass may stop early, or keep turning for longer than it should. The details of why this happens would take an article in themselves, but your airship captain will simply use some rules of thumb: when turning northerly, roll out early; when turning southerly, roll out late. The compass will then do its own thing for a few seconds before settling on your magnetic heading - fly straight and at a constant speed to let it settle down before turning again.
(Of course, if you happen to have an inexperienced person having to take the controls, shenanigans can ensue as they play the old game of "chase the compass heading")
ASI - gives your speed through the air. It works by having an open aperture into a tube in the direction of flight (a pitot tube) comparing the pressure of the incoming air with a static head aperture. The difference in pressure gives you your indicated airspeed. (Note - do not ever be tempted to blow into the pitot tube to check the ASI is working. After you do, it usually won't be).
However, indicated airspeed isn't exactly correct. The density of the air around means that any given indicated airspeed will be faster (in true airspeed) in lower density air than heavier density air. Density varies... well, I'll cover that below.
Altimeter - This is usually just a sensitive aneroid barometer, but calibrated to feet (or metres) rather than hectopascals or millibars or inches of mercury. Remember that one millibar difference in pressure equates to approximately 30 feet difference in altitude. These typically have a knob to adjust the setting, so the pilot or captain can zero it on the ground, or turn it to a given reference pressure to give a reference altitude (eg changing from a known QFE to QNH, as per the last article). Of course, if the air pressure changes for any reason other than a change in altitude (for example, as pressure changes across a country, again as per the last article, or if the pressure changes with a change in weather), it will give a further inaccuracy. Knowledge of what air density effects occur helps an airship captain not to get this too wrong.
Knowledge of the weather - giving wind, temperature, humidity, pressure changes, and the like - is hugely important. Not just to adjust for the instruments, of course, but for safety in the air. First: the wind.
Wind: You really need to know this. Because your speed over the map (your ground speed) and your direction of travel over the ground does not just depend on how fast you are moving through the air and in which direction you're pointing. A following wind adds to your speed; a headwind reduces it, a crosswind gives you a sideways vector. Unless the air is dead still (very rare), you will always have a following wind/headwind component and/or a crosswind component. Which means that if you know exactly where you are, exactly what the wind is doing (at various altitudes - it changes with height), and your instruments are perfectly accurate (spoiler - see above; they're not), you can work everything out perfectly.
Above, you will see a useful chart showing how the winds will change with altitude (both in strength and direction - for example, the chart shows that if you go from 2,000 above sea level to 10,000 in the south-west area of the UK, the wind will veer from coming from a heading of 310 degrees at 10 knots to coming from 180 degrees at 40 knots. That could really catch you out. Relying on the direction of smoke from chimneys on the ground won't necessarily help). In the UK in 2019, the Met Office updates these every six hours. In, for example, Persia in 1920, you're probably on your own...
I won't go into painful detail over the trigonometry of the navigation, except to highlight that at airship speeds, the wind tends to be considerably more important than at airliner speeds. A forty mile-an-hour headwind when you're cruising at 100 mph true airspeed versus the same as a tailwind means that a 210 mile trip in one direction will take one and a half hours; in the other direction it'll take three and a half hours.
Apart from winds caused by pressure systems, ground features will cause winds different than the chart indicates. Katabatic wind is colder air from the tops of mountains rolling down the slopes into the lower grounds of valleys and typically happens at night. Anabatic wind is the opposite - air in valleys warming up more quickly than air at the top of hills and mountains and moving upslope thanks to uneven heating (aka a "Valley Breeze") and typically happens in the mornings.
A sea breeze comes from similar effects. As water retains heat and cold better than land, the land warms up in comparison to it during the day and cools down faster at night. Accordingly, there is a wind blowing on to shore during the day at low level, and out to sea (or lake, or other large body of water) at night (the winds at altitude will be the opposite). There will also be a downdraft over the sea (in comparison to the land) during the day; the reverse at night. The convergence zone between them can cause clouds and even thunderstorms at times.
(You will also get turbulence from ground features at low level, which can give straightforward low-level turbulence, hill lift, wave effect, and even rotor effect. And, from other aircraft, wake turbulence. I can go into these in the comments or a later article if desired)
Air Density - This, above all else, is of paramount interest to an airship captain. After all, the way his vessel flies is by having an average density below that of the air below it - it seeks out the altitude at which the air's density is exactly the same as that of the airship.
The fundamentals of physics dictate that density decreases with pressure, and increases with temperature. And, less intuitively, as relative humidity increases, density decreases. So, if your airship is heading to a warmer area, expect to go up, if you don't do anything about it. Going into a humid area, you'll naturally descend. Travelling across areas where you don't know the temperature changes, humidity changes, and pressure changes, especially in poor visibility, is likely to help turn an airship captain prematurely grey.
Remember as well, that a one hectoPascal change (equivalent to 0.03 inches of mercury, if your alternate history uses those units) leads to a 30 feet change in altitude. A few hPa here or there won't matter too much if you're clear of the ground by hundreds of feet, but if you're cutting it fine on your route, or hugging the ground for any other reason... well, as the character 'Jamie' says in Beyond the Sunset: "Always remember that the ground always has right of way."
As the density decreases (higher temperature, or higher humidity), your ASI will be reading low (so you're going faster than it says) and your altimeter will be reading high (so you're lower than you think). This, also, can catch out an inexperienced pilot or captain (you have someone unfamiliar with the instruments at the controls thanks to whatever has happened in the story? Oh dear...)
It's also useful to pay attention to the temperature details in these charts, and not just for the comfort of the passengers. When the temperature drops below the dew point, moist air condenses into clouds. For ease of reading charts and estimating, you just need to know that dry air cools at about 3 degrees Celsius per 1000 feet. Saturated air cools at about 1.5 degrees Celsius per 1000 feet. When the rate at which the temperature is dropping with altitude suddenly reduces, expect clouds.
And, in those clouds, you really want to know the windspeed and direction, and the ground air pressure (as per last week's charts). Fine if you're in a time and place covered by the Met Office charts; not so easy if not.
If you are away from Met Office assistance, the specifics of clouds and weather features can help you out - but I've gone on too long already. If desired, I can cover these in a later article (comment in the forum if you'd like to see these).
Andy Cooke has written the sci-fi Endeavour trilogy (The End and Afterwards, Diamond in the Dark, Beyond the Sunset) and the political alternate history Lectern books (The Fourth Lectern, The Fifth Lectern), published by SLP