Why don't modern airlines have battery backup power for their instruments?

Hello Everyone,
Watching a ton of Crash Investigation shows and one thing that has always seems so odd to me is that modern airliners don’t have backup batteries for power.

When a jet loses its engines it loses the ability to make electrical power. And because most modern passenger aircraft no longer use analog instruments, instead opting for glass displays they lose everything except the very basics.

The basics are kept going by a small drop down propeller that windmills in the slipstream to provide minimal juice for some basic instruments and perhaps the radio.

With modern battery tech why isn’t there back up batteries installed that would allow pilots to retain full use of all their instruments in the event of engine loss? What am I missing here,

The “basic instruments” are, in fact, analog instruments.

Batteries have weight, and sufficient batteries to run “full” backups might be prohibitive. They would also require testing and certification, which is hella expensive. So much so I sincerely doubt already built airliners would be retrofitted with such a system, were it ever developed.

As far as I know modern airplanes do have batteries. Not enough to run all the electrical systems at once but just the necessities. When a plane is first turned on it uses the battery to power up before the engines can be started.

Most big airliners also have a ram air turbine which can be used to power critical systems during an engine failure.

Battery fires grounded the entire Boeing 787 fleet in 2013, including a fire in a battery that powered the plane’s auxiliary power unit. Boeing never exactly “fixed” the problem. Instead, they encased the batteries in fireproof containers to protect surrounding components.

You’ve deeply misunderstood the reality. Whether that’s because those vids suck or you’re not following the explanation I can’t say.

First off, one engine’s generator can power 100% of the normal electrics, perhaps less some galleys and some inflight entertainment. So unless both engines or both generators have failed, you’re not relying on the battery for anything. If both engines have failed, you’re going to be landing soon. If just one engine or engine-driven generator generator is lost, the cockpit looks and works exactly like normal.

If both engine generators are out, the auxiliary power unit (APU) we use for electricity at the gate can be started and provide electricity for the whole airplane. Just like it does at the gate every day. The cockpit and all supporting system would be fully powered. Depending on the specific model of airplane, the APU may not be available at very high altitude. Generally older near-obsolete airliners have more weaknesses in this regard.

If all 3 of those are out, all airliners have a battery system that is guaranteed to provide 30 minutes of battery power to an adequate set of instruments. The 30 minutes is for an almost worn-out battery in a state of discharge. Most batteries will last longer than that; more like 60-90 minutes, maybe even 2 hours if the battery is near-new and fully charged when the AC power dies.

The 30 minute standard is enough to fly a couple hundred miles and descend from altitude to a normal landing before it gets dark. For folks over civilized countries that’s almost certainly plenty.

In the case of a modern jet that does somehow find itself down to the battery, essentially the copilot’s instruments quit as does some of the backstage redundancy. For example, 1 GPS receiver stays powered, not the usual 2 or 3. So the Captain’s looking at all his usual stuff and the FO is looking over his/her shoulder.

Finally, there’s a last ditch separately internally powered standby horizon, airspeed, and altimeter that’ll run for another 10-20 minutes after everything else dies. These are small auxiliary instruments and it’s difficult to fly well using just them. But it’ll get the job done in all but the most adverse situation. But by the time they’re all you’ve got, you’re already out in the 1:several billion odds.

@Friedo points out RATs. Airliners that are certified for long haul overwater may not be able to reach an airport in the typical 30 minutes allowed for by just the battery. So there needs to be a way to stretch the battery. Enter the RAT which will provide enough power to avoid using any battery as long as you’re at a speed above landing speed.

Other solutions to the extended flight without engine/APU generators problem are generators turned by hydraulic power. If the engines are running the hydraulics are pressurized. And like the RAT, a hydraulically driven generator will provide power as long as the engines run.

As a reminder, when all the engines quit the game is over once you’ve glided down to the land/sea. Which will take significantly less time than the 30 minutes the battery supplies.

So in priority order we have:

  1. One generator per engine (787s have 2 per engine).
  2. One generator per APU (787s have 2) .
  3. For long haul airplanes, 1 RAT or a hydraulically driven generator or both.
  4. One 30+ minute battery.
  5. One standby flight instrument package with another 10-20 minutes run time.

Running out of power for instruments is simply not a problem.

Obviously you can always posit enough cascading failures to overwhelm all this stuff. What if lightning strikes and melts every wire? What if a fire starts in the cockpit behind the main instrument panel? But that’s not the way to bet.

I’d be curious to hear of an accident that came from truly running out of electrons.

I’m sorry I can’t remember the details such as airline and flight number of the crash. But the one that amazed me was a flight (modern) that lost all three engines due to fuel starvation. There was a massive fuel leak in a night flight, crew noticed tanks were unbalanced so they transferred fuel to low tank causing the remaining fuel to flow out. Long story short all three engines shut down due to lack of fuel and the crew loses most instruments.

According to the documentary they can’t even tell what their airspeed or altitude is. They have radio contact with ATC and ATC can supply them with airspeed but not altitude. It is stated that ATC relies on the planes transponder to relay altitude information and the transponder is one piece of equipment not powered by the little ram air prop.

So, if I take the show at face value I learn that:

A: A $100 million airplane doesn’t come with a few analog backup altitude and airspeed instruments.

B: ATC radar cannot guage the altitude of a contact

C: In case of compete engine failure everyone is screwed because airlplane designers decided we don’t need no stinking flight information.

Please tell me none of this is true.

I’m not doubting your story from the vid. But there’s probably more here than just lack of design redundancy. The event you tell of probably had maintenance or weather or piloting on top as the real causes. Not that that’s an excuse, just that if your implicit assumption is that we can engineer our way out of all downstream shortcomings, the answer is not always, or at least not always affordably in terms of reliability, weight, performance, or dollars.

3-engine airplane is a big clue. Could be a US B-727, DC-10, L1011, British Hawker Siddely Trident, or a Soviet TU-154. All of which were late 50s to mid 60s designs with weaker all-engine out redundancy because of the belief that losing all 3 was real unlikely.

As to your points:

A. Yes, there is backup power to the primary instruments and backup instruments as well. As to not knowing airspeed or altitude, that sounds like an unrelated malfunction or else they somehow kept flying past the life of the battery.

B. ATC radar has no ability to directly detect height of targets. Military air defense radar usually can, but the resolution is poor. Like “below 10K”, “10-20K”, “20-30K”, “>30K”.

Depending on the details of the specific airplane design, the entire transponder, including the altitude reporting to ATC function may drop off when you’re down to battery only. At that point you’re working real hard to find an airport and ATC is trying real hard to get everything outta your way. Your specific altitude is not really of much value to ATC.

Your own standby altimeter should be working. It may be a couple hundred feet out; the tolerances are looser. But it shouldn’t be dead.

C: As I pointed out above, not quite so cut and dried. In a 727 with all engines out I know the APU is not an option and you’re down to the battery for 30 minutes of acceptable instruments. But you’ll have descended to the surface long before that time runs out. I would expect the longer range DC-10 & L1011 to have some additional power source. I know the L1011 had a RAT and either or both types might have been able to start the APU inflight. I have no clue about the Brits or Soviets.

Later model airplanes, having only 2 engines have more layers of redundancy. And much more reliable engines as such. However no engine runs much when the fuel runs out. Running out of fuel is a very ugly problem to have.

Bottom line: None of this redundancy prevents the crew from becoming confused or overwhelmed by the unfamiliar situation. Nor prevents the airplane from being an old ratty thing where the various backup systems haven't been tested in years and don't work when needed. Or the battery being nearly dead because the battery charger system hasn't been working and the battery itself is overdue for replacement and won't hold much of a charge.

For sure, when you’re 100% out of electrons in a jet you’ve got a real situation on your hands. It better be a sunny day over land with airports every 50 miles or you’re probably dead. Even if the engines are still running. Kill all the engines and now we’re just debating about the details of the ensuing crash. Will it be a mostly controlled landing on an airport? On flat empty land? In a city? In the ocean? Or will they lose control in the dark or clouds or fly unwittingly into the side of a mountain?

There cannot be a foolproof answer to every possible scenario. What if the right wing takes critical damage at the root?

What the designers, the regulators and the pilots do is add enough layers of redundancy that minor everyday shit is a total non event, rare once a decade things are still non-events unless very bad luck, and the truly unmanageable game-over-barring-miracle things happen once per 10 billion flight hours.

Just now doing some research. I did learn that the separately battery-backed up instruments in addition to the main battery backed up instruments were first required by FAA after an accident that happened in 1969.

So depending on when the event you were referring to happened, we might be talking past one another in time.

I know a lot of those Air Disaster shows like to focus on the accidents that produced a real regulatory, engineering, or procedural change as a result. Better those stories than yet another runway overrun in slippery conditions or whatever.

So it may be that the particular episode you saw told of an event that was pivotal in the increase of redundancy. The goal being of course to prevent the same thing happening the same way again.

That sounds a lot like Air Transat Flight 236 but I can’t be sure. Even so, it is my understanding (and any Big Iron pilots here can correct me if I’m wrong) that airliners still have a pitot-static system for powering back up instruments and for such systems airspeed, at the very least, requires no power at all. It does require a pitot tube, but those are present in 2’s and 3’s on such airplanes. So they should definitely have airspeed.

Since you haven’t indicated the name of the documentary you watched there is no way for any of us to watch it and verify whether or not you remember it correctly or whether or not the documentary itself is full of solid facts or sloppy shit.

Actually, there are at least two instances I am aware of where airplanes entirely without engine power glided to safe landings on minimal necessary instruments.

What is actually “minimal necessary” versus what the general public thinks is necessary are two different lists.

The OP says his episode’s event was a 3-engine airplane. So not Air Transat 236.

As to pitot/static (“PS”) … Generally the standby instruments’ PS probes & plumbing and … are totally separated from the primary instruments’ PS probes & plumbing. Because one of the purposes of the standby instruments is totally unrelated to electrical failures. Its to be an independent source of probably reliable info in the event of a PS problem with the main instruments.

The main instrument systems, even if old-style round mechanical gauges definitely need electricity to work. The PS probes themselves output electronic data, not some example of air at some actual pressure flowing down a physical hose. At least not on machines designed since the early 1960s. That data is routed to “air data computers” for distribution to the consuming black boxes.

In the case of the standby instruments, they typically consume the raw air pressure via a hose although they too are computers using LCD screens to display the info. And needing power to do so. Which is where the 30 minute aircraft main battery comes in, followed in some airplanes by by a last-ditch battery inside the instrument itself.

A shortcoming with all these PS systems in electrical failure scenarios is that you can eventually get to a place where there is no pitot heat. Which means encountering a cloud at altitude will pretty quickly freeze up your last airspeed indicator. But by then you’ve already exhausted all your generator power, and your 30 minutes of battery power and you’re dealing with a situation with odds like one in a few billion.

As always, thanks for your insights!

[Bold mine]

Here’s the stupid question:

Izzit cause the landing lights, yeah?

I’m making fun of myself but I’m a bit serious. The main battery can power all the landing lights? I figured it would do all the cockpit electronics but I didn’t even think about the lights.

Oh… and PS- thanks for writing out what an APU is. (I’m reduced to thanking Dopers for writing in clear, unambiguous way–that’s how bad the writing has become here in general.)

It’ll be one of these:

If it definitely was three engines, then it may have been this L-1011:

But this was a morning flight with an oil problem, not a night flight with a fuel leak.

The OP’s description of the sequence of events sounds like a perfect match for Air Transat 236, even though it wasn’t a three-engined aircraft:

Not even close. Sorry. The landing lights are insanely powerful and the battery couldn’t run them for more than a few seconds. Instead, by “before it gets dark” we mean two things.

In the traditional pre-electronic days, when the battery dies you have no cockpit lights to see the still-functioning mechanical instruments by. By the perversity of Fate these severe electrical problems always seem to happen at night. So there you are in near pitch blackness unable to see the still-funtioning basic instruments necessary for survival. Everyone was and still is required to carry a flashlight, but back in the days of D cells and incandescent bulbs those didn’t last long. Nor could you be counted on to have yours at the ready when darkness suddenly occurs. And there’s the problem of how to hold it and also fly, which is very much a two-handed endeavor.

In the modern electronic LCD-screen based world, “it gets dark” refers to staring at blank screens where your instruments used to be. Even at noontime that’s a dead empty feeling. Like looking into the face of a corpse, not the living person who was there a moment ago.

Although your mistake does remind me of a fine joke from learning to fly light airplanes so many years ago:

Q: What’s the procedure for doing a forced landing off-airport after an engine failure at night?

A: Glide down to 50 feet above the ground. Turn on the landing light. If you like what you see directly ahead, land. If you don’t like what you see directly ahead, turn off the light.


FWIW I was watching a video on the TBM 940 (a GA turboprop) where they noted the plane no longer has a typical compass (think the thing floating in water or oil…no electrics whatsoever) but replaced it with a multifunction LCD display device that, among other things, is a compass.

They said that this unit has a 4-hour battery backup built-in and if you can’t get on the ground in four hours you have other problems.

So, clearly some aviation equipment can and do have battery backup. Whether each unit has to have its own or you need a centralized battery to run everything I don’t know. Probably different for GA and commercial planes.

The OPs description was of a 3 engined modern airliner. Those don’t exist so I’m guessing he misremembered one of the details and that it was indeed the Air Transat incident.

Bob Stevens cartoon: A fighter pilot is flying on a dark night and stormy when he loses electrical power. He prays to God for some light. Immediately there is a large flash of lightning. The pilot says, ‘Thank you. But could you make it not so bright, and not so quick?’ (Or something like that. It’s been a while since I’ve seen that book.)