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Another consequence of very high flight to maintenance ratios in aircraft – vulnerability to disasters

Posted by altandmain on October 15, 2018

This article discusses how flight to maintenance ratios can leave air forces exposed to the unexpected, including natural disasters.

A high flight to maintenance ratio is one of the big issues in fighter aircraft that are complex. It is often an issue that does not go acknowledged during the procurement stage of military spending, but has immense consequences. Lower is better in this case, because it means less maintenance is needed per hour of flight.

Flight to maintenance ratio, roughly means that for every hour of flight, how many man maintenance hours does one need on average to keep the aircraft operational? A high flight to maintenance ratio means that more man hours are needed for every hour of flight.

The B2 stealth bomber, for example has the worst flight to maintenance ratio, and as a result seldom flies.

Hurricanes and other losses

Recently, a hurricane struck the United States. What occurred is that the hurricane, Hurricane Michael, took a sudden turn that was not expected and changed course towards the Florida Panhandle, where Tyndall Air Base is located. Tyndall Air Force Base is a major US Air Force base, where many F-22 stealth fighters are stationed.

There were 55 F-22 aircraft stationed there, but only 33 were moved out and the status of the remaining 22 are not known. It would seem that some were destroyed by the hurricane.

It is likely based on the comments that the USAF could simply not evacuate them quickly enough.

Air Force officials have not disclosed the whereabouts of the remaining 22 planes, other than to say that a number of aircraft were left at the base because of maintenance or safety reasons.

An Air Force spokeswoman, Maj. Malinda Singleton, would not confirm that any of the aircraft left behind were F-22s.

But photos and video from the wreckage of the base showed the distinctive contours of the F-22’s squared tail fins and angled vertical stabilizers amid a jumble of rubble in the base’s largest building, Hangar 5. Another photo shows the distinctive jet in a smaller hangar that had its doors and a wall ripped off by wind.

All of the hangars at the base were damaged, Major Singleton said Friday. “We anticipate the aircraft parked inside may be damaged as well,” she said, “but we won’t know the extent until our crews can safely enter those hangars and make an assessment.”

It is likely based on the comments that the USAF could simply not evacuate them quickly enough. The extent of the damage, if you read the full article, is not yet known, but appears to be extremely extensive.

The reason why aircraft like the F-22 are going to be affected by this is because stealth aircraft have a set of unique characteristics that make them vulnerable:

  1. A very high flight to maintenance ratio means that they simply cannot take off quickly enough. They are down for maintenance when they need to move most urgently.
  2. There are fewer of them (since there are less than 200 F-22s due to the costs to manufacture them and because the program was terminated early), so any losses will be a larger percentage of total fleet losses.
  3. Where they can evacuate to is limited. Stealth aircraft like the F-22 or B2 need special climate controlled hangars and have a lengthy supply chain.
  4. The sheer complexity means that there will be unexpected downtown of these aircraft. From the article below:

The high-tech F-22 is notoriously finicky and not always flight-worthy. An Air Force report this year found that on average, only about 49 percent of F-22s were mission ready at any given time — the lowest rate of any fighter in the Air Force. The total value of the 22 fighters that may remain at Tyndall is about $7.5 billion.

This is inherently exposed in not just a war against a competent enemy, but also in peacetime against natural disasters.

A brutal reality of this world is that disasters that occur suddenly will happen. Whether they be hurricane/typhoons, tornadoes, earthquakes, fires, or any other disaster, a smaller fleet with a high flight to maintenance ratio means that the entire fleet of aircraft is far more vulnerable to being caught by surprise by these events.

The F-35 would also be affected

The F-35 JSF is significant in that it will be replacing the majority of aircraft in not just the USAF fleet, but also several NATO allies.

The F-35 also suffers from a high flight to maintenance ratio.

Four years into their operational career, F-35 fighters are expected to require between 41.75 and 50.1 maintenance man-hours (MMH) per flight hours, or about three times as many as most fighter aircraft currently operated by Western air forces.

Considering both the F-35 and F-22 are in this situation, this will leave the fleets of Western air forces far more vulnerable, not just to enemies in a war, but also to natural disasters and other unexpected events.

What would be the solution?

The solution is to procure cheaper aircraft (so that more aircraft can be procured and losses would be a small percentage of the fleet). These aircraft would be more widely dispersed, so that they are harder to destroy, either by an enemy, or in this case by a natural disaster.

Picard’s FLX proposals are a good step forward to protecting the Western air forces against losses like this one.

The existing aircraft would be retired. This is not without historical precedent. The F-14 was retired in part due to its unfavorable flight to maintenance ratio.

The decision to incorporate the Super Hornet and decommission the F-14 is mainly due to high amount of maintenance required to keep the Tomcats operational. On average, an F-14 requires nearly 50 maintenance hours for every flight hour, while the Super Hornet requires five to 10 maintenance hours for every flight hour.

Not only does a more complex aircraft mean fewer aircraft due to the expensive unit costs, but it also means fewer sorties per aircraft because they will be down for maintenance. This is on top of the greater vulnerability to natural disasters.

A historical perspective

Natural disasters, in the history of war, have played a key role in swinging the outcomes of many wars in the past. It would be extremely inadvisable to think that modern armies are immune to the effects of unexpected natural disasters and other weather events.

Furthermore, if we consider the effects of a natural disaster on these aircraft, what could the effects of a surprise attack be from a competent enemy? With so few aircraft concentrated in a handful of bases, and with such a high flight to maintenance ratio, the losses could be quite bad indeed. These aircraft would be in known locations on the ground. Forget for a moment about being radar stealthy in the air and worry about being exposed on the ground. Prudence would demand that we reduce our vulnerability to such events.

In a way, the OODA loop is much slower with these aircraft. Since it takes so long to maintain each aircraft, the ability for an air for with a large percentage of its total aircraft with high flight to maintenance ratios to react to a changing battlefield. It would take more time for example, for intelligence to be relayed and then a sortie generated in response to that intelligence. There are fewer aircraft, concentrated in a handful of locations, and they need more time before going to a sortie to prepare.

Clearly the solution is to procure more aircraft that are less complex, cheaper, and easier to maintain. This will not only mean fewer losses due to unexpected events, but also a much faster OODA loop.

This is an extremely costly lesson to learn, but fortunately one that was learned in peacetime and not in a war.

On a final note, I wish the best of luck to everyone affected by Hurricane Michael.

 

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Posted in weapons | Tagged: , , | 2 Comments »

Why variable sweep wings or “swing wings” for fighter aircraft are not effective at air superiority

Posted by altandmain on August 19, 2017

Why not another variable sweep fighter?

There seems to be a lot of F-14 nostalgia around. While it may have had a great deal of impact on how the US Navy conducted fleet defense, we have to consider the effectiveness of the concept of variable sweep aircraft. It is human nature to always want to look up to the past.  The other reason may very well be that people find the F-14 to look visually attractive and want similar proposals.

The reason why we will not see future variable sweep fighters however is because there are very serious drawbacks compared to fixed wing aircraft.

Short Background

Variable sweep wings, known as “Swing wing” evolved as a solution for early jet engines. Experiments were being made as early as WW2 with wings that could change their sweep on the ground, such as the Messerschmitt P.1101.

Back then jet engines produced less thrust because they ran at lower inlet temperatures and were overall more primitive. Wings with a sharp sweep were desired for high top speed, but that left the aircraft vulnerable in dogfights, which as Vietnam revealed still happened, and also led to high take-off and landing speeds. High take off and landing speeds are less safe, which would result in increased number of crashes. They also led to long runways, limiting off  road mobility and making it easier to disable for enemy forces, as there would be a far larger airport to protect.

In Europe, there were two key projects, the Panavia Tornado, which entered service as a mult-role interceptor/bomber, and the Dassault Mirage G, which never entered production. The US would build the F-111, which was a very heavy variable sweep multi-role aircraft. The famous F-14 was derived from the F-111. The USSR made several variable sweep designs, most notably the  Mig-23 and the Su-24.

Bomber designs were also made by the US and USSR. The B1 Lancer from the US, along with the Tu-22 and Tu-160 from the USSR. All 3 bombers remain in service.

What do swing wing aircraft bring?

Their main advantage is that they can use that variable sweep wing to find the optimal wing swing angle (within their sweep limits) for a given airspeed.  This can allow for fuel savings on the climb and landing during a fighter sortie.

On aircraft carriers, they have the advantage of having very low sweep on take-off and very high sweep when bursting with full afterburner. Variable sweep wings can also be folded for compact storage without compromising wing’s structural integrity (as is the case with folding wings like on F-18E).

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On an aircraft carrier, deck-space is always going to be a bottleneck. While a carrier may look very large to an untrained eye, deck space is always at a very big premium.

So why not on fighter aircraft?

To achieve variable sweep aircraft, that requires a large gearbox in the fuselage of the aircraft. This gearbox adds a great deal of mass and makes the fuselage larger, causing drag. This means that fuel fraction on such aircraft is lowered a great deal.

In a dogfight, this heavy gearbox would mean that compared to a fixed wing, it would result in an unfavorable thrust to drag, even if the pilot could switch to what they felt was the optimal sweep right before combat. Switching the wing sweep during a dogfight would be risky, as it could cause a loss of energy.

This would mean:

  1. Higher wing loading due to mass of gearbox
  2. Faster fuel consumption due to gearbox
  3. Lower transient performance (very important in a dogfight)

This gearbox would also lead to lower G limits as well. On the F-14D, the symmetric limit at 50,000 lbs was 6.5G. The F-16  and F-15 were both capable of 9G. Navalized versions of the F-18 were capable of 7.5G, while certain land based variants of the F-18 could also perform 9G. For a comparison, Dassault Rafale can do 11G, with an ultimate limit of 16.5G.

The gearbox lowered the aircraft’s fuel fraction. An empty F-14D has a mass of 43,735 lb ( or about 19,838 kg) and can take on 16.200 lb of fuel. This results in a fuel fraction of 0,27, which is below 0,30 fuel fraction required for sufficient combat persistence.

Jet engines have become far more powerful than their 1960s and 1970s counterparts, allowing for much higher thrust to weight ratios. As such, they can achieve lower take distances, even more so on an aircraft carrier with a catapult. This fact somewhat negates swing-wing’s main advantage of high low-speed efficiency.

Modern computer control surfaces too have played a role in rendering variable wing sweep obsolete as they can adjust wing shape and size very rapidly, without the weight penalty.

Complexity and reliability problems

The more complex a system is, the more risk there is for failure.

When the US Navy opted to retire the F-14 in favor of the F-18, a big reason that was given was the appalling flight to maintenance ratio.

The decision to incorporate the Super Hornet and decommission the F-14 is mainly due to high amount of maintenance required to keep the Tomcats operational. On average, an F-14 requires nearly 50 maintenance hours for every flight hour, while the Super Hornet requires five to 10 maintenance hours for every flight hour.

I’ve been told that a newer F-14 would likely require 40 to 1 and on average, the F-18 requires 8 to 1, which is in line with the USN’s claims of 5-10 to 1. So in that regard, the F-18 would be able to generate much higher sortie rates. Keep in mind that the 50 to 1 is with after  the General Electric F110 engines were put on the F-14. Early F-14s suffered from an unreliable TF-30 engine that was prone to flame-outs.

Compounding the problem, the  high flight to maintenance ratios mean that there’s a good chance you will not have enough F-14s available when you need them the most (ex: if an enemy launches a surprise attack on your carrier battle group, you may need to scramble the aircraft very quickly).

There were other points of failure. Sometimes when one side of the gearbox worked properly and the other did not, it could lead to an “asymmetric wing sweep”.

f-14-asymmetricWhile the aircraft could fly in such a situation and land with some difficulty, this leaves a point of failure. This could also be a weakness in combat, as the hydraulics could be damaged.

Much like this F-14, under Australian service, the F-111 did encounter a similar incident, and the B1 did once as well. I suspect that under Warsaw Pact service, Soviet variable sweep designs may have too.

Conclusions

The cons simply outweigh the pros when it comes to variable wing sweep. There are very significant penalties in terms of mass, cost, and complexity for variable sweep wings. While they may bring some advantages in the take-off and can have the “optimal” sweep for each scenario, the drawbacks outweigh these to the point where we are not seeing variable wing sweep aircraft on modern aircraft.

They are simply a dead end as far as aircraft design goes. While they may have seemed like a good idea on paper, when implemented in combat aircraft, they carried significant drawbacks that outweighed any advantages they brought.

 

Posted in technology, weapons | Tagged: , , , , | 15 Comments »

 
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